Reversal of optical induction in transamination by regioisomeric bifunctionalized cyclodextrins

Two isomeric compounds have been synthesized carrying a pyridoxamine on C-6 of beta-cyclodextrin and an imidazole unit on C-6 of the neighboring gluco se residue. Each one stereoselectively transaminates phenylpyruvic acid to produce phenylalanine, and with opposite stereochemical preferences. Struct ure determinations by X-ray crystallography and NMR spectroscopy indicate t hat the imidazole units serve to block proton addition from their side, rat her than acting to protonate the transamination intermediates. Related cycl odextrin-pyridoxamine compounds had been reported carrying ethylenediamine units instead of imidazoles, and high enantioselectivities in transaminatio n were claimed. Our work indicates that these claims are incorrect, and tha t only poor selectivities are seen that are often unrelated to the position of the ethylenediamine units. Neither of these transaminating systems yet approaches the enantioselectivity seen with a pyridoxamine carrying a chira lly mounted internal base group. (C) 1999 Elsevier Science Ltd. All rights reserved.