Control of diastereoselectivity in metal-catalyzed 1,3-dipolar cycloaddition between diphenylnitrone and chiral auxiliary-substituted crotonyl amide

The 1,3-dipolar cycloaddition (1,3-DC) between diphenyl nitrone (1) and 4-(S)-benzyl-((E)-2'-butenoyl)- 1,3-oxazolidin-2-one (2) was studied in the presence of several inorganic salts whose cations behave as Lewis acid, Depending on the salt and the experimental conditions, three products (3-5) can be obtained diastereoselectively. The type of conformation assumed by 2 when coordinated to the cation, allows the attack of 1 on either the Re- or the Si-face of the dipolarophile and rationalizes the resulting diastereoselectivity. The coordination of the reagents around the cation was studied by NMR spectroscopy with 2-acetyl-4-(S)-benzyl-1,3-oxazolidin-2-one (7) as model compound, (C) 1999 Elsevier Science Ltd. All rights reserved.