Solvent-controlled diastereoselectivity in tryptophan-catalyzed Mannich reactions

Solvent effects in the L-tryptophan-catalyzed Mannich reaction between hydroxyacetone and glyoxylate imines have been examined. The use of a DMSO/1-butanol (4:1 v/v) mixture as solvent at rt provided the expected Mannich adducts in good yields, high anti-diastereoselectivity (up to 10.3:1 anti/syn ratio) and excellent enantioselectivities (up to >99.9% ee for the anti isomer).