Monohalogénocyclopropanation des esters et des acides α-éthyléniques diversement alkylés. Activation et stéréosélection antithermodynamique

The halogenocyclopropanation of α,β-unsaturated acids and their esters through a reaction with dichloromethane and MeLi/LiBr proceeds with excellent yields. With the esters, the only halogenocyclopropanic adducts obtained are the hydroxylated ones while the acids lead to carbonylated adducts as major products, even when there is an excess of MeLi. With the acids, the cyclopropanation takes place selectively with a lithiated hydrate of ketone type intermediate favouring the formation of the ring. The stability of the primary product of the cycloaddition is such that it does not react massively with the methyllithium and that the halogenocyclopropane ketones can be isolated after hydrolysis. With the esters, the reaction is also activated by the primary formation of an hemiacetal; however, the cycloaddition is more equilibrated between this intermediate and the alkoxide which results from the later action of the MeLi. In both cases, a net syn stereoselectivity is observed in the unsaturated alcohols in spite of the opposite steric and stereoelectronic effects operating when the alkyl and functional groups are in a cis relation around the double bond. Key words: halogenocyclopropanation, activation, syn stereoselectivity. [Journal translation]