Halogenocycloprop anation des dioxolannes ethyleniques et des enones correspondantes par les monohalogenocarbenoides

The monohalocyclopropanation of conjugated enones and their dioxolanes has been achieved by monohalocarbenoîds generated from methylene chloride and gem-dichloroethane. In both cases, besides expected chloro-adducts, corresponding bromo-adducts are obtained. Formation of these adducts involves halogen exchange by the primary chlorocarbenoid with lithium bromide associated with methyllithium. Conjugated enones give trans α-halocyclopropanic alcohols stereospecifically while unsaturated dioxolanes lead to cis and trans α-halocyclopropyl ketones with, generally, trans prédominating. The determination of configuration has been achieved by spectroscopic methods specially the LIS effect in NMR. Stereoselectivity effect of temperature and nature of the methyllithium reagent, are studied and discussed.