δ Lactones from δ-ketoesters-II : Mechanism changes in alkylation reactions and substituent effects on stereoselection

Abstract Alkylation of methyl 4-methyl 5-oxo 5-phenyl (m and p-X substituted) pentanoates 1a-b give cis and trans tetrahydro 5,6-dimethyl 6-phenyl 2H pyran-2-ones. LFER of isomer ratios as function of the X substituent on the phenyl ring is seen in MeLi-Et2O. The lactone ratios of reactions in THF with MeMgCl are not affected by the X phenyl substituent, while a more complex situation is showed by reactions of MeMgl in Et2O and benzene. Changes in the cis: trans-ratios with variations in reactant and solvent are discussed in terms of equilibrium between folded and unfolded conformations in transition states