Acidity lithium carbenoid generation

The tin-lithium exchange is also suitable for the generation of a-lithiated oxiranes 53245-247 jjyg jQ jjjg enhanced acidity of a carbon atom incorporated into a three-membered ring, the metalation of epoxides by treatment with various alkyllithium reagents of lithium amide bases also permits one to obtain carbenoids 53 in situ (equation 35) °. 

A lato modirication of the original Kdbrich procedure entails the generation of caibenoids in the presence of an added equivalent of lithium bromide salt (equation 22). Apparently, the Lewis acid character of the added salt helps to stabilize the carbenoid by coordination wiA the halogen atom. The lithium salt is also implicated in Lewis acid complexation to the oxygen atom of the G=>0 bond, and seems to assist in the cyclization step as well. 

A one-carbcMi ring enlargement approach for the synthesis of 8-lactones has been developed by Satoh and Kurihara [123] (Scheme 76). Reaction of the lithium carbanion of chloromethyl phenyl sulfoxide with lactone 336 afforded a diastereo-meric mixture of the hemiacetal 337. Treatment of this adduct with three equivalents of KH in THF generated potassium enolate 338, which on addition of four equivalents of t-BuLi underwent an alkylidene carbenoid rearrangement to give alkynolate 339. Protonation of 339 by the addition of sulfuric acid facilitated the formation of m-hydroxy ketene 340, which underwent an intramolecular cyclization to afford 8-lactone 341. 

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