Abstract
ZnCl2-triggered lactonization reactions of γ,δ-epoxy-β-silyloxy esters with remote alkoxy groups (CH2)nOR (n = 2, 3), producing five- and/or six-membered ring lactones, have been investigated. Under such conditions, the regiochemistry of the reaction is governed by the cis or trans nature of the starting epoxy ester and also by its C(3) stereochemistry. An X-ray crystallographic structure of a γ-lactone (compound 24′) and a 13C NMR correlation for the C(4) carbon chemical shifts of all γ-lactones were obtained and used to determine the regioselectivity of the epoxide ring-opening. A mechanistic hypothesis, involving an oxocarbenium ion as a common intermediate, is presented in interpretation of the results.
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