The Lewis acid-promoted reactions of 2,2-dialkoxycyclopropanecarboxylic esters 4a-c with aldehydes and unsymmetrical ketones to give gamma-lactones were investigated. TiBr, is an excellent catalyst and gives cis 3,4-substituted gamma-lactones in good yields with high diastereoselectivity. SnBr4 promotes the reaction of 4a-c with aldehydes with high cis-selectivity, but does not promote the reaction of 4a with unsymmetrical ketones. ZrCl4 is moderately trans-selective in the reaction of 4a with aldehydes, while being moderately cis-selective in the reaction of 4a with unsymmetrical ketones. Cis-gamma-lactones can be converted into their trans-isomers by treatment with NaOEt in EtOH.
Chemoenzymatic and yeast-catalysed synthesis of diastereomeric ethyl γ-phenyl and γ-(n-pyridyl)paraconates
The synthesis of γ-phenyl and γ-(n-pyridyl)paraconates was accomplished by chemical reduction of their respective ketodiester precursors followed by cyclisation of the resulting hydroxy diester intermediates. The cis- and trans-lactones thus obtained were separated and separately subjected to enzymatic hydrolysis with HLAP. The cis-lactonic esters had enantiomeric excesses ranging from 94% to 99%,