Catalytic Enantioselective Construction of Sulfur-Containing

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Catalytic Enantioselective Construction of SulfurContaining Tetrasubstituted Carbon Stereocenters Jin-Sheng Yu, Hong-Mei Huang, Pei-Gang Ding, Xiao-Si Hu, Feng Zhou, and Jian Zhou ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01496 • Publication Date (Web): 05 Jul 2016 Downloaded from http://pubs.acs.org on July 7, 2016

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Catalytic Enantioselective Construction of Sulfur-Containing Tetrasubstituted Carbon Stereocenters Jin-Sheng Yu,† Hong-Mei Huang,‡ Pei-Gang Ding,† Xiao-Si Hu,† Feng Zhou,† and Jian Zhou*,†,§ †

Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular ‡ Engineering, East China Normal University, Shanghai, 200062, P R China; College of Chemistry and § Material Sciences, Sichuan Normal University, Chengdu, Sichuan, 610066, P. R. China; State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, P R China. E-mail: [email protected]

Abstract. Sulfur-containing tetrasubstituted carbon stereocenters widely present in natural products, drugs and biologically active compounds. Catalytic enantioselective construction of such fully substituted carbon stereocenters is of current interests, with four major synthetic strategies being developed. This review summarizes the advances in this field, discusses in detail the advantages and limitation of each synthetic strategy, and outlines the synthetic opportunities still open. Keywords: Sulfur-containing tetrasubstituted carbon stereocenters, enantioselective C-S bond forming reactions, S-containing synthons, organocatalysis, metal catalysis. 1. Introduction The synthesis of optically active organic sulfur compounds is of current interests, owing to their wide occurrence in natural and various biological systems.1 For example, all of the top 10 best-selling drugs in 2012 contain sulfur, 2 and thioether structural fragments constitute a valuable molecular tool as bioisosteric replacements in rational drug design.3 In this scenario, tetrasubstituted carbon stereocenters featuring a sulfur substitution represent a prominent structural motif that widely presents in natural products, drugs and biologically active compounds. Some representative examples are shown in Figure 1. Spirobrassinin, 4 bis-N-norgliovictin, 5 thiolactomycin 6 and gliotoxin 7 are natural bioactive compounds with a broad spectrum of activities. Noticeably, chiral (N, S)-ketal motifs, featuring a sulfur or a polysulfide bridge that observed in gliotoxin, are widely present in a large family of natural products and bioactive compounds.8 Tazobactam is a β-lactamase inhibitor, playing an important role in treating clinical infectious diseases by β-lactamase producing strains. 9 Compound 1 is a gamma secretase inhibitor for treating Alzheimer’s disease.10 While it is a common sense that sulfur-containing compounds smell terrible, chiral tertiary thiol 211 is a patented flavouring agent. Polycyclic compound 3 shows potent and cell-specific antiproliferative activity. 12 Spirocyclic compound 4 is an influenza neuraminidase 1

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inhibitor.13 Oxindole based chiral (S, N)-ketal 514 and 615 belong to a group of highly active growth inhibitors for different human tumor cells. Spirothiazolidinones 716 is a potent and selective inhibitor of Mycobacterium tuberculosis protein tyrosine phosphatase B. Noticeably, the biological activities of these compounds may be greatly influenced by the absolute configuration and substituent of the S-containing carbon stereocenter. For example, the (R)-enantiomer of spirothiazolidinones 7 is almost ten times more potent than its opposite enantiomer.16 Therefore, it is highly desirable to develop methods for the catalytic enantioselective construction of S-containing tetrasubstituted carbon stereocenters in sufficient structural diversity. This will facilitate the synthesis and modification of related bioactive compounds to build synthetic libraries, which in turn accelerates structure-activity relationship studies for drug discovery and development. It is worth mentioning that such synthetic collections are very interesting for modern probe- and drug-discovery programmes, as high-throughput screening of commercial libraries has met diminishing returns.17

Figure 1. Selected natural products, drugs and bioactive compounds featuring S-containing stereocenter.

However, the catalytic enantioselective synthesis of S-containing tetrasubstituted carbon stereocenters is still a challenging task. Apart from the general challenges associated with the creation of a fully substituted carbon stereocenters such as the relatively low reactivity of precursors and difficulties in enantiofacial control due to the lesser steric dissimilarity of non-hydrogen substituents on the prochiral carbons, the strong coordinating and adsorptive properties of sulfur species may cause extra difficulties.18 For example, the interaction between a metal complex and a sulfur species might have negative influence on catalyst turnover or change the structure of active catalytic species. This is exemplified by the 2


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following two examples. Evans et al noticed that in the Diels-Alder reaction of α,β-unsaturated thiazolidine-2-thiones 8a and cyclopentadiene catalyzed by bisoxazoline/Cu(SbF6)2 complex 9 (10 mol%), the catalyst was inactivated with dienophile 8a which was recovered in 60-70% yield; however, if stoichiometric amount of catalyst 9 was used, the desired product 10a was isolated in 78% yield and 94% ee. In contrast, no catalyst turnover problem was observed when the corresponding crotonate-imide 8b was used (Scheme 1, eq 1).19

Scheme 1. The extra challenge resulted from S-containing reagents.

In the highly enantioselective thia-Michael addition using thiols catalyzed by chiral metal complex 12a/Ni(ClO4)2.6H2O, Kanemasa et al observed the time dependence of enantioselectivity. If the reaction was run for only 3 h, the desired product 13 was obtained in 70% yield and 91% ee; however, when reaction time extended to 24 h, product 13 was isolated in quant yield with diminished 80% ee (eq 2).20 The authors also confirmed that thiols might easily bind to chiral complex 12a/Ni(ClO4)2.6H2O, but this 3


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interaction was labile enough to allow the replacement of the thiol by electrophilic imide via ligand exchanging. In light of this, the time dependence of enantioselectivity implied the deterioration of the active metal catalyst during the reaction course, possibly resulting from the binding of sulfur-containing intermediate or product to the metal. Even under organocatalytic conditions, the relatively stronger nucleophilicity of sulfur might cause undesired side reactions. In the reaction of sulfenylation of 3-thiooxindole 14 and N-(sulfanyl)succinimides 15 (eq 3),21a we observed that the side products isatin 17 and disulfide 18 were easily formed, possibly because the sulfur of 3-thiooxindole attacked the incoming electrophilic sulfur to produce a sulfur stabilized carbonium ion I that led to side reactions. During the past decade, ever-increasing attention has been paid to catalytic asymmetric synthesis of S-containing tetrasubstituted carbon stereocenters. A variety of highly enantioselective reactions have been developed. Generally, these protocols can be classified into two categories, judging by whether there is a catalytic enantioselective C-S bond forming reaction or not, as shown in Figure 2. The catalytic enantioselective C-S bond forming reactions are further subdivided into nucleophilic addition reactions using sulfur nucleophiles (path a), and electrophilic sulfenylation reaction (path b). The alternative strategy which involves the conversion of S-containing prochiral carbon centers can be subdivided into enantioselective functionalization of S-containing substrates (path c) and formal addition to C=S bond (path d). The pathways a-c have been intensively studied, while path d is a newly emerged method. Both asymmetric organocatalysis and metal catalysis have demonstrated its potency in this field.

Figure 2. Synthetic strategies to S-containing tetrasubstituted carbon stereocenters.

Despite these advances, there lacks a comprehensive review article to summarize the advances in this important area. Several excellent reviews on C-S bond formations have been published,22 together with a review on asymmetric sulfa-Michael addition by Enders et al in 200723 and a review on asymmetric α-sulfenylation reactions by Jiang et al in 2014. 24 However, these reviews only cover some enantioselective C-S bond forming reactions, and are not designed to discuss catalytic enantioselective 4


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construction of S-containing tetrasubstituted carbon stereocenters. The asymmetric synthesis of tertiary thiols and thioethers was also reviewed by Clayden et al in 2011,25 but this review mainly focuses on methods using chiral starting materials or auxiliaries. Noticeably, all of these reviews did not introduce catalytic enantioselective reactions based on S-containing substrates (path c and d), which constitute an alternative important synthetic strategy. In light of this, we feel it necessary to develop a timely review article to summarize the latest achievements, discuss in depth the advantages and limitation of each strategy, outline the synthetic opportunities still open, and give the readers some inspiration to develop more useful and excellent methods for constructing such stereocenters. In the following, these exciting results are sorted by the synthetic strategies shown in Figure 2, and each section is organized according to metal-catalyzed protocols and organocatalytic versions. To help new readers in the field of asymmetric catalysis, the organocatalytic methods are organized by different activation models in each section, with a brief discussion about the possible mechanism. 2. Enantioselective addition reactions using sulfur based nucleophiles The catalytic enantioselective addition of mercaptans or thiophenols to electrophiles bearing a fully substituted prochiral sp2-hybridized carbon is an atom-economical method for the creation of S-containing tetrasubstituted carbon stereogenic centers. While metal-catalyzed enantioselective protocols meet with limited success, possibly due to the deactivation of chiral Lewis acids by sulfur-based nucleophiles, organocatalysis proves to be a fruitful tool to explore new reactions. Nevertheless, highly enantioselective methods are still very much in demand, as only Michael addition using trisubstituted electron-deficient olefins and Mannich reactions using activated ketimines are developed. In addition, the substrate scope of these protocols is also limited to a special class of activated electrophiles. 2.1 Metal-catalyzed enantioselective reactions The sulfa-Michael addition23,26 to furnish S-containing tetrasubstituted carbon stereocenters dated back to 1998, when Shibasaki et al disclosed an enantioselective sulfa-Michael addition of thiols to cyclic enones catalyzed by their multifunctional catalyst (R)-LSB.27 They found that the addition of benzyl mercaptan 11b to β-methyl cyclohexenone 19a proceeded slowly at -20 oC, in the presence of 20 mol% (R)-LSB (Scheme 2). Even with a reaction as long as 43 h, the desired product 20a was isolated in only 56% yield and 85% ee. In constrast, the corresponding reaction using cyclohexenone 19b could complete within 14 h at -40 oC with 10 mol% catalyst (R)-LSB, giving product 20b in 86% yield and 90% ee. The much lower reactivity clearly showed the difficulty in developing enantioselective addition reactions using β,β-disubstituted Michael acceptors, because both reaction partners had been simultaneously activated by the chiral catalyst according to the author’s proposal, as shown in Scheme 2.

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Scheme 2. The sulfa-Michael addition to cyclic enones catalyzed by (R)-LSB.

Later, the authors reported the kinetic resolution of 5-methylbicyclo[3.3.0]oct-1-ene-3,6-dione 21 via the sulfa-Michael addition using thiols (Scheme 3).28a In the presence of 15 mol% (R)-ALB, along with 18 mol% of 4-MeOC6H4OH as the additive, the addition reaction of thiol 11c to enone 21 worked at room temperature to give adduct 22 in 48% yield with 78% ee, with (R)-21 being recovered in 49% yield with 77% ee. Furthermore, the resulting adduct 22 could be readily transformed to (S)-21 in 91% yield upon treating with m-chloroperoxybenzoic acid (m-CPBA), followed by saturated aqueous NaHCO3. Despite room for further improvement in enantioselectivity, this work first used molecular catalyst for the enantioselective synthesis of 21 that is a key synthon in the total synthesis of coriolin28b by Trost & Curran. Both enantiomers of 21 could be accessed. The role of phenolic additive in improving

enantioselectivity might result from its binding to Li atom of (R)-ALB, which contributed to an improved chiral pocket.

Scheme 3. Catalytic kinetic resolution of 5-methylbicyclo[3.3.0]oct-1-ene-3,6-dione 21.

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2.2 Organocatalyzed versions

The past two decades have witnessed the renaissance of enantioselective organocatalysis, along with the establishment of a variety of powerful organocatalytic activation models. Many of them are very helpful to develop enantioselective nucleophilic addition using sulfur based nucleophiles, as they are free of catalyst deactivation by nucleophilic sulfur species. In this context, iminium catalysis,29 bifunctional tertiary amine/H-bond donor catalysis,30 phosphoric acid catalysis,31 and non-bonding NHC catalysis32 have demonstrated their value in the reaction development. Nevertheless, when it comes to the construction of S-containing tetrasubstituted carbon stereocenters, successful examples are still limited, and only two types of reaction have been developed, namely the Michael addition and the Mannich reaction. 2.2.1 Michael addition In 2006, asymmetric iminium catalysis was employed by Córdova and coworkers to develop a sulfa-Michael addition initiated tandem synthesis of tetrahydrothioxanthones (Scheme 4).33 Under the catalysis of 20 mol% prolinol 24a, the reaction of 2-mercaptobenzaldehyde 23 and β-methyl cyclohexanone 19a gave product 25a in 71% yield with only 35% ee. The low ee value reflects the difficulty in achieving high enantioselectivity in the initial Michael addition, as the following aldol condensation was not the enantio-determining step. On the other hand, the corresponding reaction using cyclohexenone 19a furnished the tetrahydrothioxanthone 25b in better enantioselectivity (62% ee). This once again demonstrates the difficulty in realizing excellent enantiocontrol in the sulfa-Michael addition of β,β-disubstituted enones, due to the lesser steric dissimilarity of two substituents on the prochiral center (methyl vs methylene group). The reaction possibly proceeds via the formation of iminium intermediate which effectively activates the enone to facilitate the Michael addition and following aldol condensation.

Scheme 4. Tandem sulfa-Michael/aldol reaction of 23 and cyclic enones. 7


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In 2012, the Melchiorre group presented a remarkable organocatalytic 1,6-additions of alkyl thiols 11 to cyclic dienone 26 by employing a novel vinylogous iminium ion activation.34 The merger of chiral quinine derived primary amine 27 and chiral N-Boc protected L-valine 28 proved to be a powerful catalyst for this reaction, giving the adducts in good yields and excellent ee values. Interestingly, a cascade involving both 1,6- and 1,4-addition of thiol was also achieved when thiol was used in large excess, providing adducts 29 featuring a S-substituted tetrasubstituted carbon stereocenter in excellent ee value, albeit with moderate yields and dr values (Scheme 5). The acid co-catalyst played an important role in this protocol, as the initial 1,6-addition was almost terminated in the absence of acid co-catalyst, and the use of chiral acid 28 afforded better enantiocontrol than achiral acids such as benzoic acid.

Scheme 5. Cascade reactions involving sequential 1,6- and 1,4-addition of thiols.

Bifunctional tertiary amine-H bond donor catalysis30 also proves to be a viable tool for reaction development, as the tertiary amine moiety may deprotonatively activate nucleophilic thiols, while the H-bond donors of catalyst are capable of stabilizing the transition state via H-bonding interaction and activating the electrophiles. In 2009, Xiao et al established a highly enantioselective Michael reaction of thiols 11 to β-substituted ethyl nitroacrylate 30 (Scheme 6). 35a Only 0.3 mol% of quinine derived bifunctional thiourea 31 could effectively mediate the reaction and afforded the α-sulfenylated β-nitro esters 32 in excellent yields and ee values. The thus obtained products 32 were valuable precursors to α-thio-β2,2-amino acid derivatives, as evidenced by the synthesis of 33 in 67% overall yield without erosion of ee by a three-step transformation, along with a novel β-peptide 34. The high efficiency of this protocol might originate from the synergistic activation of both reaction partners, as shown in Scheme 6.

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Scheme 6. The sulfa-Michael addition of β,β β-disubstituted nitroalkenes.

In 2013, Xu and coworkers developed a tandem Michael addition/aldol reaction sequence using 1,4-dithiane-2,5-diol 35 and β-CF3-β-disubstituted enones 36 for efficient synthesis of functionalized tetrahydrothiophenes 38 with three carbon stereocenters including a S-containing tetrasubstituted one (Scheme 7).36 The quinine derived bifunctional squaramide 37 was identified as the catalyst of choice in terms of both reactivity and stereoselectivity, allowing the synthesis of tetrahydrothiophenes 38 in good yields and good to high ee values. The authors proposed that both mercaptoacetaldehyde I generated in situ and enones 36 were synergistically activated by bifunctional catalyst 37 to produce intermediate II, which facilitated the sulfa-Michael addition to form the enolate intermediate III. A subsequent intramolecular aldol reaction afforded the tetrahydrothiophenes 38 and regenerated the catalyst 37.

Scheme 7. Cascade sulfa-Michael/aldol reaction of 35 and β-CF3-β-disubstituted enones. 9


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Later in 2014, also by bifunctional tertiary amine catalysis, Jiang and coworkers developed a highly enantioselective conjugate addition of mercaptans 11 to β-CF3-β-substituted oxazolidinone enoates 39 (Scheme 8).37 Under the catalysis of 10 mol% quinine derived bifunctional squaramide 37, along with the co-existence of KH2PO4 (2.0 equivs), various trifluoromethylated tertiary thioethers 40 could be accessed in excellent yield and ee values. The addition of the inorganic base KH2PO4 could accelerate this reaction without erosion of enantioselectivity. The opposite enantiomers of adducts could be obtained by using the pseudo-enantiomeric quinidine derived catalyst. These optically active Michael adducts had been converted to different S-containing compounds, after replacement of the 2-oxazolindinone group.

Scheme 8. Conjugate addition of thiols to amide activated β-CF3-β β-substituted olefins.

Very recently, in the exploration of N-heterocyclic carbenes (NHCs) as non-covalent organocatalyst, Huang and coworkers accomplished an enantioselective sulfa-Michael addition reaction of thiols 11 to β-trifluoromethylated nitroalkenes 41 and enones 36 (Scheme 9).38 Despite their strong Brønsted basicity, the potential of NHCs in deprotonative activation directed asymmetric catalysis is much less studied. Interestingly, the chiral carbene generated in situ from triazolium salt 42 and LiHMDS was identified as a powerful catalyst for the present conjugate reaction, furnishing the desired products featuring a trifluoromethylated sulfur-containing tetrasubstituted carbon stereocenter in high to excellent yields and enantioselectivities. Hexafluoroisopropyl alcohol (HFIP) was used to facilitate the protonation of the enolate generated via the initial Michael addition, to suppress the retra-Michael addition. Based on mechanistic studies, it was proposed that chiral NHC catalyst served as a Brønsted base to deprotonatively activate the thiol via the formation of a hydrogen bonded complex, whilst the Michael acceptor formed π-π stacking interaction with aryl substitutent of catalyst.

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Scheme 9. The sulfa-Michael addition catalyzed by NHC catalyst.

2.2.2 Mannich-type reactions In addition to sulfa-Michael addition, the Mannich reaction of thiols or thiophenols with activated ketimines also emerged as a promising strategy to access optically active (S, N)-ketals that are present in biologically active compounds. For example, oxindole based (S, N)-acetals are interesting targets for medicinal research, as exemplified by spiro compounds 5-7 shown in Figure 1. In 2015, two nice protocols have been independently developed by the Nakamura and Enders group, respectively. Nakamura et al utilized a quinine derived bifunctional sulfonamide 46 to realize an efficient asymmetric addition of thiols 11 to isatin derived ketimines 45, with the addition of TMSOH as a protonation reagent, giving the desired (S, N)-acetals 47 in excellent yields and ee values (Scheme 10).39 Noticeably, the pyridinesulfonyl group of the catalyst 46 was important for high reactivity and enantioselectivity, as the replacement of the 2-pyridyl group to p-tolyl or 2-thienyl group resulted in much inferior results. The authors proposed that 2-pyridyl group might form an intramolecular H-bonding interaction with the N-H bond of sulfonamide group to assist the activation of ketimines, as depicted in Scheme 10.

Scheme 10. The Mannich reaction of thiols to isatin ketimine catalyzed by a chiral base. 11


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Simultaneously, Enders and coworkers utilized chiral phosphoric acid catalysis to achieve a highly enantioselective addition of thiols 11 to N-Boc isatin ketimines 48, with the use of only 1 mol% phosphoric acid 49, a variety of oxindole based (S, N)-acetals 50 were obtained in high yields and ee values (Scheme 11).40 Remarkably, both thiols and thiophenols were viable substrates, and a gram-scale synthesis was also performed without erosion of both yield and ee. A dual activation transition state was proposed in Scheme 11.

Scheme 11. The Mannich reaction of thiols to isatin ketimine catalyzed by chiral phosphoric acid.

3. Electrophilic sulfenylation The enantioselective electrophilic sulfenylation of activated methine compounds or their equivalents is an alternative C-S bond forming strategy to construct sulfur-containing tetrasubstituted carbon stereocenters.24 As electrophilic sulfur reagents are employed instead of sulfur nucleophiles, chiral Lewis acid catalysis has found wide application in this way, and organocatalytic protocols no longer dominate in this research. 3.1 Chiral metal catalyzed versions In 2005, Jereb and Togni reported a chiral Ti(IV) complex 53 catalyzed asymmetric sulfenylation of β-ketoesters 52 with phenylsulfenyl chloride 51, with a catalyst loading of 0.8-5.0 mol% (Scheme 12).41a Noticeably, even with the generation of a stoichiometric amount of HCl byproduct, this protocol still afforded optically active sulfur-containing products 54 in 80-95% yields and 52-88% ee values in the absence of any base. High enantioselectivity was obtained when α-methyl or α-fluoro β-ketoesters 52 with a bulkyl ester group were used. The β-keto amides were viable substrates as well.41b The chiral Ti(IV) complex could also catalyze the sulfenylation of β-ketoesters using phthalimide-N-sulfenyl chloride, albeit with moderate enantioselectivity.42 12


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Scheme 12. Chiral metal Ti (IV) catalyzed enantioseleective sulfenylation of β-ketoesters. Later in 2009, Shibata et al reported an enantioselective synthesis of α-fluoro-α-sulfenyl-β-ketoesters 54 catalyzed by complex DBFOX 12a/Ni(ClO4)2·6H2O (Scheme 13).43 The sulfenylation of α-fluoro β-ketoesters 52 and phenylsulfenyl chloride 51 worked well in the presence of 10 mol% catalyst, to give the desired products in good yields and high ee values. When treated with DAST, product 54a could be converted to trifluorinated α-sulfenylcarboxylate 55 in 89% yield. It was a useful intermediate for the synthesis of fluorinated isosteric analogue of SM32 (56). The addition of 4Å MS was important for reaction development, otherwise no reaction occurred. The catalyst system was also suitable for cyclic and acyclic non-fluorinated β-ketoesters.

Scheme 13. Chiral metal Ni (II) catalyzed enantioselective sulfenylation of β-ketoesters.

By cooperative catalysis of chiral N, N’-dioxide 59/Sc(OTf)3 complex and inorganic base (Na2CO3 or K2CO3), Feng et al achieved the first catalytic enantioselective construction of C3 sulfur-containing tetrasubstituted carbon stereocenter of oxindoles (Scheme 14).44 The sulfenylation of unprotected 3-alkyl or aryl oxindoles 57 by N-(phenylthio)phthalimide 58a worked smoothly to give 3-phenylthiooxindoles 60 in excellent yields and ee values. Notably, the cooperative catalysis turned out to be crucial for this sulfenylation, as chiral complex failed to catalyze the reaction by itself, and Na2CO3 could afford the desire racemic product only in 46% yield, in sharp contrast to the 87% yield and 94% ee achieved by the 13


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merger of chiral Lewis acid and Na2CO3. Therefore, it was proposed that both the enolate, produced by deprotonative activation of 3-substituted oxindoles 57 by base, and the sulfenylating reagent 58a coordinated to the chiral Sc(III) complex, to form a favorable transition state.

Scheme 14. Chiral N,N’-dioxide-Sc(OTf)3 complex catalyzed sulfenylation of 3-substituted oxindoles.

Later in 2014, Gade et al disclosed a highly enantioselective electrophilic trifluoromethylthiolation of β-ketoesters 61 using trifluoromethylthiolated hypervalent iodine reagent 62 (which was the revised structure by Buchward45), catalyzed by a chiral tridentate pincer ligand 12b/Cu(OTf)2 complex (Scheme 15).46 A variety of enantioenriched cyclic α-SCF3-β-ketoesters 63 were obtained in 81-93% yields and 64-99% ee. The products could be easily converted to optically pure α-SCF3-β-hydroxyesters bearing two adjacent quaternary stereocenters by reacting with Grignard reagents. Presumably, the coordination of β-ketoesters to chiral Cu(II) catalyst formed a Cu-bound planarized ester-enolate, and the attack of SCF3 reagent from Re-face was favored.

Scheme 15. Chiral Cu(II) catalyzed asymmetric trifluoromethylthiolation of β-ketoesters. 14


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3.2 Organocatalytic versions

In parallel with the development of chiral Lewis acids mediated protocols, several organocatalytic activation models, including enamine catalysis,47 bifunctional Brønsted base catalysis,30 phase-transfer catalysis (PTC) 48 and Lewis base catalysis, 49 also demonstrated their value in the enantioselective sulfenylation reactions. The enamine catalysis was first utilized by Wang and coworkers to develop a sulfenylation of aldehydes and ketones catalyzed by a chiral pyrrolidine trifluoromethanesulfonamide, but the ee value of products was not reported.50 Meanwhile, Jørgensen et al. successfully employed this activation model to realize a highly enantioselective sulfenylation of aldehydes.51 They also tried the sulfenylation of α-branched aldehyde 64a by 1-benzylsulfanyl-1,2,4-triazole 65a, catalyzed by L-prolinol silyl ether 66 in combination with o-nitrobenzoic acid 67a (Scheme 16). The desired product 68 was obtained in 84% yield with 61% ee. The obviously inferior enantioselectivity as compared with that obtained by using α-linear aldehydes once again showed the challenge in forming S-containing tetrasubstituted carbon stereogenic centers.

Scheme 16. Organocatalytic asymmetric sulfenylation of aldehydes.

Later on, the Jørgensen group further developed chiral Brønsted base catalyzed enantioselective sulfenylation of active methine compounds. In the presence of 10 mol% dimeric cinchona alkaloid derivative (DHQD)2PYR, the sulfenylation of 1,3-dicarbonyl compounds 61 worked smoothly to provide optically active α-sulfenylated products 69 in 66-95% yields with 51-89% ee (Scheme 17).52 A variety of active methine compounds were viable substrates, including lactones, lactams, β-keto esters and β-diketones, and both aryl and alkyl substituted electrophilic sulfenylating reagents 65 worked well under these conditions. A possible transition model was proposed in Scheme 17. The protonated catalyst interacted with the enolate derived from 61 via H-bonding interaction, which then attacked electrophilic sulfenylating reagents 65. In addition, the optically active α-sulfenylated β-hydroxy esters were obtained in moderate to good dr values by the reduction of the corresponding α-sulfenylated β-keto esters using BH3/Me2S. 15


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Scheme 17. Organocatalytic enantioselective sulfenylation of 1,3-dicarbonyl compounds.

Later in 2009, Polaske and Olenyuk realized an enantioselective sulfenylation of substituted piperazine-2,5-diones 70 with 65. The corresponding products 71 were obtained in moderate to good yields and enantiomeric excesses in the presence of 10 mol% quinine (Scheme 18).53

Scheme 18. Enantioselective sulfenylation of substituted piperazine-2,5-diones 70.

Owing to the importance of oxindoles bearing a C3 sulfur-containing tetrasubstituted carbon stereocenter in pharmaceutically active molecules (Figure 1), the development of organocatalytic methods to this structural motif has attracted much attention. In 2012, three research groups independently reported the asymmetric sulfenylation of 3-substituted oxindoles.54 The Enders group disclosed that the use of 5 mol% bifunctional squaramide 74 efficiently mediated enantioselective sulfenylation of N-Boc 3-alkyl or 3-aryloxindoles 72 using N-(sulfanyl)phthalimides 58, giving the 3-sulfenylated quaternary oxindoles 73 in 86-98% yield with 55-96% ee (Scheme 19).54a Concurrently, Li and Cheng also realized a similar sulfenylation reaction of N-Boc 3-substituted oxindoles 72 using cheap quinidine as the catalyst, albeit the reactions were run at -80 oC.54b However, both protocols were limited to highly active N-Boc 3-substituted oxindoles 72. 16


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Scheme 19. Enantioselective sulfenylations of N-Boc-protected oxindoles with N-(sulfanyl)phthalimides.

Subsequently, Jiang and coworkers reported a (DHQD)2PHAL catalyzed highly enantioselective sulfenylation of N-benzyl 3-aryloxindoles 75 using N-(sulfanyl)succinimides 15 at 30 oC (eq 1, Scheme 20).54c The opposite enantiomer of the corresponding sulfenylated oxindoles 76 could be easily accessed by using the pseudo-enantiomeric (DHQ)2PHAL as the catalyst. Later on, the authors further extended the substrate scope of this enantioselective sulfenylation by using chiral bicyclic guanidine 77. Both 3-alkyloxindoles and benzofuran-2(3H)-ones were employed and afforded the corresponding adducts 76 in good to excellent yields and ee values (eq 2).55 Upon treatment of m-CPBA, the adduct 76a was readily converted to the sulfone 78 in 91% yield without erosion of ee.

Scheme 20. Enantioselective sulfenylations of N-Bn oxindoles with N-(sulfanyl)succinimides.

17


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Most recently, an efficient sulfenylation of 3-pyrrolyl oxindoles 79 with 15 was disclosed by Yuan and coworkers, enabling the synthesis of oxindole based (N, S)-ketals 80 (Scheme 21).56 Cinchonidine was identified as a powerful catalyst for this reaction, affording optically active 3-thio-3-pyrrolyl-oxindoles 80 in high yields and ee values. However, the replacement of the N-(alkylsulfanyl)succinimides by N-(arylsulfanyl)succinimides resulted in moderate yields and ee values.

Scheme 21. Organocatalytic sulfenylation of 3-pyrrolyl oxindoles with N-(sulfanyl)succinimides.

Zhou and coworkers achieved the first highly enantioselective synthesis of dithioketals 83 via organocatalytic sulfenylation (Scheme 22).21a Commercially available dihydroquinine 82 proved to be the catalyst of choice for the sulfenylation of various S-containing methine compounds 81 using N-(sulfanyl)succinimides 15. A number of chiral dithioketals 83 could be obtained in high structural diversity, with good to excellent yields and ee values. More importantly, the undesired side reaction via S-nucleophilic way was restrained by using bifunctional acid-base catalysis.

Scheme 22. Organocatalytic sulfenylation for the synthesis of chiral dithioketals.

In 2014, Jiang and coworkers realized a nice organocatalytic enantioselective α-sulfenylations of azlactones 84 using N-(sulfanyl)succinimides 15, catalyzed by 5 mol% bifunctional squaramide 37 (eq 1, Scheme 23).57 The desired (S, N)-ketals 85 were prepared in high yields and ee values in the presence of 4 18


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Å MS as an additive. Furthermore, α-sulfur substituted α-amino acid derivatives 88 could be synthesized from adducts 85a without loss of ee. Later on, they further realized an α-sulfenylation of 5H-oxazol-4-one 86, allowing highly enantioselective synthesis of the α-sulfenylated (O, S)-ketals 87 (eq 2).58 Notably, various N-(aryl/benzyl/alkylthio)succinimides 15 were well tolerated in both cases.

Scheme 23. Enantioselective sulfenylation of azlactones and 5H-oxazol-4-one.

Owing to the high lipophilicity and high electron-withdrawing character of the SCF3 group, the enantioselective introduction of an SCF3 group is of great interest to the pharmaceutical and agrochemical industries. 59 In this context, two research groups independently reported the enantioselective trifluoromethylthiolation of β-ketoesters 61 in 2013. Rueping et al reported a highly enantioselective version using N-trifluoromethylthiophthalimide 58b (Scheme 24).60 The use of quinidine or quinine as the catalyst allowed the synthesis of both enantiomers of the corresponding α-SCF3 substituted β-ketoesters 63 in good yields with excellent ee values.

Scheme 24. Enantioselective trifluoromethylthiolation reactions of β-ketoesters with 66b.

Simultaneously, Shen et al developed a new electrophilic trifluoromethylthiolated hypervalent iodine reagent 62 for the enantioselective trifluoromethylthiolation of β-ketoesters 61.61 In the presence of 20 mol% quinine, various five-membered cyclic β-ketoesters 61 worked well to give the α-SCF3 substituted β-ketoesters 63 in high yields and ee values (Scheme 25). It was found that the hydroxyl group of quinine 19


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was crucial for good reactivity. The use of catalyst 89 without the free hydroxyl group resulted in only trace amount of product; in contrast, the product 63a was achieved in 90% yield and 92% ee in the presence of quinine. Based on initial mechanism study, a dual activation transition state was proposed.

Scheme 25. Enantioselective trifluoromethylthiolation reactions of β-ketoesters with 62.

One year later, Rueping et al accomplished an enantioselective trifluoromethylthiolation of N-Boc 3-aryloxindoles 72 using N-trifluoromethylthiophthalimide 58b, catalyzed by dimeric cinchona alkaloid derivative (DHQD)2PYR (eq 1, Scheme 26).62 This protocol allowed the synthesis of optically active oxindole derivatives 90, bearing a SCF3 group, in 75-90% yields with 84-95% ee. Shortly after, Tan and Liu reported the same reaction by using electrophilic trifluoromethylthiolated reagents generated in situ from simple and readily available trichloroisocyanuric acid 91 and AgSCF3, affording various 3-SCF3 substituted oxindoles 92 in good yields and excellent ee in the presence of (DHQD)2PYR in THF (eq 2).63 Mechanistic studies suggested that CF3S-SCF3 might be one of the active electrophilic SCF3 species. This nice protocol provides a convenient strategy to exploit enantioselective trifluoromethylthiolation reactions.

Scheme 26. Organocatalytic trifluoromethylthiolation of 3-aryloxindoles. 20


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Along with our interests in preparing optically active thioketal/acetals, 64 we also found that dihydroquinine 82 as the catalyst was workable for the enantioselective trifluoromethylthiolation of sulfur based active methine compounds 81 using N-SCF3 succinimide 15b, giving structurally diverse dithioketals 93 featuring a SCF3 group in good yields and high to excellent ee values (Scheme 27).21a

Scheme 27. Enantioselective trifluoromethylthiolation of S-containing methine compounds.

In addition to tertiary amines, bifunctional chiral secondary amines are capable of serving as general base catalysts for enantioselective sulfenylation. Zhu et al pioneered the use of α,α-diaryl prolinol 24b to mediate sulfenylation of β-keto esters using N-(arylthio)phthalimide 58. The desired products 94 were obtained in 68-98% yields and 30-97% ee (Scheme 28).65 Initial mechanistic studies suggested that chiral secondary amine 24b functioned as H-bonding acceptor to interact with the enol form of β-keto esters, and the resulting intermediate attacked the sulfenylating reagents 58. Noticeably, although chiral secondary amines as enamine or iminium catalysts have been intensively studied, their potential as Brønsted base or nucleophilic catalyst are much less explored. This work represents an early successful example of chiral secondary amine as general base catalyst. By the same catalysis pattern, Zhu and Cheng further developed a highly enantioselective sulfenylation of β-keto phosphonates to adduct 95 with both a S and a P atom attached to the carbon stereocenter.66 In addition, Fang and Zhu realized the synthesis of S-containing quaternary α-amino acid derivatives 96 from α-substituted nitroacetates in good yields and ee values.67

Scheme 28. α,α α-Diaryl prolinol 24b catalyzed enantioselective sulfenylations. 21


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Enantioselective phase transfer catalysis (PTC)48 is also a viable tool. In 2013, Maruoka and coworkers designed a novel bifunctional phosphonium bromide catalyst 97a bearing an amide moiety, which found utility in the sulfenylation of β-keto esters 61 with 58 (Scheme 29).68 This constituted an interesting example of base-free PTC pioneered by Maruoka group.48 The use of only 0.1 mol% catalyst 97a could provide the α-sulfenylated β-keto esters 69 in excellent yields and ee values. Control experiments showed that the amide group of catalyst 97a was crucial for the high reactivity and enantioselectivity, and the quaternary phosphonium moiety was also essential for reactivity. In view of this, a transition state was proposed, which is shown in Scheme 29.

Scheme 29. Enantioselective sulfenylations catalyzed by bifunctional phosphonium bromide 97a.

In enantioselective trifluoromethylthiolation of β-ketoesters using reagent 62, Shen and coworkers also found that the use of quaternary ammonium salt 98 in combination with K2CO3 was essential for the good yield and enantioselectivity for the trifluoromethylthiolation of less active six- or seven-membered cyclic β-ketoesters 61 (Scheme 30).62

Scheme 30. Enantioselective trifluoromethylthiolation by chiral PTC catalysis.

Apart from the aforementioned catalysis patterns, chiral Lewis base catalysis49 also emerged as a promising strategy for electrophilic sulfenylation of enol silyl ethers, a type of widely used nucleophiles that found limited application in the enantioselective sulfenylation. With their continuation in chiral Lewis base catalysis, Denmark and coworkers disclosed an enantioselective α-sulfenylation of ketone derived 22


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enoxysilanes using N-phenylthiosaccharin 100 as the sulfur source, catalyzed by their developed chiral selenophosphoramide 101. 69 One example of α-sulfenylation of trimethylsilyl enol ether furnished S-containing tetrasubstituted carbon stereocenter was also reported, namely the conversion of enol silyl ether 99 to 2-methyltetralone 102 bearing an α-phenylthio group in 65% yield and 70% ee (Scheme 31). Based on DFT calculations, the authors proposed that Lewis base 101 firstly reacted with N-phenylthiosaccharin 100 to give the Lewis base-bound sulfenyl cation, and then the thus formed active sulfenylating reagent reacted with ether 99. Finally, the saccharin anion removed the trimethylsilyl group to afford the corresponding α-sulfenylated product 102.

Scheme 31. Enantioselective sulfenylation of enol silyl ether. 4. Enantioselective functionalization of S-containing substrates While the enantioselective C-S bond formation22 provides a straightforward method for the formation of tetrasubstituted carbon stereocenter featuring a sulfur substitution, the alternative strategy that based on the functionalization of S-containing starting materials also turns out to be fruitful. In particular, by using pre-organized S-based substrates or dithioesters, it is possible to stereoselectively construct S-containing heterocyclic compounds. 4.1 Chiral metal catalyzed version Early in 1998, Aggarwal et al presented the enantioselective Diels-Alder reaction of α-thioacrylates 103 with cyclopentadiene catalyzed by chiral Box 104/Cu(OTf)2 complex, which afforded the desired cycloadducts 105 in up to 92% yield and 95% ee values for endo isomers (Scheme 32)70. The nature of ester and sulfenyl substituents great influenced the selectivity of the reaction. High enantioselectivity was obtained when the acrylates 103 had a phenylthio group, along with an ethyl, isopropyl or CF3CH2 ester group. As depicted in Scheme 32, a possible transition state was proposed that the chiral Box-Cu(II) 23


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complex coordinated with dieneophiles 103 via binding one of the two enantiotopic lone pairs of sulfur atom and the carbonyl group of esters, which then reacted with cyclopentadiene to give the adducts 105. In addition, the (1S, 4S)-norbornenone 106 could be easily accessed from compound 105a.

Scheme 32. Enantioselective Diels-Alder reaction of α-thioacrylates.

In addition to the synthesis of chiral norbornenone, the synthetic value of optically active compounds featuring an S-containing tetrasubstituted carbon stereocenter was vividly demonstrated by the following two examples. In 2012, Shibasaki and Kumagai reported a highly enantioselective direct aldol reaction of α-sulfanyl lactones 107 and various aldehydes 64 by employing a double activation catalysis strategy based on chemoselective activation via soft-soft interaction. It was found that the merger of 3-5 mol% of chiral (R)-108/AgPF6 complex and DBU turned out to be the best choice. Accordingly, adducts 109 bearing an S-containing tetrasubstituted carbon stereocenter were obtained in good yields and excellent stereoselectivities (Scheme 33).71 Based on a series of control experiments and mechanistic studies, the authors proposed that chiral (R)-108/AgPF6 complex selectively activated α-sulfanyl lactones 107 in the presence of aldehydes 64 through the coordination of the sulfur to soft Lewis acid Ag(I), which was very helpful for the subsequent preferential deprotonation by hard Brønsted base DBU to produce enolate II to react with aldehydes. Notably, this method had been applied for the total synthesis of viridiofungin A and NA 808, which were known to be serine palmitoyl transferase (SPT) inhibitors.

24


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Scheme 33. Direct catalytic asymmetric aldol reaction of α-sulfanyl lactones.

One year later, they extended this cooperative activation catalytic system to a direct asymmetric Mannich-type reaction of α-sulfanyl lactone 107 with aldimines 110. The combination of chiral (S)-108/AgPF6 complex and DBU furnished the desired adducts 111 featuring a β-amino-α-methylthio moiety in high to excellent yields and stereoselectivities (Scheme 34).72 The thus obtained product 111 was applied for the synthesis of optically pure trisubstituted aziridine 112, which could be further converted to α,α-disubstituted α-amino acid derivative 113.

Scheme 34. Direct asymmetric Mannich-type reaction of α-sulfanyl lactones. 25


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Recently, Chen and Hartwig established an enantioselective Ir-catalyzed allylic substitution of 5H-thiazol-4-one 114 with cinnamyl tert-butyl or methyl carbonate 115 (Scheme 35).73 In the presence of 2 mol% preformed chiral Ir catalyst 116 and 0.6 equiv of Mg(NiPr2)2, various substituted 5H-thiazol-4-one 114 worked well to give the allylated products 117 in 75-96% yield, 4:1-12:1 dr, and 98->99% ee. The base significantly influenced the diastereoselectivity of the reaction, and only 1.2:1 and 1.3:1 dr value were obtained if using quinine or Et2Zn to replace Mg(NiPr2)2. Furthermore, the resulting product 117a was readily transformed to chiral tertiary thioesters 118 under basic condition.

Scheme 35. Asymmetric allylation substitution of 5H-thiazol-4-one. 4.2 Organocatalyzed version Organocatalytic activation models such as nucleophilic catalysis,49a iminium29 & enamine catalysis,47 and Brønsted base catalysis30d have found application in enantioselective elaboration of S-containing substrates to forge the corresponding tetrasubstituted carbon stereocenters via cycloaddition reaction, Mannich reaction or Michael addition. 4.2.1 Cycloaddition reactions In 2011, Fujiwara and Fu developed a highly asymmetric [3+2] cycloaddition of allenoates 119 with heteroatom substituted electron-poor olefins mediated by a newly designed chiral phosphepine 120.74 The use of α-sulfur substituted acrylate 103a allowed the synthesis of cyclopentenes 121 bearing a S-containing quaternary stereocenter in 68-90% yield, 6:1-7:1 dr and 97-98% ee at room temperature. As shown in Scheme 36, the reaction was possibly initiated by the nucleophilic attack of phosphepine 120 to allenoate 119, which was believed to be the turnover-limiting step. When treated with Hg(O2CCF3)2, the resulting tertiary thioester 121 could be easily unmasked to a tertiary thiol 122 in 70% yield without erosion of ee value. 26


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Scheme 36. Asymmetric [3+2] cycloaddition of allenoates. Later in 2012, Ishihara and coworkers reported that their triamine catalyst 124 in combination with C6F5SO3H could realize the enantioselective Diels-Alder reaction of α-carbamoylthio substituted enals 123, affording adducts 125 in good yields and ee values (Scheme 37).75 The carbamoyl group of the products could be readily removed by reductive cleavage, as shown by the conversion of 125a to 126 in 71% yield. Possibly, the primary amine of the catalyst reacted with α-(carbamoylthio)acroleins 123 to generate the iminium intermediate, whilst the H-bonding interactions between acyl group of 123 and the ammonium proton of catalyst stabilized the transition state. The active iminium intermediate then readily underwent Diels-Alder reaction with dienes.

Scheme 37. Asymmetric Diels-Alder reaction of α-(carbamoylthio)acroleins. 27


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Concurrently, Ye and coworkers employed cascade enamine catalysis/iminium catalysis to realize a formal [4+2] cycloaddition of rhodanine 128 and enones 127, providing a facile access to chiral spirocyclohexanonerhodanine 130 that merges the pharmaceutically relevant rhodanine moiety into a spirocyclic unit (Scheme 38).76 Under the catalysis of 10 mol% chiral diamine 129 in combination with a suitable acid cocatalyst, the tandem reaction worked well to afford spirocyclohexanonerhodanine 130 in 43-95% yield, 5:1->20:1 dr, and 88-99% ee.

Scheme 38. Asymmetric organocatalyzed tandem reaction of rhodanine and enones.

Later on, they established trienamine catalysis mediated enantioselective Diels-Alder reaction of rhodanine 128 with 2,4-dienals 131. The merging of diphenylprolinol silyl ether (R)-66 with 20 mol% o-FC6H4CO2H (67b) allowed the reaction to work well to furnish the desired adducts 132 in high yields and excellent stereoselectivities (Scheme 39).77

Scheme 39. Organocatalyzed Diels-Alder reaction of rhodanine and dienals. 28


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4.2.2 Amination reaction The direct amination of active S-containing methine compounds constitutes an interesting way for the synthesis of (N, S)-ketals that are widely present in drugs and bioactive compounds. In our continuous efforts in the enantioselective synthesis of oxindole derivatives,78,79 we disclosed a highly enantioselective amination reaction of 3-thiooxindoles 81 using di-tert-butyl azodicarboxylate 133a (DBAD) for the synthesis of optically active oxindole based (S, N)-ketal in 2013. The use of 10 mol% β-ICD (134) effectively mediated this reaction, affording the desired (S, N)-ketal 135 in 68-98% yield with 47-94% ee (Scheme 40).79a This represented an early successful example of enantioselective synthesis of oxindole derivatives featuring two heteroatoms at the C3 position, a type of prominent structural motif widely present in pharmaceutically active compounds.14-16

Scheme 40. Enantioselective amination of 3-thiooxindoles.

In 2013, Palomo et al developed an efficient amination reaction of 5H-thiazol-4-one 114 with DBAD (133a), using the newly designed ureidopeptide derived bifunctional chiral Brønsted base 136a (Scheme 41).80 Chiral (S, N)-ketals 137 were prepared in good yield and ee values. Notably, the quinoline moiety of substrate 114 played a crucial role, as 2-quinolyl substituted thiazolone achieved better enantioselectivity and yield than that bearing a 2-naphthyl group. Possibly, there was H-bonding interaction between the quinoline moiety and one of the three N-H bonds of catalyst 136a.

Scheme 41. Enantioselective amination of 5H-thiazol-4-ones. 29


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Recently, Wang et al reported the deprotonative activation of rhodanines by chiral Brønsted base for amination reaction. Quinine was identified as a highly enantioselective catalyst for the α-amination of 5-substitued rhodanines 138 with diethyl azodicarboxylate 133b (DEAD), allowing efficient synthesis of rhodanine based chiral (S, N)-ketals 139 in excellent yields and ee values (Scheme 42).81

Scheme 42. Enantioselective amination of 5-substitued rhodanines.

4.2.3 Michael additions In 2012, Ye and coworkers optimized a bulkyl chiral primary amine 139 as a powerful catalyst for the highly enantioselective Michael addition of 5-substitued rhodanines 138 to enones 127 (Scheme 43).82 Both acyclic enones and cyclic enones worked well to give the adducts 140 in high to excellent yields, diastereo- and enantioselectivities. The resulting adducts 140 could be converted to various optically active rhodanine derivatives, as exemplified by the transformation of 140a to 141 and 142. A possible transition state was shown in Scheme 43. The primary amine of catalyst 139 activated enone 127 through iminium activation, while a tertiary amine activated rhodanines 138 by deprotonation.

Scheme 43. Asymmetric Michael addition of 5-substitued rhodanines 30


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Later on, Veselý et al reported an enantioselective Michael/Michael/aldol tandem reaction for the highly stereoselective synthesis of spirocyclic rhodanines 144 bearing a S-containing tetrasubstituted carbon stereocenter (Scheme 44).83 In the presence of chiral secondary amine (S)-66 and benzoic acid 67c (20 mol%, each), the initial Michael addition of N-phenylrhodanine 138a to enals 143 proceeded via the formation of iminium intermediate I, affording enamine interemdiate III that further underwent an intramolecular aldol-condensation to give the desired spirocyclic compounds 144. A double Michael cascade reaction of N-phenylrhodanine 138a and dienone was also realized by using cinchona derived primary amine to afford the corresponding spirocyclic products bearing a rhodanine moiety.

Scheme 44. Enantioslective cascade reaction of N-phenylrhodanine with enals.

Sheng and coworkers recently realized a highly enantioselective Michael-Michael cascade reaction of 3-thiooxindoles 81 with enals 143 (Scheme 45).84 The diphenylprolinyl silyl ether (S)-66 functioned as both iminium and enamine catalyst to enable this cascade reaction in a similar mechanism as described in Scheme 44. Highly functionalized chiral spirocyclic oxindole-tetrahydrothiopyran derivatives 145 featuring four consecutive stereogenic centers were synthesized in good yields and excellent stereoselectivities. Notably, the corresponding spirooxindoles 145 was found to be a potent p53-MDM2 protein-protein interaction inhibitors with good antitumor activity.

Scheme 45. Enantioselective Michael-Michael cascade reaction of 3-thiooxindoles with enals.

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Apart from aminocatalysis, bifunctional Brønsted base catalysis is also a widely used tool for the Michael additions using S-based active methine compounds. In 2013, Palomo and coworkers applied their ureidopeptide derived bifunctional catalyst 136b to realize the enantioselective addition reaction of 5H-thiazol-4-one 114 to nitroolefins 146 (Scheme 46).81 Under the catalysis of 20 mol% 136b, the desired products 147 were achieved in good yields and high stereoselectivities. Similar to their developed amination reaction in Scheme 41, the quinoline moiety of 114 was also crucial for obtaining good reaction outcome. The corresponding adduct 147a can be converted to α,α-disubstituted α-mercapto amide 148, which could be further transformed to tetrahydrothiopyran-fused isoxazoline 149. This protocol provided a new method to chiral tertiary thiols for which limited enantioselective synthetic methodologies have been reported.

O O N N

N H

O R

S

+

NO2

R1

114

146 R1 = aryl, alkyl

R = Me, Et, C7H13, Bn

N

O N H

N H

O R1 NO2

N

N N

136b (20 mol%) MeO

S

R 147

CH2Cl2, -60 oC

22 examples, 40-96% >92:8-95:5 dr, 76-98% ee

Synthetic application O Ph NO2

N N

O 1. HCl (6 N), dioxane

S 2. NaOH (2 N), dioxane 147a

Ph

1. NaH, CH2=CHCH2I THF, rt

H2N HS

NO2

2. TMSCl, Et3N

148, 72% yield

Ph Me

N

H2N O

S

O

H 149, 64% yield

Scheme 46. Enantioselective Michael reactions of 5H-thiazolone to nitroolefins.

The Palomo group further developed an enantioselective addition of 5H-thiazolone 114 to α’-silyloxy enone 150 as enoate surrogate catalyzed by a bifunctional tertiary amine-squaramide 151 (Scheme 47).85 The reactivity of this transformation was not high, as the use of 20 mol% catalyst 151 was required even carrying the reaction at room temperature. Nevertheless, the desired adducts 152 were obtained in good to high yields and excellent ee values. The synthetic value of the Michael adducts was shown by the synthesis of thiolactone 153 in 89% yield via two steps.

Scheme 47. Enantioselective Michael reactions of 5H-thiazolone to α’-silyloxy enone. 32


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Lu et al reported the Michael addition of 3-thiooxindoles 81 to nitroolefins 146 catalyzed by a threonine-incorporating multifunctional quinine catalyst 154, affording the 3-sulfenylated oxindoles 155 in excellent yields and ee values (Scheme 48).86 Various 3-alkylthiooxindoles 81 worked well; however, the 3-phenylthiooxindole provided the desired product 155 in only 5:1 dr value and 79% ee. Noticeably, the gram-scale synthesis was realized without loss of yield and ee under standard conditions. In addition, the resulting adduct 155a could be readily elaborated into the tertiary thiols 156 under the catalysis of Hg(OAc)2, and the furoindoline 158 was also achieved from 155b via four steps conversions.

Scheme 48. Enantioselective Michael addition of 3-thiooxindoles.

In parallel with the enantioselective amination of 3-thiooxindoles 81 with DBAD,79a we also studied the same Michael addition using our developed cinchona alkaloid derived bifunctional phosphoramide 159a87. In the presence of 1-5 mol% phosphoramide 159a that was accessed in two steps, both 3-arylthio and 3-alkylthiooxindoles 81 worked smoothly to give the corresponding oxindoles 155 in high to excellent dr value and excellent ee values (Scheme 49).88 The phosphoramide moiety of catalyst 159a was crucial for obtaining high dr and ee value, as 159b with the N-H bond of phosphoramide masked by a methyl group proved to be obviously inferior in terms of reactivity and stereoselectivity. Based on this result, a dual activation model was proposed. The tertiary amine moiety activated 3-thiooxindoles by deprotonation, whilst the N-H bond of phosphoramide functioned as H-bonding donor to activate the nitroolefins and organize a favorable reaction transition state. This first confirmed that the phosphoramide, a type of H-bond donors with limited application in the development of bifunctional organocatalyst, was an effective H-bond donor to develop bifucntional tertiary amine catalysts.

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Scheme 49. Enantioselective Michael addition of 3-thiooxindole by bifunctional phosphoramide.

Lu and Lan recently devised a highly enantioselective γ-addition reaction of 5H-thiazol-4-ones 114 to allenonates 160 (Scheme 50).89 Various substituted 5H-thiazol-4-one derivatives 162 could be obtained in high yields and excellent ee, catalyzed by 10 mol% amino acid derived phosphine 161. By control experiments and DFT calculations, a possible catalytic cycle was proposed. The initial attack of phosphine to the allenonates 160 generated an intermediate I, which interacted with thiazolone 114 to form an intermediate II, with the enolate bounded to the N-H bond of catalyst via H-bonding interaction, which subsequently attacked the γ-carbon of allenoate to give intermediate III. The following proton transfer produced intermediate IV, which then regenerated the phosphine catalyst 161 and afforded the adduct 162.

Scheme 50. Enantioselective γ-addition reaction of 5H-thiazol-4-ones to allenonates. 34


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5. Enantioselective formal addition to C=S bond Optically active S-containing heterocycles characterized by the attachment of the sulfur atom to the tetrasubstituted carbon stereocenter are difficult to prepare by other methods in a single transformation. However, the enantioselective formal addition to C=S bond has recently emerged as a powerful strategy to prepare such heterocyclic compounds. In 2010, Gulea and coworkers described the first catalytic asymmetric thia-Diels-Alder reaction of 1,2-diene and dithioesters 163, providing a facile access to optically active dihydrothiopyrans 165 (Scheme 51).90 Under the catalysis of 5 mol% chiral bisoxazoline 164 derived Cu(OTf)2 complex, the thia-Diels-Alder reaction of dithiooxalate 163 and 2,3-dimethyl-1,2-butadiene allowed the synthesis of S-containing heterocycle 165 in up to 82% ee. The substituent of dithioesters 163 had a big influence on the enantioselectivity of cycloadducts, as tert-butyl ester substituted dithioesters afforded the adduct 165b in only 10% ee, in sharp contrast to the 82% ee of 165a derived from ethyl ester substituted dithioesters. A possible working model featuring the bidentate coordination of dithioesters to copper was proposed to rationalize the observed stereochemistry.

Scheme 51. Enantioselective thia-Diels-Alder reaction catalyzed by Lewis acid catalyst.

Later in 2013, Jørgensen et al developed a highly enantioselective thio-Diels-Alder reaction between dienals 131 and dithioesters 163 mediated by asymmetric trienamine catalysis (Scheme 52).91 In the presence of 5-20 mol% catalyst (R)-66 and 20 mol% benzoic acid 67c, a variety of structurally diverse dihydrothiopyrans, and bi- and tricyclic S-containing heterocycles 166 could be accessed in high yields and stereoselectivities. By DFT calculations, a stepwise mechanism involving zwitterionic intermediate was proposed. And the combination of the electron-withdrawing ester group and SR5 group directed the reaction toward an initial attack of trienamine at the sulfur atom of dithioesters. Furthermore, the resulting adducts could be readily transformed to diverse S-containing chiral heterocycles.

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Scheme 52. Eantioselective thia-Diels-Alder reaction catalyzed by trienamine catalysis.

6. Conclusions and Outlook

The past decades have witnessed the development of a variety of catalytic enantioselective methods for the construction of sulfur-containing tetrasubstituted carbon stereocenters. Both metal catalysis and organocatalysis have shown their viability in developing new reactions by using any of the four strategies outlined in Figure 2. Nevertheless, despite significant ongoing progresses, this research field is still at its early stage, with ample synthetic opportunities to explore. First, it is important to exploit new reaction types, along with the expansion of the substrate scope of most of known protocols. Currently, although four synthetic strategies have been exploited, the reaction types developed by each strategy are still very limited. For instance, the nucleophilic addition reactions using sulfur based nucleophiles are confined to conjugate addition and Mannich-type reactions using highly active electrophiles. As for the enantioselective formal nucleophilic addition reaction to C=S bond, only two examples based on dithioesters are known. Therefore, the development of new reaction types would be an exciting direction in this field. Second, the potential of metal catalysis in new reaction development waits for further exploration. As 36


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depicted above, although metal catalysis is widely used to develop electrophilic sulfenylation reaction, this powerful tool have found very limited application in exploiting new reactions by other three strategies shown in Figure 2. A plausible reason is the interaction between sulfur-containing species and metals exerts negative influences on the reaction development. However, as exemplified by Shibasaki and Kumagai’s work shown in Scheme 33, it is possible to take advantage of the interaction of metal catalyst with sulfur to activate the S-containing nucleophiles with relative high pKa value for deprotonation by using catalytic amount of base, paving the way for the enantioselective synthesis. This suggests the use of multicatalyst system consisting of transition metal catalyst and organic small-molecule catalyst will be a powerful strategy for reaction design.92 As can be expected, with the development of new efficient chiral catalyst systems and synthetic methodologies, more practical and highly efficient protocols for the catalytic enantioselective construction of sulfur-containing tetrasubstituted carbon stereocenters will be established, which benefits for the drug discovery.

ACKNOWLEDGMENT We are grateful for financial support from the 973 program (2015CB856600) and NSFC (21472049, 21502053).

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Catalytic Enantioselective Construction of Sulfur-Containing

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