Abstract
The Lewis acid-promoted reactions of 2,2-dialkoxycyclopropanecarboxylic esters 4a-c with aldehydes and unsymmetrical ketones to give γ-lactones were investigated. TiBr 4 is an excellent catalyst and gives cis 3,4-substituted γ-lactones in good yields with high diastereoselectivity. SnBr 4 promotes the reaction of 4a-c with aldehydes with high cis-selectivity, but does not promote the reaction of 4a with unsymmetrical ketones. ZrCl 4 is moderately trans-selective in the reaction of 4a with unsymmetrical ketones. Cis γ-lactones can be converted into their trans-isomers by treatment with NaOEt in EtOH
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