DIASTEREOSELECTIVITY OF ORGANOMETALLIC ADDITIONS TO NITRONES BEARING STEREOGENIC N-SUBSTITUENTS

The diastereoselectivity of organometallic additions to nitrones bearing stereogenic a-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 (e.g. 22 and 23) bearing the potentially chelating β-methoxyalkyl group. However, the opposite selectivity resulted from the reaction of methylmagnesium bromide with the corresponding silyl ether (27). The relative stereochemistry of selected hydroxylamine adducts was established by reduction of their phosphate and carbonate derivatives to known amines (37a,b and 39), by periodate cleavage of a β-hydroxy hydroxylamine (41b), and by various correlations (Scheme II). The high facial diastereoselectivity observed with the N-(β-methoxyalkyl)nitrones is explained by a simple chelation model (Scheme III). © 1990, American Chemical Society. All rights reserved.