Entrainer Effects on Enantiodifferentiating Photocyclization of 5-Hydroxy-1,1-diphenylpentene in Near-Critical and Supercritical Carbon Dioxide

Enantiodifferentiating photocyclization of 5-hydroxy-1-diphenyl-1-pentene (1) sensitized by bis(1,2;4,5-di-O-isopropylidene-alpha-fructopyranosyl) 1,4-naphthalenedicarboxylate (2) was performed in near-critical and supercritical carbon dioxide media containing organic entrainers to obtain a chiral tetrahydrofuran derivative (3) in enantiomeric excess (ee) higher than those obtained in conventional organic solvents. Interestingly, the entrainer-driven ee enhancement did not depend on the entrainer polarity, which is in contrast to the behavior of the ee observed upon selective solvation in nonpolar organic solvents. This indicates that entrainer clustering around the intervening exciplex is essential :in order to keep the intimate sensitizer- substrate contact within the exciplex. Therefore, the clustering itself, rather than its property, is more crucial to prevent the dissociative diffusion to gaseous CO2. The wider allowance in choosing the entrainer enables us to use more green solvents for achieving the ee enhancement, while reducing the environmental risk.