2-Azabicyclo hexadiene

Azabicyclo[2.2.0]hexa-2,5-diene, pentakis-(pentafluoroethyl)-synthesis, 2, 304 2-Azabicyclop.2.0]hexadiene reactivity, 7, 360 thermal isomerization, 7, 360 2-Azabicyclo[2.2.0]hexa-2,5-diene synthesis, 2, 304 1 -Azabicyclo[3.2.0]hexadiene synthesis, 7, 361 1 - Azabicyclo[2.2.0]hexane reactions, 7, 344 ring strain... 

Michael additions to acceptor-substituted dienes are often followed by (spontaneous or induced) cyclizations. This was already noted by Vorlander and Groebel4 who obtained a substituted 1,3-cyclohexanedione by treatment of 6-phenyl-3,5-hexadien-2-one with diethyl malonate (equation 5). Obviously, the 1,4-addition product which is formed initially then undergoes cyclization, ester hydrolysis and decarboxylation. Similarly, reaction of methyl sorbate with methyl 4-nitrobutyrate gave the 1,6-adduct which was reductively cyclized to 6-methyl-l-azabicyclo[5.3.0]decane18 (equation 6). 

Detailed structural studies of bicyclic azetines have been difficult to carry out because of the instability inherent in this type of ring system. The presence of fluorine-containing substituents though can stabilize some bicyclic azetines sufficiently to obtain physical data in support of these structures. X-ray crystallographic examination of palladium complex (117) showed that each l-azabicyclo[2.2.0]hexadiene ring is planar. Selected bond angles and distances for (117) are listed in Table 9 (76CPB2219). 

Bicyclic azetines tend to be highly unstable species. The more strained the bicyclic system the greater the reactivity. Azetines of the 2-azabicyclo[2.2.0]hexadiene structure have been suggested as transient intermediates in photolytic conversion of pyridines (77JCS(P2)1148, 78JOC944) and of pyrimidines (81JCS(P1)943). 

Under thermal conditions bicyclic azetines undergo facile valence bond tautomerization. Typically, l-azabicyclo[2.2.0]hexadienes give pyridines (equation 16) (76CPB2219), 1,2-diazabicyclo[3.2.0]heptadiene (126) gives diazepine (127) (77CJC56) and 2-... 

The structures of these stable azaprismane derivatives were established by their rearomatization into pyridine derivatives. To account for the structures of the azaprismanes obtained, rearrangement of an initially produced 2-azabicyclo[2.2.0]hexadiene to the 1-azaprismanes was suggested. ... 

Ultraviolet irradiation of pyridines can prodnce highly strained species that can lead to isomerised pyridines or can be trapped. The three picolines and the three cyano-substituted pyridines constitute photochemical triads irradiation of any isomer, in the vaponr phase at 254 nm, results in the formation of all three isomers. From pyridines and from 2-pyridones 2-azabicyclo[2.2.0]-hexadienes and -hexenones can be obtained in the case of pyridines these are nsnally nnstable and revert thermally to the aromatic heterocycle. Pyridone-derived bicycles are relatively stable, 4-alkoxy- and -acyloxy-pyridones are converted in particnlarly good yields. Irradiation of iV-methyl-2-pyridone in aqueous solution prodnces a mixture of regio- and stereoisomeric 4n pins 4n photo-dimers. ... 

It was demonstrated that fH]-diethoxymethane could be used as a deuteroformaldehyde equivalent with benzylamine hydrochloride and cyclo-hexadiene in the aqueous aza Diels-Alder reaction. The monolabelled 3-fH]-2-benzyl-2-azabicyclo[2.2.2]oct-5-ene is obtained in 32% yield [2] ... 

Reactions. Further studies on the valence-bond isomerizations of perfluoro-alkyl-pyridines, reported last year, have shown that photolysis of the perfluoroalkylated derivative [28 R = CF(CF3)2, R = CF2CF3, R = Cp3] in CF2CICFCI2 yields a mixture of the l-azabicyclo[2.2.0]hexadiene (29) and the... 

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