Catalytic enantio- and diastereo-selective addition of diethylzinc to racemic α-thio- and α-seleno-aldehydes: asymmetric synthesis of optically active vicinal thio- and seleno-alcohols

Abstract

Chiral 1,2-disubstituted ferrocenyl amino alcohols 2 and 3[(–)- and (+)-DFPE] catalysed enantioand diastereo-selective addition of diethylzinc reagent to racemic α-thio- and α-seleno-aldehydes 1 to afford optically active vicinal thio- and seleno-alcohols 4–7. In the presence of (R,S)-catalyst 2, the R-enantiomer of 1 selectively reacted with diethylzinc to afford vicinal (3S,4R)-thio- and selenoalcohols 4 with high diastereoselectivity. On the other hand, ethylation of the S-enantiomer of 1 proceeded with low selectivity and low yields. Similarly, the (S,R)-catalyst 3[(+)-DFPE] catalysed ethylation of racemic α-thioaldehydes 1 to afford vicinal (3R,4S)-thioalcohols 6 with high stereoselectivity. The recovered unchanged aldehydes 1 were completely racemized after work-up.