Umpolung dithioacetals

As known from the Umpolung reactions, dithioacetals can be deprotonated efficiently among others [230, 306], a recent application is 122 [307] (Scheme 1.54). Even basic aluminum oxide can catalyze these isomerizations. One older example reports the isomerization of an S-propargyl phosphanesulfide to the allene [308],... 

The term Umpolung is most often used to describe the reversal of the carbon polarity when going from a carbonyl group to the carbanion of a dithioacetal. 

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... 

Table 8.19. Ketene Dithioacetal 385 as a Nucleophilic Umpolung Reagent...

B.V. 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... 

C=0. It is therefore susceptible to nucleophilic attack at carbon. Methods have been developed by which this polarization is effectively reversed so that the carbon atom itself becomes the nucleophilic centre. Such an inversion is known as umpolung. An example is provided by the 1,3-dithiane system. An aldehyde may be converted into a cyclic dithioacetal by reaction with propane-1,3-dithiol in the presence of an acid. The two adjacent electronegative sulphur atoms make the C—H bond of this acetal rather acidic. Treatment with butyllithium therefore affords a lithio derivative in which the carbon atom is susceptible to electrophilic attack. The 1,3-dithiane system is reconverted into a carbonyl group by acid hydrolysis in the presence of mercury(II) ions, which complex with the dithiol. The RCO group in the original aldehyde is thus equivalent to R—C=0 (Fig. 3.10). 

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