Szlwnzary Some 2-substituted-3H-azepines are reported to photoisomerize to 2-azabicyclo [3,2,0] hepta-2,g-dienes.PHOTOCHEMICAL intramolecular cycloadditions of 3H-azepines (I) should be disrotatory processes because of the constraints imposed by the seven membered ring. Simple considerations of the initial interactions developing in the highest occupied orbital of the first excited state on proceeding along the reaction co-ordinates? leads to the conclusion that 2-azabicyclo [3,2,0]hepta-2,6-dienes (11) and 6-azabicyclo [3,2,0]hepta-2,6-dienes (111) are allowed products. Irradiation of dilute pentane solutions of (Ia) (medium pressure Hanovia lamp, Corex filter) yielded (IIa) (70%). Similar reactions of (Ib) and (Ic) using low-pressure mercury lamps gave (IIb) (45%) and (IIc) (60%). No products of the type (111) were identified in any irradiations.: A brown polymer accounted for the remainder of the reactant.The structures assigned to (IIa), (IIb), and (IIc) are consistent with elemental analyses and u.v., i.r., and n.m.r.$ spectral data. The ring-proton portions of the n.m.r.(I I )x (111)
(la) X = -NMe2
(I b) X.= -NH2(Ic) X =: -0Et2 spectra, which are strikingly similar, is particularly decisive in distinguishing between the isomeric 2-aza-bicyclo-[3,2,0 J hepta-2,6-dienes (11) and 6-aza-bicyclo [3,2,0]hepta-2,g-dienes (111). Each compound has a 1H doublet in the range of T 3-60-3.64 and a 1H triplet in the range of T 3-8-3.89 with no additional absorptions below T 5-00. Since the t Arguments for the validity of such an approach when dealing with molecules in which there is little symmetry were presented in a $ No volatile products other than those reported were detected except on photolysis of 2-ethoxy-SH-azepine, when an Its structure cannot alter the basic observation of selectivity in the Spectra of (IIa) and (IIc) in CCI, and of (IIb) in review article (ref. l), "Conservation of Orbital Symmetry").uncharacterised impurity in the product was present (<3%). photoisomerization.[ %H,] pyridine.N.m.r. spectra were measured at 60 MHz with Me,Si as the internal standard.
