PHOTOSENSITIZED <2 + 2>-CYCLOADDITION OF 3-ACETYL-2<3H>-OXAZOLONE TO MALEIC ANHYDRIDE, DIMETHYLMALEIC ANHYDRIDE, AND 1,6-ANHYDRO-4-O-BENZYL-2,3-DIDEOXY-β-D-erythro-2-HEXENOPYRANOSE