The photocycloaddition of 3-keto-4, 6-dienic steroids to seven s-trans, one s-cis, and one cis Diels-Alder dienes has been studied. The products obtained do not correlate with either the predominant configuration of the diene nor its triplet energy, but do correlate with the diene ionization potential. The dienes, butadiene, 2-methylbutadiene, 1-acetoxybutadiene, and 2, 4-dimethyl-l, 3-pentadiene, all undergo two competing cycloadditions. The first is a [2 + 2 + 2 + 2] cycloaddition on the a face of the steroid where both dienone double bonds add across the diene double bonds to form three annulated cyclobutane rings. The structure was confirmed by the isolation of the cyclobutanone derived from 1-acetoxybutadiene. The alternative [2 + 2 + 2 + 2] adduct, yielding a 5:4:5 ring system, was also obtained in a low yield with 2, 3-dimethylbutadiene. The adducts with terminally substituted dienes are formed in a tail-to-tail manner. The second type of cycloaddition observed is a photo-Diels-Alder reaction to yield [4 + 2] adducts. The addition across the α, βdienone double bond yields exclusively the symmetry-allowed trans-4α, 5β[4 + 2] adduct in a head-to-tail manner. This reaction is ascribed to an excited-state reaction rather than a ground-state trans double bond. Accompanying this product were tail-to-tail cis [4 + 2] cycloadditions across the γ, δ double bond. A transition occurs at approximately 8.4 eV, where the products are best described as being formed through a diradical with initial bonding at C4 of the dienone, followed by ring closure to generate cis and trans [2 + 2] as well as cis-4α, 5β [4 + 2] adducts. This transition is characterized by 1-vinylcyclohexene, which gave a mixture of all the observed adducts with the exception of the [2 + 2 + 2 + 2] adduct. Below approximately 8.4 eV, the [2 + 2 + 2 + 2] and γ, δ [4 + 2] adducts were not formed. Thus, trans, trans-2, 4-hexadiene, 1, 1'-bicyclohexenyl, and 1, 3-cyclohexadiene, normally a very reactive Diels-Alder diene, gave mainly [2 + 2] adducts across the α, βdienone double bond, together with lesser amounts of the cis [4 + 2] adduct. The dienone does not phosphoresce but does fluoresce and the fluorescence is quenched by trans, trons-2, 4-hexadiene. The photocycloadditions are efficiently, and differentially, quenched by 3, 3, 4, 4-tetramethyl- l, 2-diazetidine 1, 2-dioxide. The reactions are postulated to occur through the ππ* triplet excited state of the dienone. The cycloadditions are discussed in terms of Epiotis and Shaik's theoretical ππ* triplet photocycloaddition model. © 1979, American Chemical Society. All rights reserved.
