Gamma-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 lambda > 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 gamma-CD, the neutral 2,3-NA was selected as a typical reactant in this work. UV-vis, fluorescence, and (1)H NMR analysis reveal that gamma-CD can encapsulate 2,3-NA to make a stable 2:1 inclusion complex 2,3-NA@gamma-CD both in aqueous solution and in the solid state. Irradiation of 2,3-NA@gamma-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 gamma-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@gamma-CD under ambient temperature and pressure. All of the observations indicate that native gamma-CD with hydrophobic interaction only is capable of regulating the orientation of naphthalene analogue 2,3-NA within the cavity of gamma-CD, and thereby leading to the highest ee value of 48% obtained so far for the photocyclodimerization with native gamma-CD in solution.