ChemInform Abstract: Cyclopropanation with Dibromomethane under Grignard and Barbier Conditions.

Abstract

Cyclopropanation with Dibromomethane under Grignard and Barbier Conditions. -Dibromomethane (II) together with a tertiary Grignard reagent cyclopropanate (homo)allylic magnesium and lithium alcoholates efficiently. The cyclopropanation under Grignard conditions afford higher syn-selectivities at ambient or higher temperatures than those obtained by other methods in the case of substrates having substituents in the α-position. Lithium allyl alcoholates provide the best cyclopropanation rates, especially in the case of relatively unsubstituted allyl or sensitive α-tertiary allyl alcoholates or under Barbier conditions. Lithium (homo)allyl alcoholates are cyclopropanated directly with (II) and magnesium under Barbier conditions at higher temperatures, cf. (I)→(VII). In tandem reactions carbinols (X), (XVI), and (XXI) are obtained from allyloxylithium or -magnesium intermediates, generated in situ by alkylation of conjugated aldehydes, ketones, or vinyloxiranes like (VIII), (XIV), and (XIX).