γ-Cyclodextrin-Directed Enantioselective Photocyclodimerization of Methyl 3-Methoxyl-2-naphthoate

Irradiation of methyl 3-methoxyl-2-naphthoate (2,3-NA) in methanol solution with light λ > 280 nm produces anti-head-to-head cubane-like photocyclodimer 1 and [4+4] intermediate 2, which is, to the best of our knowledge, the first incidence of directly obtaining the intermediate in photocyclodimerization of naphthalene analogues. X-ray crystal structural analysis has realized the chirality of 1, and the optically pure enantiomers 1_a and 1_b have been achieved by HPLC resolution. To understand the fundamental photocyclodimerization of naphthalene analogues mediated by native γ-CD, the neutral 2,3-NA was selected as a typical reactant in this work. UV−vis, fluorescence, and ¹H NMR analysis reveal that γ-CD can encapsulate 2,3-NA to make a stable 2:1 inclusion complex 2,3-NA@γ-CD both in aqueous solution and in the solid state. Irradiation of 2,3-NA@γ-CD results in photocyclodimerization with remarkable selectivity and efficiency, whereas no photocyclodimers could be detected in host-free aqueous solution and the neat solids. More importantly, the use of native γ-CD as a chiral reaction vessel turns out to be an effective and versatile strategy for the enantioselective photocyclodimerization of 2,3-NA. The ee values of 48% in aqueous solution and up to 34% in the solid state for anti-head-to-head photocyclodimer 1 have been achieved upon irradiation of the inclusion complex of 2,3-NA@γ-CD under ambient temperature and pressure. All of the observations indicate that native γ-CD with hydrophobic interaction only is capable of regulating the orientation of naphthalene analogue 2,3-NA within the cavity of γ-CD, and thereby leading to the highest ee value of 48% obtained so far for the photocyclodimerization with native γ-CD in solution.