6533b824fe1ef96bd1280a36

RESEARCH PRODUCT

Photocyclization of a bichromophoric phenol/olefi system substituted at the methylene spacer - Zwitterions versus H-bridged intermediates in the excited state proton transfer

subject

description

Photolysis of (E)- and (Z)-2-(1-ethyl-3-phenylpropenyl)phenol (1 and 2) under a variety of conditions produces a mixture of dihydrobenzopyrans 3 and 4, dihydrobenzofuran 5, cyclopropanes 6 and 7, and rearranged olefin 8. Acidic treatment of 1 and 2 gives a mixture of the six-membered ring products 3 and 4. Remarkable differences, associated with the nature of the precursor and the reaction conditions, are observed in the regiochemistry of cyclization and in the stereochemistry of the dihydrobenzopyrans (3/4 ratio). This points in favour of an H-bridged intermediate, rather than a fully zwitterionic species, as the direct precursor of the photocyclized products.