Substituent Effect on the Diastereoselectivity in the Chelation‐Controlled Radical Reactions of γ‐(p‐Substituted‐benzyloxy)‐α‐methylene Esters with Alkyl Iodides.

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Abstract A pronounced substituent effect on the diastereoselectivity in the chelation controlled radical reactions of ethyl γ-( p -substituted-benzyloxy)-α-methylenecarboxylates with alkyl iodides was observed. The syn -selectivity increased in the order of electron-donating ability NO 2 3 i -Pr, Me, OMe of the p -substituent, and the plot of the log( syn / anti ) versus Hammett sigma constants gave a linear correlation. The complexation experiments of the substrates with Lewis acid using 1 H NMR spectroscopy and the competition experiments between p -isopropylbenzyloxy and p -trifluoromethylbenzyloxy esters showed that the electron-donating p -isopropyl group stabilized the seven-membered chelate ring to give high syn -selectivity.