Origins of stereoselectivity in the chelation-controlled addition of alkyl radicals to α-methylene-γ-oxycarboxylic acid esters

Abstract

The diastereoselectivity in the chelation-controlled alkyl radical (R³˙) additions to α-methylene-γ-oxycarboxylic acid esters 1–12 was rationalised by analysing the low-energy conformers of radical intermediate models 46–59 and 66–69, which resemble the structures of early transition states of the exothermic transfer of a H-atom or allyl group. The sense of diastereoselectivity depends principally on the conformations of the sharply folded seven-membered chelate ring and the dihedral angle OC–O–C of the ester moiety. The transfer reaction to the radical intermediate bearing an ethoxy group with Z-geometry (dihedral angle ca. 0°) occurs predominantly on the exposed outside face of the radical centre, whereas the ethoxy group with E-geometry (dihedral angle ca. 180°) shields the outside face of the radical centre and lowers the diastereoselectivity. The diastereoselectivity depends also on the conformation of the CH₂R³ group attached to the radical centre. When the CH₂–R³ bond is perpendicular to the radical face, R³ shields the outside face of the radical centre. The intermediate bearing the ethoxy group with Z-geometry and the CH₂–R³ bond parallel to the radical face affords the highest syn-selectivity in the reactions of γ-methoxymethoxy and γ-benzyloxy esters.