Conjugate Addition of Alkyl Thiols

Significance

The authors report a catalytic enantioselective conjugate addition of simple alkyl ­thiols to α,β-unsaturated N-acylated oxazolidin-2-ones using quinidine derivative QD. The thiourea moiety of this catalyst is crucial to ensure satisfactory reactivity and enantioselectivity, since 6′-OH quinidine derivative QD′ only gave the adduct in 15% yield and 74.5:25.5 er after 24 h while QD provided the product in nearly quantitative yield and 92.5:7.5 er after 2 h. Under optimized conditions (in chloroform at -20 or -50 ˚C), the sulfa-Michael addition of alkyl thiols such as benzyl ­thiols, allyl thiols to β-alkyl or aryl α,β-unsaturated N-acylated oxazolidin-2-ones furnishes the corresponding sulfides in 84-99% yields and high enantioselectivities (up to 98:2 er). Conversion of one of the 1,4-adducts into a β-mercapto ester is also demonstrated by the authors.