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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).

Cyclopropane derivatives Q 0021Tandem Cyclopropanation with Dibromomethane under Grignard Conditions.-Treatment of allylic and homoallylic alcoholates with CH 2 Br 2 and tBuMgCl allows an efficient access to cyclopropyl carbinols and their homologues. The reaction of α-substituted substrates proceeds with good to high syn-selectivity. One-pot formation of cyclopropyl carbinols form aldehydes, ketones, esters, as well as oxiranes is possible.-(BRUNNER, G.; EBERHARD, L.; OETIKER, J.; SCHROEDER*, F.; J. Org. Chem. 73 (2008) 19, 7543-7554; Richstoff-Forsch., Givaudan AG, CH-8600 Duebendorf, Switz.; Eng.) -Jannicke 08-076