(-ALPHA-PINENE AND (-)-ALPHA-PINENE AS CHIRAL RECOGNITION PROBES WITHNATURAL CYCLODEXTRINS AND THEIR PERMETHYLATED DERIVATIVES - AN AQUEOUS NMR-STUDY())

The binding of (1R)-(+)- and (1S)-(-)-alpha-pinene to alpha-, beta- an d gamma-cyclodextrin and to the corresponding permethylated derivative s TM alpha-, TM beta- and TM gamma-cyclodextrin, has been studied in a queous solution by NMR spectroscopy. The stoichiometries and the assoc iation constants have been measured, The signals of both (+)- and (-)- alpha-pinene were found to follow the Slow exchange, whereas those of the cyclodextrins the fast exchange regime, indicating that the type o f exchange is independent of the magnitude of binding constants, which vary along the series, and also a result of other factors, in additio n to the associated rate constants, The structures of the respective c omplexes have been derived from 2D ROESY experiments, The cyclodextrin s preferentially bind with the (1S)-(-)-alpha-pinene, but only alpha-C D exhibits remarkable enantioselectivity, These observations show clea rly that formation of a hydrogen bond or the presence of an aromatic r ing, previously invoked as some of the requirements for enantioselecti vity with CDs, are not necessary, In addition to enantioselectivity, s ite selectivity was also observed for alpha- and TM alpha-CD with the preferred (1S)-(-)-enantiomer, the stoichiometry being 2:1 in both cas es.