Acyl anion synthon 1,3-dithiane anions

Likewise 1,3-dithianes can be deprotonated by alkyl lithium bases and the resulting anions are strong nucleophiles. The ditliiane group can be hydrolyzed back to the carbonyl group. Thus the dithiane serves as a synthon for the acyl anion. 

The acyl anion (R-C=0) is not stable as such, but when an aldehyde is converted into a 1,3-dithiane by reaction with propane-1,3-dithiol and then treated with base, it forms an acyl anion equivalent, and hence is susceptible to attack by electrophilic reagents (see Section 5.9). Two extensive compilations of formyl and acyl anion synthons together with references to their reactions... 

We have met the acyl anion or d1 synthon in chapter 23 but for the disconnection 8 on 1,4-diketones we need a d1 reagent that will do conjugate additions on enones such as 36. Sadly that eliminates dithians from consideration as they are too basic (hard) and tend to add direct to carbonyl groups. 

Disconnection of the TM at the newly formed C-C bond gives an illogical synthon (an acyl anion, for which a 1,3-dithiane anion is the synthetic equivalent). An alternate, stepwise approach to the retrosynthesis involves first doing a functional group interconversion, followed by a logical disconnection. Either... 

In summary, the title compound, readily available from diethyl oxalate, is a synthetically useful synthon in various C-C bondforming reactions, requiring only catalytic amounts of mild Lewis acids such as zinc chloride. Mtematives to such an acyl anion equivalent include dithiane derivatives of glyoxylic acid esters, which have not been tested as widely and which require some synthetic effort in the final deprotection step. ... 

If the equivalent of a negatively charged carbonyl group is added to another carbonyl, then a 1,2-dioxygenated species is produced. The reagents that can do this are called acyl anion equivalents—they are the synthons for [XC(=0)]. We have already met a number of such species, the cyanide ion, the anions of alkynes, and dithianes. 

---