Enhancement of Diastereoselectivity in Photodimerization of Alkyl 2-Naphthoates with Chiral Auxiliaries via Inclusion within γ-Cyclodextrin Cavities

Irradiations of alkyl 2-naphthoates are known to result
in four isomeric “cubane-like” photodimers: antiHH-2, synHH-2, antiHT-2, and synHT-2 where
the antiHH-2, antiHT-2, and synHT-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 antiHH-2. The absolute configuration of each diastereomer
of antiHH-2 has been established
for the first time here. The diastereomeric excesses (de's) of antiHH-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.