Aldehydes are asymmetrically acylated by a two step sequence that is initia
ted by a homologation step to 1,1-heterodisubstituted alkenes followed by a
symmetric dihydroxylation. Thus, ketene O,S-acetals are efficiently prepare
d from aldehydes by a Peterson olefination with lithiated methoxy-phenylthi
otrimethylsilyl methane 14 as the C-1 source. Although they are dihydroxyla
ted with the Sharpless catalyst with moderate to good enantioselectivity (6
2-80% ee), the process is not efficient owing to the low chemical yields of
the desired alpha-hydroxy methyl esters (7-37%). Use of the corresponding
sulfoxide 24 or sulfon 25 led to an improved chemical yield of alpha-hydrox
y methyl ester 19, but the stereoselectivity was diminished. In contrast, i
ntermediate ketene O,O-acetals are prepared by a Horner-Wittig reaction wit
h phosphine oxide 31 and are dihydroxylated both with good chemical and ste
reochemical yield. The concept is applicable to aromatic, aliphatic, and ch
iral aldehydes. For example, this short sequence allows exclusive and indep
endent preparation of both diastereomeric heptoses 69a and 69b.