REACTIVITY OF MAXIMALLY CONJUGATED AZEPINES

Ethyl 1//-azepine-1-carboxylate is known to react with nitrosobenzene in benzene to form a [6 -I- 2]tc adduct together with a small yield (17%) of the [4 + 2]n adduct 80TL319 . If the reaction is carried out in refluxing chloroform, the yield ration is reversed, the [4 -I- 2]jt adduct now being formed in the higher yield (41%) and the [6 -I- 2]n adduct in 22% yield 92H(34)497 . The improved yields are attributed to stabilization of ionic intermediate (19) by solvation by chloroform. 

In addition to functioning as the polyene component in Diels-Alder reactions of 1//-azepines, a dienophilic character is demonstrated by the ease with which they also function as the dienophile in inverse cycloaddition reactions. In the reaction of ethoxy 1//-azepine-1-carboxylate with 3,4,5,6-tetrachloro-l,2-benzoquinone, both [2- -4]jt and [6- -4]jt adducts are formed in a hetero Diels- 

Alder reaction 86BCJ2485 . The two adducts (21) and (22) are stable, remaining unchanged after heating at 180°C in benzene. On the other hand, the [6 + A]n adduct (20), the major product of the reaction, is converted into (22) in 63% yield at 90 °C. 

This ability of ethyl and methyl l//-azepine-l-carboxylate to participate as an enophile and a dienophile is demonstrated in a further report describing their reactions with methyl pyrone-3-carboxylate and its 5-isomer 84BCJ3483 . On heating at 80°C for 68 hours, methyl 1/f-azepine-l-carboxylate and the pyrone-3-isomer yielded the [2 + 4]7t adduct (23) and the azepine [6 + 4]tc dimer. Under similar conditions ethyl 1/f-azepine-l-carboxylate and the corresponding pyrone-5-isomer gave a mixture of the [2 + 4]7t and [6 + 4]7t adducts (24) and (25) in yields of 25% and 20%, respectively. At 110°C the products consisted of the azepine [6 + 6[n dimer and a small amount of 3-ethyl 7-methyl 3/7-3-benzazepine-3,7-dicarboxylate (26), arising from (24) by extrusion of carbon dioxide. In a further reaction the same authors have demonstrated the formation of (27) by reaction with tetrachlorocyclopentadienone dimethyl acetal. 

The reaction of methyl 1/7-azepine-l-carboxylate with A,a-diphenylnitrone yields [2 -I- 3] dipolar cycloaddition products (28) as a mixture of exo- and e /o-isomers 92H(34)497 , the structures of which were assigned with the aid of LAOCOON III simulated NMR spectra. The almost equal distribution of isomers at C5 and the absence of any tra 5-product favors a concerted 1,3-dipolar addition rather than the involvement of a stepwise ionic mechanism, since in a stepwise reaction the conformational flexibility of the azepine ring would allow the second bond to be formed on either face of the azepine ring. 

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