The chiral α-alkylthio-α-alkylmalonate derivative is one of the most potentially valuable intermediates to synthesize biologically active natural products and pharmaceuticals. There have been many synthetic approaches to construct sulfur-bearing quaternary chiral carbon centers, mostly by the direct sulfa-Michael addition. However, sulfenylation has many drawbacks in preparing substrates and modifying functional groups. To expand the scope of research, our research team designed a novel synthetic method for constructing sulfur-bearing chiral carbon centers by the asymmetric sulfenylation of α-alkylmalonates via phase-transfer catalysis. Our research team recently reported the enantiomeric α-alkylation of malonates with high chemical yield and optical purity via phase-transfer catalysis for the first time and successfully proved its effectiveness through synthesizing various chiral building blocks bearing quaternary carbon centers using it. The phase-transfer catalysis occurs under relatively moderate conditions and its reagents and solvents are economical and environmentally sustainable. The enantioselective phase-transfer catalytic sulfenylation of α-alkylmalonate was accomplished in the presence of the phase-transfer catalyst, (1S,2S,4S,5R)-2-((R)-(allyloxy)(6-methoxyquinolin-4-yl)methyl)-5-ethyl-1-(2,3,4-trifluorobenzyl)quinuclidin-1-ium bromide, to afford the corresponding α-alkylthio-α-alkylmalonates in high chemical (up to 97%) and optical (up to 90%) yields which can be readily converted to versatile chiral intermediates.
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Enantioselective synthesis of α-alkylthio-α-alkylmalonate via phase-transfer catalytic sulfenylation
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