1,3-Dipolar cycloaddition of methoxycarbonylnitrile, furannitrile and substituted benzene nitrile oxides (X = 4-CH3, 4-OCH3, 3-OCH3, 4-Cl, 3-Cl, 2,4-diCl, 4-F) to dimethyl 7-(diphenylmethylene)bicyclo[2.2.1]hept-2-ene-5,6-dicarboxylate (XII) led exclusively to exo cycloadducts IV, on irradiation with a low-pressure mercury lamp afforded the required 4-substituted derivatives of dimethyl 2,2-diphenyl-5-formyl-3-azabicyclo[4.3.0]nona-4,9-diene-7,8-dicarboxylate (XI) as sole products. Selectivity of the photorearrangement of isoxazolines IV to enaminoaldehydes XI is due to a stabilization of the biradical VII by the overlap of the radical-electron with π-electrons of the C=C double bond and by two benzene rings. Quantum yields of the photorearrangement, established from the deficit of the starting IV, were found to be greater than those for the analogous oxabicyclic derivatives XVI. The reaction mechanism is proposed and the dependence on various solvents is pointed out; this reaction sequence, viz. the 1,3-dipolar cycloaddition followed by a photochemical rearrangement provides a new method for obtaining derivatives of tetrahydro-2H-pyridine from the well available cyclopentadiene.
p-Hydroxyphenacyl photoremovable protecting groups — Robust photochemistry despite substituent diversity
