Diethyl ether added as an entrainer (cosolvent) to near- and supercritical CO2 significantly enhanced the enantioselectivity of photocyclization of 5,5-diphenyl-4-penten-1-ol sensitized by saccharide naphthalenedicarboxylate to give a cyclization product in enantiomeric excesses much larger than those obtained in conventional organic solvents, revealing the unique features of nc- and sc-CO2 as well as the critical role of entrainer clustering to the intervening diastereomeric exciplex pair.