PHOTOLYSIS OF 5,5-DIBENZYL-DELTA(3)-1,3,4-OXADIAZOLINES

Photolysis of dibenzyl-Delta(3)-1,3,4-oxadiazolines (3) in the presenc e of dimethyl acetylenedicarboxylate (DMAD) gives only modest yields o f the expected symmetrical 3,3-dibenzylcyclopropenes (4), but these ar e accompanied by more than six byproducts, including unsymmetrical cyc lopropenes, methylenecyclopropanes, and various pyrazoles. The origin of this array of products can be explained by a series of steps starti ng with photolysis of 3 to form a diazoalkane that undergoes 1,3-dipol ar cycloaddition to DMAD, generating a 3H-pyrazole as initial product. The latter is further photolyzed to a symmetrical cyclopropene in com petition with benzyl group migration by thermal 1,5-sigmatropic or ion -pair rearrangement to afford a 4H-pyrazole, The 4H-pyrazole in turn u ndergoes photolysis to an unsymmetrical cyclopropene, which rearranges to a methylenecyclopropane. The 4H-pyrazole also undergoes autoxidati on, in the presence of air, to afford a benzoyl-4H-pyrazole Additional ly, in competition with rearrangement, the various pyrazoles lose a be nzyl group or a methoxycarbonyl group to afford pyrazoles with one les s substituent.