Catalytic Asymmetric Reactions with N,O-Aminals - American

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Catalytic Asymmetric Reactions with N,O-Aminals Yiyong Huang, Chen Cai, xing yang, Zongchao Lv, and Uwe -- Schneider ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01725 • Publication Date (Web): 15 Jul 2016 Downloaded from http://pubs.acs.org on July 16, 2016

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Catalytic Asymmetric Reactions with N,O-Aminals Yi-Yong Huang,†,* Chen Cai,† Xing Yang,† Zong-Chao Lv† and Uwe Schneider‡,* †

Department of Chemistry, School of Chemistry, Chemical Engineering and Life Science, Wuhan University of Technology, Wuhan 430070, P.R. China. ‡ EaStCHEM School of Chemistry, The University of Edinburgh, The King’s Buildings, David Brewster Road, Edinburgh EH9 3FJ, UK. ABSTRACT: N,O-aminals, molecules bearing a geminally N,O-substituted (stereogenic) carbon center, have been recently recognized as an important class of building blocks in organic synthesis. As direct precursors of imines and iminium ions, N,O-aminals were converted through asymmetric organocatalysis or metal catalysis to diverse enantiomerically enriched compounds including N-heterocycles. Furthermore, cyclic N,O-hemiaminals acted as acyclic amino aldehyde surrogates, which were transformed to enantioenriched products otherwise challenging to access. Finally, cyclic N,O-aminals were formed in situ as key intermediates in asymmetric catalysis. In this review, we introduce a wide array of catalytic asymmetric protocols involving the use of four distinct types of N,O-aminals as starting materials or key intermediates. KEYWORDS: Aminal, hemiaminal, carbinolamine, acetal, asymmetric catalysis, heterocycles, iminium ion, imine 1.

INTRODUCTION N,O-aminals, also termed carbinolamines, N,O-acetals, or N,O-hemiaminals, represent an interesting class of organic compounds comprising a (stereogenic) carbon center that is geminally substituted by an alkoxy (or hydroxy) and an amino group, respectively: −C(OR)(NR’2)−.1 R may be an organic group or a hydrogen atom. N,O-aminals are common structural motifs embedded within diverse biologically important natural products and pharmaceuticals.2 Typically, N,O-aminals are synthesized from the corresponding α-amido sulfones through nucleophilic substitution of the sulfonyl by an alkoxy group,3 or from the corresponding imines through nucleophilic addition of an alkoxy group.4 Moreover, N,O-aminals have been described as intermediates during imine formation, i.e., through nucleophilic addition of an amine to aldehydes or ketones with subsequent proton transfer.5 Reversely, Nprotected imines have been prepared from the corresponding N,O-aminals through formal elimination of the alcohol.6 Compared to the corresponding imines,7 N,O-aminals often display a higher stability and are thus more convenient to manipulate. Indeed, N,Oaminals can be purified by column chromatography on silica gel, whereas imines are prone to hydrolysis under the same conditions. Certain imines have proved to be very unstable and cannot be isolated, i.e., C-alkyl imines with an enolizable α-hydrogen atom tend to tautomerize to the undesired aliphatic enecarbamates.8 In turn, the catalytic in situ generation of reactive imines from the corresponding bench-stable N,O-aminals serves as an elegant way to address this issue. As listed in Figure 1, N,O-aminals may be classified into acyclic (A) and cyclic (B, C, D) structures. Aminals of type A (acyclic), B (lactam-type), and C (cyclic amide-type) have been used directly as starting materials in asymmetric catalysis. In contrast, aminals of type D, herein called latent aminals, have been shown to serve as key intermediates in asymmetric catalysis being generated in situ from the corresponding cyclic imines or amine derivatives. This review is focused on catalytic asymmetric C−C or C−N bond formations involving N,O-aminals as reagents. Within each category of aminals, individual examples of asymmetric catalysis have been illustrated according to the specific reaction type.

Figure 1. Distinct Types of N,O-Aminals Iminium ions, commonly formed through condensation of aldehydes or ketones with a secondary amine or by protonation of imines under Brønsted acidic conditions, have been studied for almost one century.9 Yet, the application of acyclic or cyclic N,O(hemi)aminals as iminium ion precursors in asymmetric catalysis was only recently developed. This asymmetric transformation relies on the catalytic use of enantiomerically enriched Brønsted acids, hydrogen-bond donors, or Lewis acids, which may trigger the in situ generation of highly electrophilic iminium ions through elimination of –OR (R = H or organic group; Scheme 1a). Subsequently, nucleophiles add in an intermolecular or intramolecular manner to give the corresponding enantioenriched products. In

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addition, a notable acid-catalyzed asymmetric rearrangement of N,O(alkenyl)-aminals to the corresponding β-amino aldehydes has been implemented (Scheme 1b). This transformation was shown to proceed through an initial C–O bond cleavage followed by asymmetric C–C bond formation between the in situ-formed enol and imines. Furthermore, cyclic N,O-hemiaminals can undergo ring-opening in order to act as acyclic amino aldehyde surrogates, which may formally react with nucleophiles or electrophiles in an asymmetric bond formation to form ultimately enantiomerically enriched cyclic amine derivatives (Scheme 1c). Finally, in the latent aminal strategy, N,O-aminals can be generated in situ from the corresponding cyclic imines or amine derivatives to undergo asymmetric C–C bond formation with nucleophiles (Scheme 1d). Common to all these transformations is that N,O-aminals have proved to be useful reagents and building blocks to synthesize complex highly functionalized molecules, including biologically active natural products and pharmaceuticals. With the goal to provide readers with a comprehensive overview and outlook for this important research field, the most representative examples of asymmetric catalysis with aminals have been summarized. To the best of our knowledge, such a topical literature review in the field of asymmetric catalysis is not yet available.

Scheme 1. Distinct Catalytic Asymmetric Transformations of N,O-Aminals 2. 2.1

CATALYTIC ASYMMETRIC REACTIONS USING N,O-AMINALS Use of Acyclic N,O-Aminals (A)

Acyclic N,O-aminals of type A, containing aromatic or aliphatic (saturated and unsaturated) groups, have been used in asymmetric catalysis displaying important advantages over the corresponding imines. In this context, various types of enantioenriched amine derivatives were readily accessible from A through catalytic asymmetric C−C bond formations, i.e., Mannich reaction, azaPetasis−Ferrier rearrangement, nucleophilic alkylation, and Suzuki−Miyaura cross-coupling. 2.1.1 Catalytic Asymmetric Mannich Reactions The classic three-component Mannich reaction of an amine, an aldehyde, and a ketone has been well documented for the introduction of an aminomethyl group in α position to the ketone’s carbonyl group.10 However, the α-aminomethylation of aldehydes to give β-amino aldehydes, being useful precursors for γ-amino alcohols and β-amino acids, has proved to be more challenging. In 2006, Gellman et al.11 and Córdova et al.12 reported independently that the use of α-aminomethyl ether 1 as a methylene iminium surrogate –instead of formaldehyde-derived imines– was pivotal to address the key issue (Scheme 2). In this scenario, aldehydes 2 and the enantiomerically enriched proline derivative 3 formed in situ the corresponding chiral enamines, which underwent asymmetric Mannich reaction with the iminium ion, generated in situ from 1, to give –after carbonyl reduction− γ-amino alcohols 4 in up to 87% yield with up to 98% ee. The latter was converted to the corresponding Boc-β2-homonorvaline product without loss of optical purity.


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Scheme 2. Asymmetric Mannich Reaction with Aldehydes The catalytic asymmetric Mannich reaction has also proved to be versatile for a two-carbon homologation strategy.13 Herein, the careful selection of substrates may be crucial to favor the intended C−C bond formation versus a potential over-reaction. Indeed, the use of aromatic N,O-aminals rac-5 –instead of imines− turned out to be the key to the success in Terada’s Mannich reaction with enecarbamates 6 to give the corresponding β-amino aminals 7 in up to 87% yield, albeit with low diastereoselectivity (Scheme 3).14 Subsequent reduction of aminals 7 resulted in the formation of 1,3-diamine derivatives 8 with up to 98% ee. It was proposed that the catalytic use of the enantiomerically enriched phosphoric acid (R)-9a15 or (R)-9b triggered the smooth conversion of rac-5 to the corresponding imine I (loss of MeOH; Scheme 3). The latter may then be activated by (R)-9 to generate a chiral ion pair with high reactivity towards 6 to give the corresponding β-amino imine II, which would be trapped by MeOH to provide 7. Likewise, aliphatic aminal rac-5a was shown to react with enecarbamate 6a to give intermediate 7a, which was trapped in an azaFriedel−Crafts reaction with indole to provide predominantly syn-8a in 70% yield with >99% ee.16

Scheme 3. Asymmetric Mannich Reaction with Enecarbamates Propargyl amines are important synthetic handles for further derivatisation to more complex amine compounds. The most popular method to synthesize propargyl amines relies on catalytic alkynylation of imines.17 An alternative under-developed strategy involves the use of C-alkynyl imine building blocks. Considering the difficult preparation of these imines, Shao et al. used the corresponding N-protected C-alkynyl aminals rac-10 in novel asymmetric Mannich reactions with β-dicarbonyl compounds18 11 and 12 (Scheme 4).19 These transformations proceeded in the presence of Takemoto’s enantiomerically enriched bifunctional thiourea 13 at an elevated temperature to give the corresponding functionalized propargyl amines 14 and 15 in 74−96% yields with 83−97% ee. The catalytic activation of substrate rac-10 by the chiral catalyst 13 was proposed to proceed via hydrogen-bonded complex I, which may undergo elimination of EtOH to form the chirally modified activated imine II being susceptible for nucleophilic addition (Scheme 4). It is noted that the use of cyclic β-ketoester 12 was shown to give a substantially higher syn-selectivity for products 15 (compared to the use of monothio malonate 13, which provided 14 with only moderate diastereoselectivity).


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Scheme 4. Asymmetric Mannich Reaction with β-Dicarbonyl Derivatives 2.1.2 Catalytic Asymmetric Aza-Petasis−Ferrier Rearrangement The asymmetric Mannich reaction between aldehydes and imines via enamine catalysis represents one of the most important methodologies to build enantioenriched β-amino aldehydes.20 Contrary to aromatic and glyoxylate-derived aldimines,21 aliphatic aldimines with an enolizable α-hydrogen atom are prone to tautomerize to their enamine form, which limits their synthetic utility as an electrophile. This issue was elegantly addressed by Terada et al. in 2009 by using aliphatic N,O(allyl)-aminals rac-16 as imine surrogates in the presence of a nickel catalyst and chiral BINOL-derived phosphoric acid (R)-9 (Scheme 5).22 The sequence starts with a nickel-catalyzed isomerization to form the corresponding N,O(alkenyl)-aminals rac-(Z)-17, which may undergo an asymmetric aza-Petasis−Ferrier rearrangement to give β-amino aldehydes 18 in high yields with up to 95% anti-selectivity and >99% ee. This rearrangement was shown to proceed through sequential C−O bond cleavage and C−C bond formation23 with a critical fast consumption of the in situ generated aliphatic imines. Interestingly, an anomalous temperature effect was observed in Terada’s study (Scheme 5). Indeed, based on control experiments with enantiopure aminals (S)-(Z)-17 and (R)-(Z)-17, the C−O bond cleavage proved to be kinetically enantioconvergent. At an increased temperature of up to 40 oC, Si-face attack proved to be favorable over Re-face attack (transition state I vs. transition state II), thus leading predominantly to (2R,3R)-18. Additional computational studies were fully consistent with these experimental results.24

Scheme 5. Asymmetric Aza-Petasis−Ferrier Rearrangement of N,O(Alkenyl)-Aminals 2.1.3 Catalytic Asymmetric Nucleophilic Alkylations Enantiomerically enriched N-containing molecules bearing a trifluoromethyl group (CF3) at the stereogenic center are of high importance.25 In 2006, Charette et al. envisioned the asymmetric synthesis of chiral quaternary α-trifluoromethyl amines through catalytic addition of dialkyl zinc reagents to trifluoromethyl ketimines. However, these ketimines proved to be too unstable for being isolated. In turn, the corresponding aminals rac-19 were used as stable imine equivalents together with dialkyl zinc reagents26 in the presence of a catalyst system composed of copper(II) and diphosphine (R,R)-20 under previously established conditions.27 N-


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Protected α-CF3 amine derivatives 21 were formed in 71−89% yields with 91−99% ee (Scheme 6).28 The targeted α-CF3 amine 22 was easily obtained through N-deprotection under acidic conditions.

Scheme 6. Asymmetric Alkylation with Zinc Reagents Catalytic asymmetric allylation of imines with allyl boron reagents, entailing C–C bond formation between C(sp2) and C(sp3) centers, has been drawing great attention.29 With the aim to develop a novel asymmetric indium(I) catalysis, Kobayashi et al. reported a rare asymmetric C(sp3)–C(sp3) bond formation between N,O-aminals rac-23 and an allyl boronic ester to give the corresponding homoallylic amine derivatives 25 in 88−99% yields with 72−97% ee (Scheme 7).30 Key to the success proved to be the combined catalytic use of indium(I) chloride and silver BINOL-phosphate (R)-24. Control experiments using an enantiomerically pure aminal 23 –in the presence of a racemic silver salt 24– provided exclusively product rac-25 and/or recovered 23 in enantiomerically enriched form. These data strongly suggested a reaction pathway via chiral iminium ion pair catalysis,31 i.e., asymmetric C–C bond formation between iminium ion and allyl indium species directed by an enantioenriched counteranion (Scheme 7).

Scheme 7. Asymmetric Hosomi–Sakurai Allylation with a Boron Reagent 2.1.4 Catalytic Asymmetric Suzuki−Miyaura Cross-Coupling The asymmetric construction of stereogenic carbon centers possessing a CF3 group has remained a significant and challenging synthetic task. Efficient asymmetric reactions with trifluoromethyl building blocks were shown to deliver various enantioenriched CF3 products under mild conditions.32 In 2013, Lautens et al. demonstrated an asymmetric Suzuki–Miyaura cross-coupling between trifluoromethyl N,O-aminals rac-26 and aryl boroxines in the presence of a chiral catalyst system composed of palladium(II) and pyridyl-oxazoline 27 (Scheme 8).33 α-CF3 aryl methyl amines 2834 were obtained in 59−91% yields with 63−97% ee. The nature of the aromatic substituents within aminals rac-26 and/or the boroxines was shown to critically influence both yield and optical purity of products 28.

Scheme 8. Asymmetric Suzuki−Miyaura Cross-Coupling 2.2

Use of Lactam-Type N,O-Aminals (B)

Cyclic N,O-aminals B of the lactam-type35 –including cyclic (thio)hydantoins and sulfonamides– constitute another interesting class of imine surrogates that have been exploited in asymmetric catalysis for the preparation of diverse polycyclic structures. The-


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se types of starting materials may be aminals (with a geminal alkoxy group) or hemiaminals (with a geminal hydroxy group). The use of a suitable chiral Brønsted acid or hydrogen-bond donor may generate the corresponding enantioenriched iminium ion intermediate, which would be trapped by an appropriate nucleophile. Alternatively, chiral metal catalysts may trigger the in situ formation of the corresponding imines prior to asymmetric C–C bond formation. 2.2.1 Catalytic Asymmetric Pictet–Spengler Reactions The Pictet–Spengler reaction, a special case of an intramolecular aza-Friedel–Crafts reaction, is an intriguing transformation to build tetrahydro-β-carboline or tetrahydroisoquinoline moieties.36 The conventional Pictet–Spengler reaction relies on the condensation of a β-arylethyl amine with an aldehydes or a ketone to give the required electrophilic species, which can be attacked intramolecularly by the electron-rich aromatic ring. In contrast, Jacobsen et al. used indole-tethered N,O-hemiaminals rac-29 and trimethylsilyl chloride (TMSCl), in the presence of the enantioenriched thiourea catalyst 30, to give the corresponding isoindolo-[2,1a]isoquinolines 3137 in 51−94% yields with 81−99% ee (Scheme 9).38 TMSCl proved to be necessary to form in situ chlorolactams 29’. Based on the well-known anion-binding properties of thioureas,39 chiral N-acyl iminium chloride–thiourea complex I may be generated from 29’ and 30. I may undergo asymmetric intramolecular C–C bond formation –via II (pathway A) or directly (pathway B)– to form III, which would aromatize to give 31. A beautiful example was the straightforward access to 31a, which was reduced to the natural product (+)-harmicine without loss of optical purity.

Scheme 9. Asymmetric Pictet–Spengler Reaction of Indole-Tethered N,O-Hemiaminals Subsequently, Jacobsen et al. reported the use of pyrrole-tethered N,O-hemiaminals rac-32 in the same type of asymmetric intramolecular C–C bond formation catalyzed by the same chiral thiourea 30 (Scheme 10).40 Hydroxylactams rac-32 were readily synthesized in situ through imide alkylation with alkyl lithium reagents. Depending on the R group on pyrrole’s N-atom, the C–C bond formation occurred selectively at C2 (R = H) or C4 (R = TIPS). Such regioselectivity has been reported for pyrroles previously.41 Eventually, pyrrolo-indolizidinones 33 (R = H) and pyrrolo-quinolizidinones 34 (R = TIPS) were obtained in up to 86% yield with up to 96% ee. It is noted that other types of aromatic hydroxylactams, bearing methoxylated benzene, thiophene, or benzothiophene derivatives, were considered not suitable for this protocol.

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Scheme 10. Asymmetric Pictet–Spengler Reaction of Pyrrole-Tethered N,O-Hemiaminals Owing to the prevalence of polycyclic motifs in various bioactive natural products and pharmaceutical molecules, the development of efficient annulation methodologies is one of the most fundamental yet significant issues in synthetic organic chemistry.42 In this context, Jacobsen et al. successfully exploited both the anion-binding capacity of an enantioenriched thiourea and potential cation–π interactions within key reaction intermediates. Indeed, N,O-hemiaminals rac-35, bearing a tether with an alkene and an aromatic ring, were reacted in the presence of a catalyst system composed of chiral thiourea 36 and HCl, to give –in a Pictet– Spengler bis-annulation– the corresponding tetracycles 37 in 51−77% yields with 89−94% ee (Scheme 11).43 This elegant transformation has provided not only a new direct entry to polycycles, but has also redirected the design of enantiomerically enriched catalysts towards non-covalent interactions.

Scheme 11. Asymmetric Pictet–Spengler Bis-Annulation of Functionalized N,O-Hemiaminals 2.2.2 Catalytic Asymmetric Aza-Friedel–Crafts Reactions As stated before, hydroxylactam-derived iminium ions were shown to form in situ enantioenriched ion pairs with chloride– thiourea complexes for efficient asymmetric intramolecular C–C bond formation. In view of expanding this concept to an intermolecular version, Jacobsen et al. used indoles 38 and O-acetylated N,O-aminals rac-39 in the presence of a chiral Schiff base-derived thiourea 40 (Scheme 12).44 In this context, indoles displayed a nucleophilic aza-Friedel–Crafts reactivity.45 Aminals rac-39 were used for an increased solubility compared to the corresponding hemiaminals. 3-Functionalized indoles 41 were obtained in 12−93% yields with 85−97% ee. It is noted that for optimal reactivity, a catalytic amount of water was required together with a superstoichiometric amount of trimethyl silyl chloride (for electron-rich indoles) or boron trichloride (for electron-poor indoles; in situ generation of HCl).

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Scheme 12. Asymmetric Aza-Friedel–Crafts Reaction with O-Acetylated N,O-Aminals In addition to chiral hydrogen-bond donors, i.e., thioureas, chiral Brønsted acids such as phosphoric acid derivatives were shown to be suitable catalysts for the asymmetric intermolecular aza-Friedel–Crafts reaction. In 2010, Rueping et al. anticipated that quaternary hemiaminals rac-42 could be catalytically activated by Brønsted acid (R)-43 to form in situ –through dehydration– the corresponding chiral N-acyl iminium ion pair,46 which could be trapped with indole as a nucleophile (Scheme 13).47 This C–C bondforming strategy afforded γ,γ’-disubstituted γ-lactams 44 in 20−93% yields with 53−86% ee. Adducts 44 constitute precursors to the corresponding enantioenriched γ-amino acids (through lactam hydrolysis).

Scheme 13. Asymmetric Aza-Friedel–Crafts Reaction with Quaternary N,O-Hemiaminals Following the same strategy, Zhou et al. developed asymmetric aza-Friedel–Crafts reactions between indoles and a tertiary N,Ohemiaminal rac-45 using the phosphoric acid catalyst (R)-46 (Scheme 14).48 Accordingly, 3-functionalized indoles 47 were obtained in 90−99% yields with 32−83% ee. The optical purity was shown to be improved significantly through a single recrystallization, or under the modified catalysis conditions reported by Masson and coworkers.49 Subsequently, Zhou et al. applied the same concept to the catalytic activation of quaternary N,O-hemiaminals rac-48 using phosphoric acid (S)-49 (Scheme 15).50 3Functionalized indoles 50, bearing a quaternary stereogenic center, were formed in 59−99% yields with 56−95% ee. Control experiments revealed that the lactam proton in rac-48 was critical for high asymmetric induction. This protocol affords a reliable access to enantioenriched indole derivatives 50 possessing isoindolin-1-one scaffolds, which exist in many bioactive molecules.51 Recently, Gredičak et al. extended this strategy to the combined use of rac-48 and thiols leading to enantioenriched N,S-aminals bearing a quaternary stereogenic center.52

Scheme 14. Asymmetric Aza-Friedel–Crafts Reaction with a Tertiary N,O-Hemiaminal


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Scheme 15. Asymmetric Aza-Friedel–Crafts Reaction with a Quaternary N,O-Hemiaminal As described beforehand, the challenge of generating an enantioenriched isoindolo-[2,1-a]isoquinoline skeleton has not been addressed for less reactive simple aromatic rings in the intramolecular C–C bond formation via iminium ion pair catalysis.40 In this context, Lete et al. examined bicyclic quaternary N,O-hemiaminal rac-51, bearing an isoindolo-[2,1-a]isoquinoline backbone, under chiral Brønsted acid conditions (Scheme 16).53 In the presence of indoles and a catalytic amount of phosphoric acid (R)-9a, rac51 was converted to isoindolo-[2,1-a]isoquinolines 52 in 42−93% yields with 58−79% ee. Although the asymmetric induction was rather moderate, this elegant formation of highly fused N-heterocycles bearing a quaternary stereogenic center is fairly impressive.

Scheme 16. Asymmetric Aza-Friedel–Crafts Reaction with a Bicyclic Quaternary N,O-Hemiaminal The catalytic asymmetric aza-Friedel–Crafts reaction with ketimines is one of the most efficient approaches to optically active quaternary amine derivatives. In contrast to a great number of successful examples with aromatic imines, relatively few studies exist for the use of aliphatic imines, which may be ascribed to their lower levels of reactivity, stability, and enantiofacial discrimination. In this context, Terada et al. explored (thio)hydantoin-derived N,O-aminals rac-53 as precursors for aliphatic ketimines (Scheme 17).54 In the presence of the phosphoric acid catalyst (S)-54, it was proposed that rac-53 underwent elimination of MeOH to generate in situ the corresponding cyclic imines 53’. The latter reacted with 2-methoxyfuran to afford 5-functionalized furans 55 in 50−99% yields with 65−94% ee. Interestingly, furans 55 –bearing a quaternary α-carbon center– were converted to butenolides of type 56 without loss of optical purity. The high levels and the sense of asymmetric induction were studied using DFT calculations.

Scheme 17. Asymmetric Aza-Friedel–Crafts Reaction with (Thio)Hydantoin-Derived N,O-Aminals


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2.2.3 Catalytic Asymmetric Mannich Cyclization

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Considerable efforts have been directed toward the synthesis of enantioenriched izidine alkaloids, such as pyrrolizidine, indolizidine, and quinolizidine, for their interesting biological and pharmacological properties.55 As a straightforward and cost-effective strategy, catalytic asymmetric synthesis of these compounds has drawn much attention.56 In 2014, Koley et al. explored a versatile methodology to construct three distinct izidine libraries based on a bio-inspired intramolecular Mannich reaction.57 Indeed, according to the enzyme-catalyzed biosynthesis of these bicyclic β-amino aldehyde derivatives, an organocatalytic asymmetric Mannich cyclization was anticipated with acetal-tethered N,O-aminals rac-57 (Scheme 18). The main challenge of one-pot acetal deprotection/enamine formation/N-acyl iminium ion generation within rac-57 was overcome using sub-stoichiometric amounts of MacMillan’s enantioenriched secondary amine 58 and triflic acid. Indeed, the asymmetric Mannich cyclization occurred smoothly via chiral enamine/iminium ion intermediate I, and –after reduction of the hydrolyzed Mannich adducts II– izidines 59 were obtained as a single diastereoisomer in up to 89% yield with up to 97% ee (Scheme 18). This concept provided efficient access to izidine alkaloids, such as (–)-tashiromine, (–)-trachelanthamidine, and (–)-epilupinine.

Scheme 18. Asymmetric Mannich Cyclization with Acetal-Tethered N,O-Hemiaminals 2.2.4 Catalytic Asymmetric Transfer Hydrogenations Asymmetric transfer hydrogenation of chiral iminium ion pairs, generated in situ from the corresponding imines in the presence of an enantioenriched Brønsted acid catalyst, has been reported using Hantzsch esters as the hydrogen source.58 In this context, Zhou et al. reported the asymmetric reduction of N,O-hemiaminals rac-48 using Hantzsch ester 60 and the VAPOL-derived phosphoric acid catalyst (R)-61 (Scheme 19).59 Key to the success of this concept proved to be the smooth dehydration of N-unprotected rac-48 to generate the corresponding iminium ion intermediate. Accordingly, 3-substituted isoindolinones 62 were obtained in up to 71% yields with up to 95% ee. The moderate efficiency of this process has been ascribed to the formation of alkene by-products 63. Interestingly, control experiments using alkenes 63 or N-protected rac-48 as substrates –under otherwise identical conditions– failed to give hydrogenation products 62.

Scheme 19. Asymmetric Reduction of N,O-Hemiaminals with a Hantzsch Ester An alternative hydrogen source to 60 has proved to be 2-phenyl benzothiazoline (65; Scheme 20).60 In order to address the issues encountered during the asymmetric reduction of rac-48, Jia et al. used the phosphoric acid catalyst (S)-64 in combination with 65 as a more suitable reducing agent in this context (Scheme 20).61 Accordingly, 3-aryl isoindolinones 66 were obtained in up to 99% yield with up to 91% ee.


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Scheme 20. Asymmetric Reduction of N,O-Hemiaminals with a Benzothiazoline Almost at the same time, You et al. independently exploited the same concept for the asymmetric conversion of bicyclic N,Ohemiaminals rac-67 to enantioenriched tetrahydro-β-carboline frameworks of type 68 (Scheme 21).62 The use of the phosphoric acid catalyst (S)-49 in combination with Hantzsch ester 60 resulted in the formation of products 68 in 68–94% yields with 77–90% ee.

Scheme 21. Asymmetric Reduction of Bicyclic N,O-Hemiaminals with a Hantzsch Ester In 2013, Zhou et al. investigated the asymmetric ring-opening reduction of bicyclic N-sulfonyl oxaziridines rac-69 and rac-71 to afford the corresponding cyclic sultams 70 and 72, using molecular hydrogen and a catalyst system composed of palladium(II), (S,S’,R,R’)-TangPhos (73), and L-camphorsulfonic acid (L-CSA; Scheme 22).63 Substrates rac-69 and rac-71 feature a ring-strained bicyclic aminal substructure bearing a quaternary carbon center, which may readily undergo reductive ring-opening to give cyclic sulfonamide-type N,O-hemiaminals I. This initial activation corresponds to an N–O bond cleavage triggered by a transient [Pd]–H species and acid (HX). I may dehydrate in situ under acidic conditions to give the corresponding cyclic N-sulfonyl imines II, which would be reduced by another [Pd]–H species to provide products 70 and 72 in 80–98% yields with 90–99% ee. Further studies including the asymmetric reduction of isolated imines II gave comparable results under almost identical conditions (in the absence of 64 L-CSA), thus providing strong evidence for the imine hydrogenation pathway (Scheme 22).

Scheme 22. Asymmetric Reduction of Bicyclic N-Sulfonyl Oxaziridines with H2 2.2.5 Catalytic Asymmetric Suzuki−Miyaura Cross-Coupling The asymmetric Suzuki–Miyaura cross-coupling between imines and aryl boron reagents has drawn great interest for accessing enantiomerically enriched tertiary aryl amine derivatives.65 However, only few stable imines, e.g. N-tosyl imines, have proved to be compatible with the employed aqueous conditions. In order to substantially broaden the scope for this cross-coupling chemistry, N,O-(hemi)aminals were considered as stable and readily available imine surrogates. Nishimura et al. investigated the asymmetric cross-coupling between quaternary N,O-hemiaminals rac-48 and aryl boroxines 74 in the presence of a catalyst system composed


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of rhodium(I) and the diene ligand (S,S)-75a (Scheme 23).66 It was anticipated that 74 may act as both an arylating and a dehydrating agent. Indeed, rac-48 was proposed to undergo dehydration to generate in situ the corresponding cyclic N-acyl ketimines, which would cross-couple with a transient [Rh]–Ar2 species to afford cyclic tertiary aryl amine derivatives 76 in 65–99% yields with 91– 98% ee. It was shown that ortho-, meta-, and para-substituted aromatic rings were tolerated.

Scheme 23. Asymmetric Suzuki−Miyaura Cross-Coupling with Quaternary N,O-Hemiaminals 2.2.6 Catalytic Asymmetric [3+2] Annulation Annulation reactions have proved to be a versatile tool to construct complex molecules with new rings starting from simple materials. In their previous work, Nishimura et al. accomplished an annulation between cyclic N-sulfonyl ketimines and 1,3-dienes initiated by an iridium(I)-triggered C−H bond activation.67 Unfortunately, the asymmetric version of this transformation failed. In turn, quaternary N,O-hemiaminals rac-48 were examined in combination with dienes 77 in the presence of a catalyst system composed of iridium(I) and the diene ligand (S,S)-75b to afford spiroaminoindanes 78 in up to 91% yield with up to >99.5% ee (Scheme 24).68 It was proposed that rac-48 may undergo dehydration to form in situ cyclic N-acyl ketimines I. The chiral iridium(I) catalyst would trigger imine-directed ortho C–H bond activation in I to generate metallacycle II. The latter may react with 77 through two sequential C−C bond formations to give regioselectively 78. It is noted that the regioselectivity for the oxidative cyclization proved to be highly dependent on the substitution pattern of 77. Most recently, this catalytic protocol was successfully extended to the use of alkynes (instead of dienes).69

Scheme 24. Asymmetric [3+2] Annulation with Quaternary N,O-Hemiaminals and 1,3-Dienes 2.3

Use of Cyclic Amide-Type N,O-Aminals (C)

N-Protected N,O-aminals C of the cyclic amide-type include an N-heterocyclic backbone comprising pyrrolidine, (3,4didehydro)piperidine, and dihydroquinoline derivatives. These substrates may be aminals (with a geminal alkoxy group) or hemiaminals (with a geminal hydroxy group). In the case of aminals, the use of a suitable catalyst may generate the corresponding cyclic iminium ion intermediate with the N-protecting group not incorporated in the cycle. Such transient electrophilic species may be trapped by an appropriate nucleophile. In the case of hemiaminals, ring-opening could be triggered to form the corresponding acyclic amino aldehyde intermediate. Depending on the catalyst and the reaction conditions, such transient species may react with a nucleophile or an electrophile. Various enantioenriched N-heterocycles have been obtained via asymmetric catalysis with aminals C. 2.3.1 Catalytic Asymmetric Mannich Reactions Cyclic amide-type aminals bearing a 3,4-didehydropiperidine backbone can be considered as suitable precursors for the corresponding synthetically useful piperidinium ion intermediates.70 However, these compounds have been scarcely studied although useful enantioenriched 2-functionalized piperidine derivatives may be readily accessible through C−C bond formation with nucleophiles. In this context, Matsumura et al. investigated the asymmetric Mannich reaction between N,O-aminals 79 and diaryl malonates 80 in the presence of a catalyst system composed of copper(II) and the chiral bisoxazoline ligand 81 (Scheme 25).71 Accordingly, 2-functionalized 3,4-didehydropiperidines 82 were obtained in up to 86% yield with up to 97% ee. The suggested mechanism


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involves the in situ generation of iminium ion I and enantioenriched copper enolate II from rac-79 and 80, respectively. Based on the absolute configuration of the stereogenic center in 82, it was proposed that I may undergo Si-face attack by II.

Scheme 25. Asymmetric Mannich Reaction with Malonates In 2015, Pineschi et al.72 and Liu et al.73 independently investigated the asymmetric Mannich reaction between cyclic amide-type aminals rac-83 and aldehydes (Scheme 26). Based on catalysis principles with chiral enamine species, Liu’s system comprised a Lewis acid [copper(II)] and chiral secondary amine 84, whereas Pineschi’s system was composed of a Brønsted acid (TsOH) and chiral secondary amine 85. It was proposed that the mechanism in both cases proceeded through C−C bond formation between in situ-formed quinolinium ion and chiral enamine species. Accordingly, 2-functionalized 3,4-didehydroquinoline derivatives 86 were obtained in up to 91% yield with up to 99% ee, albeit with moderate diastereoselectivity. Importantly, Liu’s catalyst system proved to be applicable to the use of N,O-aminals based on the tetrahydroisoquinoline (rac-87) and the tetrahydro-β-carboline (rac-88) frameworks.

Scheme 26. Asymmetric Mannich Reaction with Aldehydes 2.3.2 Asymmetric Aza-Morita–Baylis–Hillman Reaction The (aza-)Morita–Baylis–Hillman [(aza-)MBH] reaction is an important C–C bond formation, which commonly employs tertiary amines or phosphines as organocatalysts to convert α,β-unsaturated carbonyl compounds to the corresponding enolates that may be trapped subsequently by aldehydes and ketones (or imines).74 In the aza-MBH reaction, N-acyl iminium ions may be used as a highly electrophilic transient species. In this context, Aggarwal et al. examined the use of pyrrolidine- and piperidine-derived N,Oaminals rac-89 in combination with α,β-unsaturated cyclic ketones, in the presence of a cooperative mediator system: an enantiomerically enriched Lewis base (sulfide 90, 1.5 equiv) and a Lewis acid (TMSOTf, 2.5 equiv; Scheme 27).75 Accordingly, αfunctionalized pyrrolidine and piperidine derivatives 9176 were obtained in 49–90% yields with 80–98% ee. Low-temperature NMR studies of a mixture comprising a cyclic enone, sulfide 90, and TMSOTf indicated that the diastereoisomeric β-sulfonium silyl enol ether intermediates Ia and Ib were formed in situ in a molar ratio of 2:1 (Scheme 27). In turn, based on the predominant formation of (S)-91, it was suggested that the enantio-determining step proceeded through a kinetically favored Re-face attack of iminium ion intermediate II by enolate Ia (anti-periplanar transition state).


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Scheme 27. Asymmetric Aza-MBH Reaction with Cyclic N,O-Aminals 2.3.3 Catalytic Asymmetric Tandem Reactions A fairly unique synthetic application of N-heterocycle-based N,O-hemiaminals rac-92 –in combination with acyclic ketones– was investigated by Kanai et al. using a catalyst system composed of copper(I) alkoxide, diphosphine (R)-93a, and water (Scheme 28).77 This type of catalytic asymmetric tandem reaction afforded the corresponding 2-functionalized five-, six-, and sevenmembered N-heterocycles 94 in 52–99% yields with 84–98% ee. In this context, it is noteworthy that the efficient asymmetric synthesis of several quinolizidine alkaloids was accomplished based a common single adduct. New insights into the reaction mechanism confirmed that these transformations did not proceed via iminium ion pair catalysis. Indeed, the proposed reaction pathway includes three distinct cascade steps: (1) aldol reaction between acyclic amino aldehyde 92’, the ring-opened form of cyclic hemiaminal rac-92, and enolate I; (2) dehydration of aldol adduct II; (3) copper-catalyzed asymmetric intramolecular aza-Michael addition within aldol condensate III to afford product 94 (C–N bond formation). Overall, cyclic hemiaminals rac-92 have been functionalized through one-pot tandem reactions at the usual 2-position, which corresponds to a formal C–O bond activation in α position to the nitrogen atom.

Scheme 28. Tandem Reactions with Cyclic N,O-Hemiaminals Involving an Asymmetric Intramolecular Aza-Michael Addition Using a different type of catalysis, cyclic hemiaminals rac-92 were functionalized through one-pot tandem reactions at the unusual 3-position, which corresponds to a formal C–H bond activation in β position to the nitrogen atom (Scheme 29). With this concept in mind, Liu et al. investigated the use of rac-92 and β-nitro styrenes in aqueous media in the presence of a catalyst system composed of enantioenriched secondary amine 3 and 4-nitro benzoic acid (Scheme 29).78 Accordingly, 3-functionalized five- and six-membered N-heterocycles 95 were obtained as a single diastereoisomer in up to 91% yield with up to 99% ee. The proposed reaction pathway includes three distinct cascade steps: (1) ring-opening of rac-92 to form in situ acyclic amino aldehyde 92’ followed by condensation with 3 to generate chiral enamine I; (2) asymmetric intermolecular Michael addition of I to a β-nitro styrene (Si-face attack; C–C bond formation); (3) in situ cyclization of the N–PG/iminium ion intermediate to afford product 95 (C–N bond formation). Interestingly, it was revealed that the addition of water proved to be critical to suppress a competitive catalyst deactivated pathway (Scheme 29). Remarkably, overall N,O-hemiaminals have been used for the first time as formally nucleophilic species.

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Scheme 29. Tandem Reactions with Cyclic N,O-Hemiaminals Involving an Asymmetric Intermolecular Michael Addition 2.3.4 Catalytic Asymmetric Suzuki−Miyaura Cross-Coupling N-Acyl quinolinium salts have emerged as a novel platform to assemble various enantioenriched 2-functionalized dihydroquinoline derivatives through C–C bond formation with a variety of C-centered nucleophiles. Typically, these cyclic iminium ions have been formed from the corresponding quinolines activated by chloroformate, or via boron-assisted ionization of N-protected 2ethoxy-1,2-dihydroquinoline rac-83 (Scheme 30).79 Based on the latter method, Doyle et al. pioneered the challenging asymmetric version in this context. The initial studies of this nickel-catalyzed arylative C–C cross-coupling between rac-83 and various aryl boronic acids failed to give high asymmetric induction.80 Two changes in the catalytic protocol proved to be critical to achieve high enantioselectivity: (1) the use of a nickel(II) pre-catalyst and chiral ligand 96 combined with a mild reducing agent (NaOPh•3H2O); (2) the slow addition of aryl boroxines. In turn, the intended C–C cross-coupling afforded 2-aryl-1,2-dihydroquinolines 9781 in up to >99% yield with up to 90% ee (Scheme 30).82 The transformation proceeded through an unusual ionic oxidative addition of quinolinium ion intermediate I83 to the chiral nickel(0) complex to give chiral π-allyl nickel(II) species II, which would undergo C– C bond formation with the corresponding boron–ate complex (reductive elimination).

Scheme 30. Asymmetric Suzuki−Miyaura Cross-Coupling with a Dihydroquinoline-Type N,O-Aminal 2.3.5 Catalytic Asymmetric Alkenylation Distinct from the transition metal-catalyzed asymmetric 2-functionalization of dihydroquinoline-type N,O-aminals, Schaus et al. disclosed an organocatalytic concept for asymmetric C–C bond formation between dihydroquinoline-type N,O-aminals rac-83 and alkenyl boron reagents (Scheme 31).84 Only the catalytic use of (+)-tartaric acid (98) –in the absence of transition metal Lewis acids– proved to be highly efficient for the asymmetric alkenylation of the prochiral center of N-acyl quinolinium ion intermediates, in situ formed from rac-83. Interestingly, strongly coordinating solvents (NMP or HMPA), in combination with a mildly acidic alcohol additive (trichloroethanol), proved to be critical for reactivity and asymmetric induction. Accordingly, 2-functionalized dihydroquinolines 99 were obtained in 70–89% yields with 76–92% ee. Control experiments indicated that tetracoordinated boron species I, generated in situ through ligand exchange between 98 and B(OEt)3, was most likely the resting state of the catalyst


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(Scheme 31). Ligand exchange between I and the alkenyl boron reagent may give chiral alkenyl boron species II, which may activate the C–O bond of rac-83 to deliver chiral quinolinium borate ion pair III. The enantio-determining step was proposed to be the intramolecular-like nucleophilic C–C bond formation within III.85

Scheme 31. Asymmetric Alkenylation of Dihydroquinoline-Type N,O-Aminals

2.4

Use of Latent or In Situ N,O-Aminals (D)

The term latent N,O-aminals D refers to the catalytic in situ generation of aminal intermediates without these being starting materials. To date, only tetrahydroisoquinoline (THIQ) and tetrahydro-β-carboline frameworks were α-functionalized according to this novel strategy for asymmetric catalysis. In such a reaction system, the catalytic process includes two distinct steps: (1) generation of the aminal intermediate from cyclic imines or amine derivatives; (2) asymmetric C–C bond formation between an in situ-generated iminium ion and a C-centered nucleophile. 2.4.1 Catalytic Asymmetric Mannich Reaction The THIQ motif is present in many biologically important compounds. Thus, the development of a facile route to enantiomerically enriched THIQs is considered particularly desirable.86 Dihydroisoquinolines (DHIQs) feature a cyclic imine structure, and diverse enantioenriched 1-functionalized THIQs may be available through asymmetric nucleophilic addition to the C=N bond.87 In this context, Sodeoka et al. developed the palladium(II)-catalyzed asymmetric Mannich reaction between DHIQs 100 and diisopropyl malonate to afford 1-functionalized THIQs 101 in 57–98% yields with 81–97% ee (Scheme 32).88 Interestingly, premixing DHIQs 100 and (Boc)2O (1.5 equiv) proved to be critical for both reactivity and asymmetric induction, which strongly suggested that an unusual mechanism was operative. It was proposed that carbonate-type N,O-aminals rac-102 were formed in situ followed by decarboxylation89 to give N,O-aminals rac-103 (Scheme 32). The enantio-determining step was assumed to be the C–C bond formation between isoquinolinium intermediate I –generated in situ from rac-103– and the chiral palladium enolate [using diphosphine (R)-93b]. Such a scenario has been discussed in a similar context.71 It is noted that comparable catalysis results were obtained using isolated N,O-aminals rac-103, which provides strong evidence for the postulated mechanism.

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Scheme 32. Asymmetric Mannich Reaction with Cyclic Imines 2.4.2 Catalytic Asymmetric C‒H Alkenylation and Arylation Asymmetric oxidative C1(sp3)‒H bond activation/functionalization of THIQs represents another straightforward approach to enantiomerically enriched 1-functionalized THIQs. Although impressive progress has been made during the last decade,90 Liu et al. has implemented the latent aminal strategy only very recently. Indeed, it is well-known that in cyclic amine derivatives iminium ions can be generated through oxidative C‒H bond activation in α position to the nitrogen atom. In this context, Liu et al. explored the generation and reactivity of tetrahydro-β-carbolinium and N-carbamoyl tetrahydropyridinium species.91 Under metal-free conditions ‒using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as an oxidant in a fluorinated alcohol‒ asymmetric alkenylation and arylation of tetrahydro-β-carbolines 104 were accomplished with the corresponding boron reagents in the presence of the chiral tartrate catalyst 105 and 106, respectively (Scheme 33).92 1-Functionalized tetrahydro-β-carbolines 107 were obtained in up to 88% yield with up to 96% ee. Likewise, N-carbamoyl tetrahydropyridines 108 were converted to functionalized tetrahydropyridines 109 in up to 86% yield with up to 96% ee. It was proposed that N,O-aminal rac-110 was formed in situ from 108 through addition of CF3CH2OH to the corresponding transient iminium ion species I (Scheme 33). This latent aminal was detected by TLC analysis, and its use in catalysis provided comparable results. Rac-110 was suggested to react with the chiral alkenyl boron species to give the critical iminium borate intermediate II for enantio-determining C–C bond formation. More recently, this strategy was applied to asymmetric alkenylation and arylation of N-acyl tetrahydroisoquinolines.93

Scheme 33. Asymmetric C‒H Alkenylation and Arylation of Cyclic Amine Derivatives 2.4.3 Catalytic Asymmetric C‒H Alkynylation In the same year, Liu et al. investigated a catalytic asymmetric cross-dehydrogenative coupling (CDC) between N-carbamoyl THIQs 111 and terminal alkynes (Scheme 34).94 In contrast to their earlier protocol,95 a catalytic amount of the chiral ligand 112 was used in combination with ytterbium(III) (1.2 equiv) in the presence of 2,2,6,6-tetramethylpiperidine N-oxide salt (T+BF4‒) as an oxidant. Accordingly, 1-alkynyl THIQs 11396 were obtained in up to 70% yield with up to 95% ee (Scheme 34). In addition to copper(I) ‒required for the alkyne reagent‒ a stoichiometric amount of EtOH proved to be critical for a high asymmetric induction, which suggested a new reaction pathway. Indeed, it was proposed that 111 may undergo oxidative C‒H bond activation to form in


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situ dihydroisoquinolinium ion intermediate I, which would be trapped by EtOH to give the reactive N,O-aminal rac-114. Subsequent activation of the latter by a Lewis acid would replace the corresponding counteranion to form iminium ion intermediate II. Accordingly, II –rather than I– would react in the enantio-determining C–C bond formation to afford 113.

Scheme 34. Asymmetric C‒H Alkynylation of Cyclic Amine Derivatives

3.

CONCLUSION

In the course of developing new efficient asymmetric synthesis protocols, the discovery and implementation of novel building blocks and synthetic handles may be one of the key lessons to learn. In this context, organic and pharmaceutical chemists have increasingly employed N,O-aminals ‒complementary to imines‒ in order address issues in contemporary asymmetric synthesis. Indeed, the last decade has witnessed significant progress in both asymmetric organocatalysis and metal catalysis using various N,Oaminals for intermolecular or intramolecular transformations. In this review, the use of four distinct types of N,O-aminals was summarized exhibiting three interesting functions of (hemi)aminals: stable iminium ion precursors; precursors to a rearrangement through prior C‒O bond cleavage; precursors to acyclic amino aldehydes through prior C‒N bond cleavage (ring-opening). In addition, cyclic N,O-aminals have been described as key reaction intermediates starting from the corresponding cyclic imines or amine derivatives. The ready accessibility of N,O-aminals in racemic form renders them an attractive alternative to imines for the asymmetric synthesis of diverse optically active nitrogen-containing compounds. The practical and mechanistic aspects of a wide array of catalytic asymmetric reactions using N,O-aminals in the presence of various catalysts have been discussed. At this stage, the popular use of chiral organocatalysts often still suffers from a high catalyst loading. In addition, in several protocols the substrate generality was not satisfactory yet; yields and asymmetric induction were sometimes quite sensitive to substitution patterns and functional groups in the substrates. These issues may be overcome by developing novel enantioenriched catalyst systems. We believe more unique properties of N,O-(hemi)aminals are to be discovered, hence facilitating new transformations. Finally, the exploration of novel classes of N,O-aminals for the construction of enantioenriched complex molecules may increasingly become a trend in medicinal chemistry research. With the remarkably fast development of catalytic asymmetric methodologies, further applications of N,O-aminals are expected to be both deepened and broadened.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; TEL: (+86)-27-8775 6662 (Yi-Yong Huang). *E-mail: [email protected]; +44 131 650 4718 (Uwe Schneider)


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 21303128, 21573169), the Fundamental Research Funds for Central Universities (WUT: 2016-IB-007, 2016-YB-007), and a Marie Curie Integration Grant by the European Union (PCIG10-GA-2011-304218). We greatly appreciate the comments and suggestions from the reviewers.

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SYNOPSIS TOC 1.

N ,O -Ami na ls R N

R1

PG

OR2 (A) n

R1

MET YM

R1 C

CA TAL Y S

I

R

AS

R

RI

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

N OR2 PG (C)

S

N

X n

OR2

R3

(B) R R1

N

PG

2

C hir al Adducts

OR (D)


Catalytic Asymmetric Reactions with N,O-Aminals - American

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