Abstract
Utilizing host-guest inclusion complexation in the solid state, alpha-monosubstituted ketones were deracemized using optically active host compounds such as (-)-trans-2,3-bis(hydroxydiphenylmethyl)-1,4-dioxaspiro[5.4]decene (1a) under alkali conditions. This new method afforded optically active alpha-monosubstituted ketones in excellent yields with high enantiomeric excesses. For example, (+/-)-2-benzylcyclohexanone (2a) and (+/-)-3-benzylhexan-2-one (3c) were converted to the R-isomer (74% ee) and the S-isomer (96% ee), respectively, in quantitative yields. An x-ray crystallographic study elucidated the structure of the inclusion complex of 1a and (R)-2a. The study showed that the host molecules 1a ingeniously includes the guest molecule (R)-2a via hydrogen bonding and van der Waals interactions. As an application of deracemization, coenzyme (R)-alpha-lipoic acid and (R)-(-)-epilachnene, the antipode of an defensive droplets from the Mexican bean beetle, Epilachna varivestis, were synthesized in short steps with>99% ee and 87% ee, respectively.
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