Carbanions cychc

Rifamycin S also undergoes conjugate addition reactions to the quinone ring by a variety of nucleophiles including ammonia, primary and secondary amines, mercaptans, carbanions, and enamines giving the C-3 substituted derivatives (38) of rifamycin SV (117,120,121). Many of the derivatives show excellent antibacterial properties (109,118,122,123). The 3-cycHc amino derivatives of rifamycin SV also inhibit the polymerase of RNA tumor vimses (123,124). 

Grignard reaction and similar transformations allow C-C bond formation without a palladium catalyst. Grignard reagents and organolithium compounds are very versatile carbanion sources used in the synthesis of acyclic, heterocychc and carbo-cychc compounds. The esters, ketones and aldehydes are more stable when the reaction takes place on solid supports than in the hquid-phase, because this immo-bihzed components are not so sensitive towards water or oxygen. In the total synthesis of (S)-zearalenone (155) on solid supports the Grignard reaction is one of the key steps (Scheme 3.16) [120]. 

A study ofthe mono- vs di-alkylation reactions of dibromide (9) with carbanions (lOc-g), covering a range of >15 p units in DMSO, has revealed that the carbanions (lOd-g) derived from the less acidic carbon acids give exclusively the bis(monoalkylated) product (11) however, carbanions (lOa-c) give the cychc product (12) of dialkylation. This dichotomy is apparently a consequence of the relative rates of formation (by proton transfer, ) and cyclization ( ) of the conjugate base ofthe monoalkylated intermediate. 

Diastereoselectivity in the addition of electrophiles to the carbanions of 2-(alkylthio)thiolane-l-oxides has been explored and a six-membered cychc transition... 

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