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

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posted on 2009-05-01, 00:00 authored by Lin Luo, Gui-Hong Liao, Xiao-Ling Wu, Lei Lei, Chen-Ho Tung, Li-Zhu Wu
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 1a and 1b 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.