Stereochemistry of grignard reactions on some conformationally mobile δ-keto esters: The effects of changing solvent and reactant

Methyl 4-methyl-5-oxo-hexanoate (4) and methyl 4-methyl-5-oxo-5-phenyl-pentanoate (5) yield mixtures of cis and trans tetrahydro-5, 6-dimethyl-6-phenyl-2H-pyran-2-one (6+7) on reacting with PhMgX and MeMgX, respectively. Ratios $\text{6}\text{7}$ were measured for reactions performed in benzene, diethyl ether (with XI) and tetrahydrofuran (with XCI). A comparison is made of the results obtained with those of methyl (2-oxo-cyclohexyl)-propionate (1). It is suggested that the ester group and the keto group can interact in the transition states of the reaction performed in the less polar solvent and that in polar solvents they are remote from one another during the reaction. These conformational changes are believed to be responsible for the observed stereochemical differences in the reactions.