The development of catalysts for Mannich-type reactions that afford anti-products with excellent diastereo- and enantioselectivities under mild conditions and low catalyst loadings (1-5 mol %) is reported. Based on principles gained from the study of (S)-proline-catalyzed Mannich-type reactions that afford enantiomerically enriched syn-products, (3R,5R)-5-methyl-3-pyrrolidinecarboxylic acid (RR35) has been designed to catalyze the direct enantioselective anti-selective Mannich-type reactions. Computational studies of the above reaction using HF/6-31G* level of theory suggested that this design would be highly effective. The catalyst was subsequently synthesized and studied in organocatalytic Mannich-type reactions between unmodified aldehydes and N-PMP-protected alpha-imino esters. In accord with the design principles and in quantitative agreement with the theoretical predictions, reactions catalyzed by this catalyst afforded anti-products in good yields with excellent diastereo- and enantioselectivities (anti:syn 94:6 to 98:2, >97 to >99% ee).
Chiral 1,2-amino alcohols and 1,2-diols are common structural motifs found in a vast array of natural and biologically active molecules. 1 Recently, significant efforts have been applied toward the development of direct catalytic asymmetric approaches to the construction of these units based on the addition of unmodified R-hydroxyketones to imines or aldehydes in Mannich-type and aldol reactions, respectively. 2,3 Although the elegant studies of Shibasaki and Trost have provided routes to both syn-and anti-1,2-amino alcohols and diols using metal-based catalysis, 2 highly enantioselective organocatalytic approaches have been limited to syn-1,2-amino alcohols and anti-1,2-diols. 3 Here we describe simple and efficient routes to highly enantiomerically enriched anti-1,2-amino alcohols and syn-1,2-diols through direct asymmetric Mannich, Mannich-type, and aldol reactions catalyzed by primary aminecontaining amino acids.To generate anti-1,2-amino alcohols and syn-1,2-diols, we sought to design novel catalysts. In the reactions of R-hydroxyketones with (S)-proline, products form via a reaction involving an (E)-enamine A for both Mannich-type and aldol reactions 3 (Scheme 1). With pyrrolidine-derived catalysts or secondary amines, (E)-enamine intermediates predominate because of steric interactions in (Z)-enamine B. The stereochemistry of the product can be explained by transition state C or D because the si face of the (E)-enamine reacts (Scheme 1a). To selectively form anti-Mannich products in reactions involving alkylaldehydes and alkanone-derived nucleophiles, we previously designed catalysts (3R,5R)-5-methyl-3-pyrrolidinecarboxylic acid and (R)-3-pyrrolidinecarboxylic acid ((R)--proline), respectively. 4,5 With the latter catalyst, reactions proceed through transition state E, and the reaction face of the (E)-enamine is reversed from that of the (S)-proline-catalyzed reaction (Scheme 1b). These catalysts were, however, less than optimal for reactions of R-hydroxyketones. 6 For reactions of R-hydroxyketones, we reasoned that the use of a (Z)-enamine in the C-C bond-forming transition state should generate anti-Mannich and syn-aldol products. In our early studies of aldol reactions involving unmodified hydroxyacetone mediated by antibody catalysis, we noted preferential reaction of a (Z)-enamine of hydroxyacetone formed with the primary amine of the lysine side chain, the key catalytic residue of the aldolase, rather than reaction through an (E)-enamine as we had observed with cyclic ketones. 7 We reasoned that, with primary amines, (Z)-enamines of R-hydroxyketones F should predominate over (E)-enamines G. 8 When (Z)-enamine F reacts in the C-C bond-forming transition state (H or I), anti-Mannich or syn-aldol products should form predominately (Scheme 1c). Studies of direct asymmetric aldol and Mannich-type reactions catalyzed by primary amine-containing amino acids have been reported. 9 However, within these studies, reactions of R-hydroxyketones were either not tested or, when tested, enantioselectivities of the ...
The development of enantioselective anti-selective Mannich-type reactions of aldehydes and ketones with imines catalyzed by 3-pyrrolidinecarboxylic acid and related pyrrolidine derivatives is reported in detail. Both (3R,5R)-5-methyl-3-pyrrolidinecarboxylic acid and (R)-3-pyrrolidinecarboxylic acid efficiently catalyzed the reactions of aldehydes with alpha-imino esters under mild conditions and afforded anti-Mannich products with high diastereo- and enantioselectivities (anti/syn up to 99:1, up to >99% ee). For the reactions of ketones with alpha-imino esters, (R)-3-pyrrolidinecarboxylic acid was an efficient catalyst (anti/syn up to >99:1, up to 99% ee). Evaluation of a series of pyrrolidine-based catalysts indicated that the acid group at the beta-position of the pyrrolidine ring of the catalyst played an important role in forwarding the carbon-carbon bond formation and in directing anti-selectivity and enantioselectivity.
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