Diastereoselective ring-opening aldol-type reaction of 2,2-dialkoxycyclopropanecarboxylic esters with carbonyl compounds. 2. Synthesis of cis-2,3-substituted-γ-lactones

Abstract

The reaction of 3-alkyl-2,2-dialkoxycyclopropanecarboxylic esters 1d–i with symmetrical ketones and formaldehyde was investigated. Cyclopropanes 1d–i react with symmetrical ketones and formaldehyde in the presence of TiCl 4 to give cis-2,3-substituted-γ-lactones in good yields with high diastereoselectivity. In the reaction of 3-ethylcyclopropane 1d, the reaction conditions hardly influenced the diastereoselectivity. Regarding the reaction of 3-methylcyclopropane 1f, however, the effect of the reaction conditions, especially the solvent, on the diastereoselectivity was observed. High chemoselectivity was also observed for the reaction of 1d with 1,4-cyclohexanedione mono-ethylene acetal ( 24). The isomerization of cis-γ-lactones by treatment with NaOEt in EtOH gives trans-isomers in good yields.