Diastereoselectivity of organometallic additions to nitrones bearing stereogenic N-substituents

Abstract

The diastereoselectivity of organometallic additions to nitrones bearing stereogenic α-arylethyl, β-methoxyalkyl, and β-(silyloxy)alkyl substituents on nitrogen has been investigated. High and complementary diastereoselectivity (90-94%) was observed in the additions of Grignard reagents to nitrones bearing the potentially chelating β-methoxyalkyl group. However, the opposite selectivity resulted from the reaction of methylmagnesium bromide with the corresponding silyl ether. The relative stereochemistry of selected hydroxylamine adducts was established by reduction of their phosphate and carbonate derivatives to known amines, by periodate cleavage of a β-hydroxy hydroxylamine, and by various correlations. The high facial diastereoselectivity observed with the N-(β-methoxyalkyl)nitrones is explained by a simple chelation model