Biellmann coupling

In view of the synthetic applications, among carbanions stabilized by only one divalent sulfur atom, allylic thiocarbanions proved to be particularly valuable, as shown with Biellmann coupling of allylic groups, applied to an elegant synthesis of squalene from farnesyl bromide [301]. In this synthesis, the retention of the allylic double bond position and stereochemistry in both the metallation-alkylation and the desulfurization steps are noteworthy. However, the results are not always as clear-cut, and... 

Allylic sulfides can be coupled with allylic bromides in a process which was first described by Biellmann and applied to the squalene synthesis. Famesyl sulfide was prepared from 2,4-dinitrophenylfar-nesyl oxide and benzenethiolate. Finally, the allylic thiocarbanion was coupled with famesyl bromide to give, in 82% yield, squalene sulfide, easily transformed to squalene with Li tNH2 (Scheme 32). 

Scheme 3.53 Biellmann allyl-allyl coupling for the synthesis of 1,5-dienes [200].

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