16-AzabicycIo hepta-2,5-diene

However, if the azepine is C-monosubstituted, e.g. 14, or unsymmetrically substituted, then two isomeric 2-azabicycloheptadienes, e. g. 15 and 16, may result corresponding to electrocyclic ring closure involving C2-C5 or C4-C7 of the azepine ring. In practice, the ratio of the two isomers formed (which may be separated by vapor phase chromatography) varies with the position of the substituent.236 In contrast, irradiation of methyl 2,5-di-tm-butyl-l//-azepine-l-carboxylatein methanol yields only methyl 3,5-di-tert-bulyl-2-azabicycIo[3.2.0]hepta-3,6-diene-2-carboxylate (81 %).70... 

Theoretically, valence isomerization of 3//-azepincs can give rise to 2-azabicyclo[3.2.0]hepta-2,6-dienes 20 and/or the isomeric fused azetines, 6-azabicycIo[3.2.0]hepta-2,6-dienes 21. Practically, however, 2-alkoxy- and 2-amino-3//-azepincs 19, on photolysis, yield the 2-azabicyclic systems 20 exclusively, since such structures are stabilized by imidate or amidine resonance.238... 

A few examples are known of the thermally reversible, photoisomerization of 4//-azepines, e.g. 22, to l-azabicycIo[3.2.0]hepta-2,6-dienes, e.g. 23.83 The yields are high (65 95%) and the alternative isomers, the 6-azabicycIo[3.2.0]hepta-3,6-dienes, resulting from C2-C6 ring closure, do not appear to be formed. 

Azabicycio [2 2 1 ]-hepta-2,5-dienes, Naphthalen-l,4-imines and Anthracen-9,10-imines... 

The reaction of (9) with dimethyl 7-oxabicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxylate (118) proceeds with formation of the cycloadduct (119) in 82% yield. Pyrolysis or photolysis of the adduct (119) resulted in a double fragmentation to give the thiophene derivative (120) and the furan (121) (75CC840). A similar reaction takes place between the mesoionic compound (9) and dimethyl 7-p-toluenesulfonyl-7-azabicycIo[2.2.1]hepta-2,5-diene-2,3-dicarboxylate which yields a 1 1 adduct of the same type as (119) (76BCJ3314). 

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