Fluoride-promoted fragmentation

A highly exo-selective asymmetric hetero Diels-Alder reaction was the key step in D.A. Evans total synthesis of (-)-epibatidine. The bicyclic cycloadduct was then subjected to a fluoride-promoted fragmentation that afforded a (f-keto ester, which was isolated exclusively as its enol tautomer. The removal of the ethoxycarbonyl functionality was achieved using the Krapcho decarboxylation. Interestingly, the presence of a metal salt was not necessary in this transformation. Simply heating the substrate in wet DMSO gave rise to the decarboxylated product in quantitative yield. 

Fluoride-promoted fragmentation of 2,3-bis(2-trimethylsilylethyl)cyclopropenethione resulted in elimination of fluorotrimcthylsilane and ethenc to give the tetrabutylammonium salt of 3-thioxocycloprop-1 -cne-1,2-dithiolate, which, by addition of sodium perchlorate solution, was converted into the sodium salt in 62% yield.The reaction of 2,3-bis(/ert-butylsulfanyl)cyclo-propenethione with two equivalents of phenyllithium in tetrahydrofuran at — 80 C for three hours gave 2-(tect-butylsulfanyl)-3-phcnylcyclopropenethionc in 85% yield. 

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