The irradiation of chiral phenylglyoxylate esters in the presence of furan within NaY, LiY, KY, CsY, and RbY showed in some cases an increase of stereoselectivity in comparison with the observed stereoselectivity when the reaction was performed in solution. The enhanced stereoselectivity can be explained assuming that the biradical intermediate of the reaction can have an advantage (it occupies a smaller volume) to assume the correct conformation able to cyclize when the reaction is performed within a zeolite. This advantage is greater for the stereoisomer able to give the observed product than for the other one. In the case of the phenylglyoxylate ester obtained from (S)-1-methyl-1-propanol, the stereoselectivity was higher with NaY than with LiY. In the case of the phenylglyoxylate ester obtained from (S)-2-methyl-1-butanol, NaY and LiY gave the same stereoselectivity. The effect of the cation can be explained considering that, in the case of the phenylglyoxylate ester obtained from (S)-1-methyl-1-propanol, the interaction between the LUMO of the alkali ion and the HSOMO of the biradical intermediates was different for SS and RS stereoisomers. In the case of the phenylglyoxylate ester obtained from (S)-2-methyl-1-butanol no difference was observed in the HSOMO of SS and RS stereoisomers of the biradical intermediates in agreement with the observed no effect of the change of alkali ions. When the reaction was performed within β-cyclodextrin a reduction of stereoselectivity was observed.
Keywords: organic photochemistry, SS biradical intermediate, anti-syn equilibration, rigidification, cyclodextrin
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