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

摘要

Irradiation of methyl 3-methoxyl-2-naphthoate (2,3-NA) in methanol solution with light λ. > 280 nmproduces anti-head-to-head cubane-like photocyclodimer 1 and [4+4] intermediate 2,, which is, to thebest of our knowledge, the first incidence of directly obtaining the intermediate in photocyclodimerizationof naphthalene analogues. X-ray crystal structural analysis has realized the chirality of 1, and the opticallypure enantiomers 1a and 1b have been achieved by HPLC resolution. To understand the fundamentalphotocyclodimerization of naphthalene analogues mediated by native y-CD, the neutral 2,3-NA wasselected as a typical reactant in this work. UV—vis, fluorescence, and ~1H NMR analysis reveal that y-CDcan encapsulate 2,3-NA to make a stable 2:1 inclusion complex 2,3-NA @ y-CD both in aqueous solutionand in the solid state. Irradiation of 2,3-NA @ y-CD results in photocyclodimerization with remarkableselectivity and efficiency, whereas no photocyclodimers could be detected in host-free aqueous solutionand the neat solids. More importantly, the use of native y-CD as a chiral reaction vessel turns out to bean effective and versatile strategy for the enantioselective photocyclodimerization of 2,3-NA. The eevalues of 48% in aqueous solution and up to 34% in the solid state for anti-head-to-head photocyclodimer1 have been achieved upon irradiation of the inclusion complex of 2,3-NA @ y-CD under ambienttemperature and pressure. All of the observations indicate that native y-CD with hydrophobic interactiononly is capable of regulating the orientation of naphthalene analogue 2,3-NA within the cavity of y-CD,and thereby leading to the highest ee value of 48% obtained so far for the photocyclodimerization withnative y-CD in solution.