Raising the ceiling of diastereoselectivity in hydrogen transfer on acyclic radicals.

We report here that the monodentate complexation of Me2AlCl to an ester group significantly enhances the selectivity of hydrogen transfer on acyclic radicals flanked by both an ester functionality and a stereogenic center, leading to C-2,C-3-anti products with high diastereoselectivity. In certain cases improved ratios were obtained using bulkier aluminum Lewis acid such as MAD (methylaluminum-di(di-2,6-tert-butyl-4-methylphenoxide). Electron spin resonance studies on these acyclic radicals indicate that Lewis acid complexation leads to conformational changes in the radicals. The stereochemistry of the preferred enolate radicals complexed with Lewis acids and their impact on the acyclic transition states involved are suggested.