Vinyl sulfides intramolecular carbolithiation

Carbocyclization of m-alkcnyl-z-methoxybcnzy I lithiums to form five- or six-membered rings has been studied 101 the five-membered ring is formed with a cis-stereochemical relationship between the methoxy substituent and the adjacent methyl group. Intramolecular carbolithiation of vinyl sulfides at — 105°C in THF has been found to occur non-stereospecifically with regard to the newly formed C—Li centre.102. The stereochemistry of selective tandem Michael addition alkylation reactions of vinylphosphonates has been explored.103... 

Advantage has been taken of the low-temperature conditions (—105°C, THF) required for intramolecular carbolithiation of vinyl sulfides (75), in order to smdy the stereochemistry of the process with regard to the new lithium-bearing stereogenic center, which remains configurationally stable. Formation of the new carbon-carbon bond is stereospecific, whereas formation of the new carbon-lithium bond is non-stereospecific. Since (Z)-(75) and ( )-(75) give rise to different ratios of the diastereoisomeric products (76b) and (77b), formed via (76a) and (77a), the kinetically controlled (76b) (77b) ratios (87 13 and 40 60, respectively) are believed to reflect the stereochemistry of the corresponding carbolithiation process. 

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