Photochemical synthesis. VI.The formation of benzosemibullvalene derivatives in the photoaddition of diphenylacetylene to methyl 2-naphthoate, a degenerate thermal isomerization of the benzosemibullvalene skeleton

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Abstract
Six compounds that comprise up to 80% of the mixture formed on ultraviolet irradiation of diphenylacetylene and methyl 2-naphthoate have been isolated and characterized by a combination of spectral and chemical methods. Two of these compounds [(4) and (7)] are derivatives of 1a,2,7,7a-tetrahydro-1,2,7-metheno-1H-cyclopropa[b]naphthalene, two [(5) and (6)] are derivatives of benzosemibullvalene, and two [(8) and (9)] are lactones derived from (6) and (4), respectively. The yields of individual products of this photoaddition vary with the ratio of the reactants and the solvent (benzene or cyclohexane). Certain aspects of the mechanism of the formation of (4) and (5) are discussed, including the intermediacy of the bicyclo[4,2,0]octa-2,4,7-trienes (12) and (37) and the role of methyl 2-naphthoate as sensitizer in the photoisomerization of (4) to (5). The thermal interconversion of the benzosemibullvalenes (5) and (6) between c. 15 and 110� is described, as well as the thermal isomerization of (4) and (7) to derivatives of benzobicyclo[4,2,0]octa-2,4,7-triene. An efficient synthesis of the lactone (8) by a novel lactonization procedure is reported.

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