PHOTOCYCLOADDITION OF CYCLOPENTENONE TO PROPENAL ACETALS

When cyclopentenone was irradiated in the presence of propenal acetals , a complex mixture of head-to-head and head-to-tail [2 + 2] photocycl oadducts was obtained. The structure and the stereochemistry of the cy cloadducts were determined by 2D NMR spectroscopy. The head-to-tail re gioisomers and the stereoisomers with ezo side chains were preferred. Low asymmetric induction was observed when propenal acetals derived fr om tartaric acid or l,and dibenzyl threitol were used.