[2+2]PHOTOCYCLOADDITION OF 5,6-SUBSTITUTED 2-OXO-2H-PYRAN-3-CARBOXYLATES WITH ALKENES

The [2+2]photocycloaddition of alkenes and 5,6-substituted 2-oxo-2H-pyran-3-carboxylates with an ester as an electron-withdrawing group was developed. The 3,4-adducts, 4,5-disubstituted 3-oxa-2-oxobicyclo[4.2.0]oct-4-ene-1-carboxylates, were obtained in moderate yields accompanied by considerable amounts of the 5,6-adducts, 1,6-disubstituted 2-oxa-3-oxobicyclo[4.2.0]oct-4-ene-4-carboxylates. These structures, including their stereochemistry, were determined by various spectral and computational analyses, and some were examined by X-ray crystallographic analysis. The determined regio- and stereoselectivities were explained by frontier orbital theory.