A Direct, Concise, and Enantioselective Synthesis of 2-Substituted 4,4,4-Trifluorobutane-1,3-diols Based on the Organocatalytic In Situ Generation of Unstable Trifluoroacetaldehyde

A direct, concise, and enantioselective synthesis of 2-substituted 4,4,4-trifluorobutane-1,3-diols based on the organocatalytic asymmetric direct aldol reaction of an ethyl hemiacetal of trifluoroacetaldehyde with various aldehydes was examined. A catalytic amount (30 mol %) of commercially available and inexpensive l-prolinamide is quite effective as an organocatalyst for the catalytic in situ generation of gaseous and unstable trifluoroacetaldehyde from its hemiacetal, and a successive asymmetric direct aldol reaction with various aldehydes in dichloromethane at 0 °C, followed by reduction with sodium borohydride, gives 2-substituted 4,4,4-trifluorobutane-1,3-diols in moderate to good yields (31-84%) with low diastereoselectivities and good to excellent enantioselectivities (64-97% ee).

Reversal of diastereoselectivity in reactions of the trifluoroacetaldehyde ethyl hemiacetal with enamines and imines: metal-free, complementary anti- and syn-selective synthesis of 4,4,4-trifluoro-1-aryl-3-hydroxy-2-methyl-1-butanones

A complete reversal of diastereoselectivity was observed for reactions of the trifluoroacetaldehyde ethyl hemiacetal with enamines and imines, derived from propiophenones, that produce 4,4,4-trifluoro-1-aryl-3-hydroxy-2-methyl-1-butanones. This process serves as the first reliable, metal-free, complementary anti- and syn-selective method to prepare 4,4,4-trifluoro-1-aryl-3-hydroxy-2-methyl-1-butanones.