title: Enhancement of diastereoselectivity in photodimerization of alkyl 2-naphthoates with chiral auxiliaries via inclusion within γ-cyclodextrin cavities.
abstract: Irradiations of alkyl 2-naphthoates are known to result in four isomeric "cubane-like" photodimers: anti(HH)-2, syn(HH)-2, anti(HT)-2, and syn(HT)-2 where the anti(HH)-2, anti(HT)-2, and syn(HT)-2 consist of pairs of diastereomers. Here, chiral auxiliary and chiral microreactor strategies have been combined to achieve high diastereoselectivity in photodimerizations of an enantiomeric pair of 2-naphthoates with (R)- and (S)-1-methoxycarbonylethyl esters as chiral auxiliaries (1R and 1S). Thus, irradiations of their γ-cyclodextrin (γ-CD) complexes have been conducted. Fluorescence, IR, and NMR spectra of both enantiomers of 1 demonstrate that their γ-CD complexes are mainly 2:2 with the molecules of 1 in head-to-head orientations. Irradiation of the complexes in the solid state mainly resulted in anti(HH)-2. The absolute configuration of each diastereomer of anti(HH)-2 has been established for the first time here. The diastereomeric excesses (de's) of anti(HH)-2 from 1R and 1S were 94% and 86%, respectively. These de's are much higher than those found from irradiations in solution (55% for 1R and 1S), where the opposite diastereomeric form is in excess! Calculations of the energies of various conformations of the head-to-head 2:2 inclusion complexes were performed using the PM3 approach. The predicted major diastereomers based on the calculation are consistent with those found experimentally.
authors.affiliation: Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, The Chinese Academy of Sciences, Beijing 100190, People's Republic of China.
authors.forename: Hong-Xia
authors.initials: HX
authors.lastname: Xu
authors.forename: Su-Fang
authors.initials: SF
authors.lastname: Cheng
authors.forename: Xiu-Jie
authors.initials: XJ
authors.lastname: Yang
authors.forename: Bin
authors.initials: B
authors.lastname: Chen
authors.forename: Yue
authors.initials: Y
authors.lastname: Chen
authors.forename: Li-Ping
authors.initials: LP
authors.lastname: Zhang
authors.forename: Li-Zhu
authors.initials: LZ
authors.lastname: Wu
authors.forename: Weihai
authors.initials: W
authors.lastname: Fang
authors.forename: Chen-Ho
authors.initials: CH
authors.lastname: Tung
authors.forename: Richard G
authors.initials: RG
authors.lastname: Weiss