Abstract

　　The occurrence of vinylcyclopropane subunit in many natural and synthetic biologically active compounds, as well as its utility as a synthetic intermediate, has promoted many strategies for its construction. In view of the difficulty associated with the regioselectivity, diastereoselectivity (cis/trans) and enantioselectivity, the highly selective synthesis of optically active multisubsituted vinylcyclopropanes in one step remains a challenging problem.
　　1. Switching of stereochemistry: Dramatic effect of HMPA on the stereoselectivity of the cyclopropanation reaction of telluronium allylides with α,β-unsaturated esters and amides.
　　Huang et al. reported that the telluronium allylides were readily available and reacted with α,β-unsaturated esters to provide the vinylcyclopropane derivatives in high yields with high stereoselectivity. Interestingly, the stereoselectivity of ylide cyclopropanation could be controlled by the choice of the base used for the formation of ylide. When KHMDS is used as the base, this cyclopropanation reaction is subject to thermodynamic control and the cis-isomer is the major product. In the presence of lithium salts, however, the reaction may proceed via a chelating six-membered ring transition state, which is formed by coordination of lithium ion with carbonyl oxygen and ylidic carbanion simultaneously. This coordination can change the stereochemical course of the reaction. This mechanistic insight suggested that it is possible to tune the stereoselectivity of such reactions by inhibition of the coordination in situ.