Enantioselectivity cydodextrins

Mechanism of Separation. There are several requirements for chiral recognition. (/) Formation of an inclusion complex between the solute and the cydodextrin cavity is needed (4,10). This has been demonstrated by performing a normal-phase separation, eg, using hexane—isopropanol mobile phase, on a J3-CD column. The enantiomeric solute is then restricted to the outside surface of the cydodextrin cavity because the hydrophobic solvent occupies the interior of the cydodextrin. (2) The inclusion complex formed should provide a rdatively "tight fit" between the hydrophobic species and the cydodextrin cavity. This is evident by the fact that J3-CD exhibits better enantioselectivity for molecules the size of biphenyl or naphthalene than it does for smaller molecules. Smaller compounds are not as rigidly held and appear to be able to move in such a manner that they experience the same average environment. (5) The chiral center, or a substituent attached to the chiral center, must be near to and interact with the mouth of the cydodextrin cavity. When these three requirements are fulfilled the possibility of chiral recognition is favorable. 

Enantioselective packings Polar or nonpolar Packings with enantioselective cages or enantioselective surfaces, microcrystalline cellulose triacetate, cellulose ester or cellulose, carbamate/sil-ica composites, optically active poly(acrylamide)/silica composites, chemically modified silicas (Pirkle phases), cydodextrine modified silicas Operated either with normal phase or reversed phase mobile phases... 

Enantioselectivity The ability of a sorbent (typically a cydodextrin) to separate chiral compounds. 

It is known that cydodextrins have a hydrophobic cavity (a binding site for aromatics) and a hydrophilic external surface. A template-directed asymmetric sulfoxidation has been attempted with various aryl alkyl sulfides. [91]. Oxidations were performed by using metachlo-roperbenzoic add in water in the presence of an excess of p-cyclodextrin. The best ee (33%) was attained for mem-(r-butyl)phenyl ethyl sulfoxide. The decrease in the amount of P-cyclodex-trin below 1 mol equiv. causes a sharp decrease in enantioselectivity because of competition with oxidation of free substrate by the oxidant. Similarly, Drabowicz and Mikolajczyk observed modest asymmetric induction (27% ee) in the oxidation of Ph-S-n-Bu with H2O2 in the presence of P-cyclodextrin [92]. 

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