A R CO CH2 Umpolung Equivalent

Dithiocarboxylic acid derivatives also provide a useful route to dithioketene acetals (another name for ketene dithioacetals).Treatment of dithiopropionic acid (376) with excess LDA (for 0-alkylation reactions 

The actual umpolung reaction that allows a ketene dithioacetal to function as if it had an electrophilic carbon a to a carbonyl is achieved by conjugate addition of nucleophiles to the ketene dithioacetal, followed by hydrolysis. Both sulfide and sulfoxide ketene derivatives can be used. Conjugate addition of an ester enol-ate derived from ferf-butyl acetate (secs. 9.2, 9.4.B, 9.7.A) to the ketene dithioacetal [CH2=C(SOMe)2] gave the stable dithioacetal anion (382). Conversion of the dithioacetal to the bis(sulfoxide) enhanced the ability of that species to function as a Michael acceptor. Subsequent transformation of 382 gave the aldehyde-ester (383).370 

Chapter 8. Nucleophilic Species That Form Carbon-Carbon Bonds 

In THF, y-substitution predominated in reactions of 391 and addition of DABCO increased that yield, presumably by coordination near the sulfur to sterically block that position. Addition of HMPA favors more a-substitution, and addition of a strongly coordinating species such as cryptate blocks the y-position, which leads to a-substitution as the sole product. 

A R(C=0)CH2CH2CH2+ Equivalent. In this umpolung equivalent, the potentially positive center is even further removed from the carbonyl (see 342 in Table 8.18). This umpolung equivalent is best achieved by reaction of a nucleophile with a vinylogous ketene dithioacetal such as 395.376 initial reaction of 

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