CATALYSIS BY BETA-CYCLODEXTRIN IN THE REACTION OF PARA-NITROPHENYL ACETATE WITH ALPHA-AMINO-ACIDS

The reactions of p-nitrophenyl acetate, 1, with both enantiomers of the following alpha-amino acids: alanine (2a), methionine (2b), leucine (2c), and tryptophan (2d), were studied in the presence of beta-cyclodextrin (beta-CD). All the reactions were catalyzed by beta-CD and no chiral discrimination was observed. The reactions of 1 with the D isomers of 2a and 2d were also studied in the presence of alpha- and gamma-CD. In both cases the catalysis increased in the order alpha < gamma < beta-CD. Also, to analyze the influence of the polar nature of the nucleophile, the reactions of 1 with both enantiomers of the methyl esters of 2a, 2c, and 2d were studied in the presence of beta-CD. The catalysis observed was significantly smaller than that for the reactions with the amino acids. It is suggested that the catalysis by CDs is due to the formation of a ternary hydrogen-bonding complex that results from the association of the amino acid with the complexed substrate. The effect of the size of the CD cavity on the observed catalysis is attributed to the geometrical requirements for the reaction. The decrease in catalysis in the reactions with the methyl esters is due to a weaker hydrogen-bonding association with these neutral nucleophiles.