Letter pubs.acs.org/OrgLett
Cite This: Org. Lett. 2018, 20, 4970−4974
Catalyst-Controlled Diastereodivergent Construction of Vicinal Sulfur-Functionalized Quaternary and Tertiary Stereocenters Lili Zhang,† Huijun Yuan,† Wei Lin,† Yuyu Cheng,‡ Pengfei Li,*,‡ and Wenjun Li*,† †
Department of Medicinal Chemistry, School of Pharmacy, Qingdao University, Qingdao 266021, P. R. China Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, P. R. China
‡
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S Supporting Information *
ABSTRACT: A catalyst-controlled diastereodivergence is described for the enantioselective conjugate addition of ohydroxyphenyl-substituted p-QMs with 5H-thiazol-4-ones. The reactions were enabled by two chiral complementary, nonenantiomeric catalysts to furnish a series of adducts possessing vicinal sulfur-functionalized quaternary and tertiary stereocenters in high yields with excellent asymmetric induction. Moreover, o-QMs generated in situ from o-hydroxybenzyl alcohols were also compatible.
S
catalyst-controlled diastereoselective and enantioselective nucleophilic addition of 5H-thiazol-4-ones, and as a continuation of our efforts in asymmetric reactions of p-QMs,12 herein, we present catalyst-controlled diastereodivergent reactions of o-hydroxyphenyl-substituted p-QMs with 5Hthiazol-4-ones. The initial conjugate addition of 5-methyl-2-phenylthiazol(5H)-one 2a to 4-(2-hydroxybenzylidene)-2,6-di-tert-butylcyclohexa-2,5-dienone 1a proceeded efficiently in CH2Cl2 at 0 °C in the presence of chiral phosphoric acid CPA-1 (Table 1, entry 1). The product 3aa was obtained in 95% yield with 52% ee and >20:1 dr, the absolute configuration of which was determined by single-crystal X-ray crystallography. After careful screening of CPAs and the conditions, the optimal reaction conditions were established, delivering 3aa in 98% yield with 96% ee and >20:1 dr (entry 6, see the Supporting Information (SI) for details). Notably, lowering the loading of CPA-6 to 1 mol % still enabled the formation of 3aa in 94% yield with 96% ee and >20:1 dr after 48 h (entry 7). Under the optimal conditions, we investigated the scope of this reaction (Table 2). It was found that this catalytic strategy was applicable to various o-hydroxyphenyl-substituted p-QMs bearing different types of substituents. Both electron-withdrawing and electron-donating groups were well tolerated to afford the corresponding adducts 3aa−fa in 83−94% yields with 86−96% ee and all >20:1 dr. The electronic nature of the substituents on the 4-position of 1b−d had little effect on the efficiency and stereoselectivity. The scope of 5H-thiazol-4-ones
tereoselectivity (diastereo- and enantioselectivity) lies at the heart of asymmetric catalysis. Owing to advances in the field of asymmetric synthesis, the absolute stereochemical outcome can be controlled by the choice of which enantiomeric form of catalyst is employed.1 However, an important issue arises when multiple stereocenters are created in a single chemical transformation, thus potentially leading to the formation of mixtures of diastereomers. An ideal, but considerable challenging, aspect of asymmetric catalysis is the ability to selectively control the formation of all of the possible stereoisomers from the same starting materials. Accordingly, much effort has been devoted to developing diastereodivergent processes.2 Although impressive results have been achieved in catalyst-controlled diastereodivergent reactions,3 it is still highly desirable to develop such processes to benefit the target- and diversity-oriented asymmetric synthesis.4 Considering their importance with interesting biological activities,5 the catalytic enantioselective construction of sulfurfunctionalized quaternary carbon stereocenters has attracted much attention.6 Accordingly, the asymmetric nucleophilic addition of 5H-thiazol-4-ones has been well established.7 However, in terms of the diastereodivergent process, only one example has been reported by Lee and Jiang.8 Therefore, developing catalytic diastereodivergent reactions of 5H-thiazol4-ones is in high demand. On the other hand, p-quinone methides (p-QMs) have proven to be a class of excellent Michael acceptors.9 In particular, o-hydroxyphenyl-substituted p-QMs have been successfully employed as 4-synthons in annulations.10 However, other types of reactions of ohydroxyphenyl-substituted p-QMs are very limited. We have successfully developed a chiral phosphoric acid catalyzed 1,6addition of o-hydroxyphenyl-substituted p-QMs.11 To develop © 2018 American Chemical Society
Received: July 4, 2018 Published: July 27, 2018 4970
DOI: 10.1021/acs.orglett.8b02088 Org. Lett. 2018, 20, 4970−4974
Letter
Organic Letters
different positions of the aromatic ring (Ar) participated in reactions smoothly to give adducts 3ab−ah in 84−97% yield with 90−96% ee and >20:1 dr. In addition, the heteroaromatic 2i−m were also compatible to afford adducts 3ai−am in 85− 93% yields with 80 → 99% ee and >20:1 dr. The alkyl group (R2) of 2 affected the outcomes and increasing the bulk of R2 resulted in relatively inferior results. Notably, 3an was still obtained in 83% yield with 82% ee and >20:1 dr. To achieve the diastereodivergent conjugate addition of 2 to 1, quite different organocatalysts, nonenantiomeric catalysts of CPAs were investigated. Importantly, the desired product 4aa, the diastereoisomer of 3aa confirmed by single-crystal X-ray crystallography, was obtained in 55% yield with 9:1 dr but with a poor enantioselectivity of 8% ee from the chiral thiourea TU1 catalyzed reaction of 1a and 2a (Table 3, entry 1).13
Table 1. Condition Optimization of CPA-Mediated Reactiona
entry
CPAs
solvent
time (h)
1 2 3 4 5 6 7e
CPA-1 CPA-2 CPA-3 CPA-4 CPA-5 CPA-6 CPA-6
CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2
6 6 6 6 6 6 48
yieldb (%) 3aa, 3aa, 3aa, 3aa, 3aa, 3aa, 3aa,
eec (%)
drd
52 66 80 94 94 96 96
>20:1 >20:1 5:1 >20:1 >20:1 >20:1 >20:1
95 94 79 90 85 98 94
Table 3. Condition Optimization of Diastereodivergent Reactiona
a Unless noted, 1a (0.05 mmol), 2a (0.06 mmol), CPA (10 mol %) in solvent (0.3 mL) at 0 °C for the time given. bIsolated yield. c Enantiomeric excess (ee) of the major enantiomer, determined by HPLC analysis using a chiral stationary phase. ddr = diastereoselectivity ratio, determined by 1H NMR. eCPA-6 (1 mol %).
Table 2. Scope of CPA-6-Mediated Conjugate Addition of 1 with 2a
entry
R1/R2/Ar
yield (%)b
1 2 3 4 5 6 7 8 9 10 11 12 13e 14e 15 16 17 18 19
H/Me/Ph 4-F/Me/Ph 4-Cl/Me/Ph 4-Me/Me/Ph 5-Br/Me/Ph 6-MeO/Me/Ph H/Me/4-FC6H4 H/Me/4-ClC6H4 H/Me/4-MeC6H4 H/Me/3-FC6H4 H/Me/3-BrC6H4 H/Me/3-MeOC6H4 H/Me/2-FC6H4 H/Me/2-thienyl H/Me/2-pyridinyl H/Et/2-pyridinyl H/n-Pr/2-pyridinyl H/Me/3-pyridinyl H/Bn/Ph
3aa, 94 3ba, 90 3ca, 89 3da, 83 3ea, 93 3fa, 89 3ab, 95 3ac, 97 3ad, 93 3ae, 94 3af, 93 3ag, 94 3ah, 84 3ai, 93 3aj, 92 3ak, 88 3al, 85 3am, 85 3an, 83
ee (%)c 96 90 86 92 96 94 94 92 90 96 96 94 96 80 98 94 88 >99 82
entry
cat.
solvent
time (h)
1 2 3 4 5 6 7e 8e,f
Tu-1 SA-1 SA-2 SA-3 SA-4 SA-5 SA-5 SA-5
CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2
24 24 24 24 24 24 24 36
drd >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1
yieldb (%) 4aa, 4aa, 4aa, 4aa, 4aa, 4aa, 4aa, 4aa,
55 76 54 53 75 80 85 83
eec (%)
drd
8 88 66 76 14 88 94 92
9:1 10:1 1:1 1:1 >20:1 13:1 15:1 15:1
a
Unless noted, 1a (0.05 mmol), 2a (0.06 mmol), catalyst (10 mol %) in the solvent (0.3 mL) at 0 °C for the time given. bIsolated yield. c Determined by HPLC analysis using a chiral stationary phase. d Determined by 1H NMR. e4 Å MS (15.0 mg) was added. fSA-5 (5 mol %).
Pleasingly, a remarkable enhancement of enantioselectivity was achieved when chiral squaramides were employed (entries 2− 5), and SA-5 was proved to be the most efficient catalyst to furnish 4aa in 80% yield with 88% ee and 13:1 dr (entry 6).14 Systematic screening studies including the effect of solvents, reaction temperature, additives, concentration, and the catalyst loading revealed that SA-5 enabled the formation of 4aa in 83% yield with 92% ee and 15:1 dr (Table 3, entry 8; see the SI for details). To give a clear comparison with the CPA-6-catalyzed reactions, the performance of catalyst SA-5 using exactly the same substrates as in Table 2 were investigated (Table 4). A series of 1 (1a, 1d, 1f−h) bearing either electron-withdrawing or electron-donating groups were successfully applied to the
a Unless noted, 1 (0.05 mmol), 2 (0.06 mmol), and CPA-6 (1 mol %) in CH2Cl2 (0.3 mL) at 0 °C for 48 h. bIsolated yields. cDetermined by HPLC analysis using a chiral stationary phase. dDetermined by 1H NMR. e−40 °C, 72 h.
was also examined. A series of 5-methyl-2-arylthiazol-(5H)ones 2b−h bearing electronically distinct substituents at 4971
DOI: 10.1021/acs.orglett.8b02088 Org. Lett. 2018, 20, 4970−4974
Letter
Organic Letters Table 4. Scope of SA-5-Mediated Conjugate Addition of 1 with 2a
entry
R1/R2/Ar
yieldb (%)
eec (%)
drd
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
H/Me/Ph 4-Me/Me/Ph 6-MeO/Me/Ph 4-Br/Me/Ph 5-MeO/Me/Ph H/Me/4-FC6H4 H/Me/4-ClC6H4 H/Me/4-MeC6H4 H/Me/3-BrC6H4 H/Me/2-FC6H4 H/Me/2-thienyl H/Me/2-pyridinyl H/Et/2-pyridinyl H/Me/3-pyridinyl H/Bn/Ph
4aa, 83 4da, 94 4fa, 93 4ga, 76 4ha, 98 4ab, 81 4ac, 85 4ad, 88 4af, 86 4ah, 87 4ai, 86 4aj, 99 4ak, 83 4am, 86 4an, 87
92 92 99 96 88 92 98 94 88 86 86 98 92 88 78
15:1 >20:1 >20:1 >20:1 >20:1 >20:1 11:1 13:1 >20:1 15:1 11:1 >20:1 18:1 12:1 6:1
Scheme 1. Further Investigations
We then surveyed the scope of the diastereoselective and enantioselective reaction between o-hydroxybenzyl alcohols 8 and 2 (Table 5). Aromatic moieties in the o-hydroxybenzyl
a
Unless noted, 1 (0.05 mmol), 2 (0.06 mmol), SA-5 (5 mol %) in CH2Cl2 (0.3 mL) at 0 °C for 36 h. bIsolated yields. cDetermined by HPLC analysis using a chiral stationary phase. dDetermined by 1H NMR.
Table 5. Scope of CPA-4-Mediated Reactionsa
diastereodivergent reaction with 2a, furnishing adducts 4aa, 4da, and 4fa−ha in 76−98% yields with 88−99% ee and 15:1 → 20:1 dr. With 1a as the Michael acceptor, the reaction was amenable to a wide scope of different substituted 5H-thiazol-4ones 2b−m, affording adducts 4ab−am in 81−99% yields with 86−98% ee and 11:1 → 20:1 dr. Obviously, the electronic nature of the substituents (Ar) did not impose an evident effect on the stereoselectivity. The reaction of 5-benzyl-2phenylthiazol-4(5H)-one 2n afforded 4an in 87% yield with 78% ee and 6:1 dr. Importantly, these results demonstrate a broadly useful method for the synthesis of 4, diastereoisomers of 3 from a wide range of substrates using catalyst SA-5. After fully establishing the scope of the catalyst-controlled diastereodivergent conjugate additions, we then performed several control experiments to gain insight into the mechanism (Scheme 1). Notably, neither CPA-6 nor SA-5 could catalyze the reaction between 5 and 2a. Furthermore, whether CPA-6 or SA-5 was employed, no reaction occurred when 6 was used to react with 2a. These results indicate that the free hydroxyl group in 1a played a key role. On the other hand, Li et al. reported that the isomerization energy of 1a and 7a was 6.7 kcal·mol−1, indicating that the transformation of p-QM to oQM was not difficult.10c To examine the possible reaction pathway, we then investigated the reactions between o-QMs and 2 (Scheme 1, bottom).15 Catalyzed by CPA-1, 2(hydroxy(4-methoxyphenyl)methyl)phenol 8a was employed to react with 2a smoothly to generate racemic products of 9aa in 64% yield. After adjusting certain parameters, the optimal conditions were established, and the desired product 9aa was obtained in 87% yield with 98% ee and 19:1 dr (see the SI for details). On the basis of these results, it is most likely that the diastereodivergent conjugate additions proceeded via a 1,4addition.
entry
R/R1/R2/Ar
yieldb (%)
eec (%)
drd
1 2 3 4e 5e 6e 7 8 9 10 11 12 13 14f 15
4-MeO/H/Me/Ph 4-MeO/4-Br/Me/Ph 4-MeO/4-Me/Me/Ph H/H/Me/Ph 4-F/H/Me/Ph 4-Cl/H/Me/Ph 3-Me/H/Me/Ph 2-Me/H/Me/Ph 4-MeO/H/Me/4-FC6H4 4-MeO/H/Me/4-ClC6H4 4-MeO/H/Me/3-BrC6H4 4-MeO/H/Me/3-MeOC6H4 4-MeO/H/Me/2-FC6H4 4-MeO/H/Me/2-pyridinyl 4-MeO/H/Bn/Ph
9aa, 87 9ba, 81 9ca, 87 9da, 95 9ea, 84 9fa, 94 9ga, 84 9ha, 82 9ab, 84 9ac, 89 9af, 86 9ag, 83 9ah, 90 9aj, 76 9an, 88
98 89 94 96 86 94 92 94 95 99 96 95 86 92 80
19:1 10:1 13:1 >20:1 13:1 17:1 18:1 17:1 13:1 >20:1 >20:1 14:1 16:1 15:1 9:1
a
Unless noted, 8 (0.05 mmol), 2 (0.06 mmol), CPA-4 (1 mol %) in toluene (0.6 mL) at 0 °C for 24 h. bIsolated yields. cDetermined by HPLC analysis using a chiral stationary phase. dDetermined by 1H NMR. e25 °C. fPhCF3 as solvent.
alcohols were well-tolerated. Substituents with different electronic features and different substitution patterns were all applicable to furnish the corresponding products 9ba−ha in 81−95% yields with 86−96% ee and 10:1 → 20:1 dr. Furthermore, the reaction was applicable to a wide range of 2 bearing different aromatic moieties, regardless of the steric and electronic nature of the substituents on the aromatic ring to furnish the corresponding products 9ab−ah in 83−90% yields with 86−99% ee and 13:1 → 20:1 dr. Heteroarylcontaining 5H-thiazol-4-ones were also used to afford product 4972
DOI: 10.1021/acs.orglett.8b02088 Org. Lett. 2018, 20, 4970−4974
Letter
Organic Letters Notes
9aj in 76% yield with 92% ee and 15:1 dr. Notably, sterically more congested 5-benzyl-2-phenylthiazol-4(5H)-one 2n generated product 9an in 88% yield with 80% ee and 9:1 dr. The absolute configurations of products were assigned on the basis of X-ray crystallographic analysis of 9ca. To demonstrate the utility of the CPA-mediated reaction, the scale up of the reaction was carried out (Scheme 2). The
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Natural Science Foundation of China (21502043), the National Key Research and Development Program of China (2016YFA0501403), the Natural Science Foundation of Shandong Province (ZR2017JL011), the Special Funds for the Development of Strategic Emerging Industries in Shenzhen (JCYJ20170817110526264), and a start-up grant from Qingdao University.
Scheme 2. Synthetic Potential
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DEDICATION Dedicated to Professor Albert Sun Chi Chan of Sun Yat-sen University.
CPA-6-mediated reaction of 1a at 1.50 mmol proceeded well under the standard conditions to generate 3aa in 92% yield with 96% ee and >20:1 dr. The CPA-4 catalyzed reaction of 8a at 1.80 mmol also worked well to furnish 9aa in 90% yield with 98% ee and 19:1 dr. Importantly, these results indicate that the asymmetric reaction of both o-hydroxyphenyl-substituted pQMs and o-QMs generated in situ from o-hydroxybenzyl alcohol with 5H-thiazol-4-ones have the potential for a largescale production. In conclusion, we have established catalyst-controlled diastereodivergent asymmetric reactions of p-QMs and oQMs with 5H-thiazol-4-ones. We have developed two catalytic systems and investigated their applicability in the reaction of ohydroxyphenyl substituted p-QMs with 5H-thiazol-4-ones. All of the reactions proceed smoothly with a wide range of substrates and furnish selective diastereoisomers possessing vicinal sulfur-functionalized quaternary and tertiary stereogenic centers with high enantiocontrol. Furthermore, this approach is applicable to the reaction of 5H-thiazol-4-ones to o-QMs generated in situ from o-hydroxybenzyl alcohols, affording adducts in high to excellent yields with high stereoselectivities. The applications of the synthetic strategy are under investigation, and additional results will be reported in due course.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b02088. Experimental section, characterization details and X-ray data (PDF) Accession Codes
CCDC 1836274−1836276 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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REFERENCES
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Corresponding Authors
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[email protected]. ORCID
Wenjun Li: 0000-0001-9045-7845 4973
DOI: 10.1021/acs.orglett.8b02088 Org. Lett. 2018, 20, 4970−4974
Letter
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DOI: 10.1021/acs.orglett.8b02088 Org. Lett. 2018, 20, 4970−4974
