ChemInform Abstract: Chiral Selection and Chiral Induction by the Use of Regiospecifically Di‐(or poly)‐Substituted Cyclodextrins.

Abstract

Regiospecific bifunctionalization of -cyclodextrins is achieved by use of rigid difunctional modifying reagents. Successive (two step) nucleophilic substitution on a regiospecifically capped cyclodextrin leads to a -cyclodextrin derivative having two different functional groups on regiospecific positions. Such compounds behave as artificial enzymes and receptors. A typical example of an artificial receptor is the cyclodextrin having amino and carboxyl groups on the regiospecific prim, prim positions, which recognizes free (unmodified) amino acids quite strongly in a neutral aqueous solution. A(Modified B6)-B[w-amino(ethylamino) --Cyclodextrin accelerates conversion from keto acids to L-amino acids in 98/2, 98/2 and 95/5 L/D ratio for phenylglycine, Phe and Trp, respectively. 1. GENERALSTRATEGY FOR ENZYME MIMICKS BYTHE USE OF MODIFIED CYCLODEXTRINS Cyclodextrins, cyclic oligomers (hexamer, heptamer, octamer, etc) of glucose, were easily obtainable from potato starch. Each cyclodextrin has a torus shape cavity inside the molecule, which is surrounded by axial C-H bonds of glucoses, making the environment hydrophobic. From the torus, hydroxyl groups are sticking out, making the molecule water soluble (see Fig. 1). Fig.1.The crystallographic structure Fig.2. The crystallographic structure of of -cyclodextrin (ref. 1) the inclusion complex of -cyclodextrin with p-nitrophenylacetoamide (ref. 2) Most probably water molecules penetrate into the cavity in an aqueous solution, as shown in X-ray crystallography. These water molecules are not satisfactorily hydrogen bonded, in other words they are destabilized to some extent, being readily driven out when a more suitable guest — a hydrophobic guest — comes to contact. Again this situation is well demonstrated by the X-ray crystallography (Fig. 2). The guest best accommodated into the cyclodextrin cavity, therefore, is to be best recognized by the C-H wall of the cavity in a way of best van der Waals surface contact.