Fused azirines

The photolysis of 4-azidopyridazines gives significant yields of the ring-expansion products, unsaturated 1,2,5-triaze-pines. Thus, 3-methoxy derivative 801 reacts, via formation of a nitrene and ring opening of an intermediate fused azirine, with methoxide or diethylamine to give 4-methoxy (or4-diethylamino)-l,2,5-triazepines 802 (X = OMe, NEt2). 

A systematic review of synthetic approaches to fused azirines is provided in CHEC-III . One of the most common routes to aziridines of this type involves a cyclization via nucleophilic attack of a nitrogen on an exocyclic sp3-hybridized carbon as exemplified in Scheme 1 <2003TL7997, 2003TA1969>. Another general approach to fused azirines involves DielsAlder reactions of carboxylate-substituted 2/7-azirines (e.g., Scheme 2) < 1997S271, 1998J(P1)2969, 2005S555, CHEC-III(1.02.3.2.2)121>. 

The reaction was applied successfully to cyclic vinyl azides to produce fused azirine rings. 1-Azido-cyclooctene (46) was converted on irradiation in 93% yield into 9-azabicyclo-[6.1.0]-non-l-(9)-ene (47). The azirine could again be hydrolysed to the aminoketone (48)... 

Heteroaryl azides frequently undergo ring expansion on photoelimination of nitrogen fused azirine intermediates have been proposed. Irradiation of the 4-azidoisoquinolines (91), for... 

An elegant application of the isoxazole -azirine rearrangement has been used in the generation of the fused azirine 64 which, on further photolysis, underwent ring expansion to the cyclic ketenimine 65 as shown in Eq. (19).66... 

In a manner formally analogous to the rearrangement shown in Scheme 33, 2-quinolylcarbene and 1-isoquinolylcarbene rearrange thermally to 1-naph-thylnitrene and 2-naphthylnitrene, respectively (Scheme 34).202,223 9-Phen-anthridylnitrene affords the cyclic carbodiimide 169, which can be prepared almost pure by pyrolysis at 490°C, condensing the product at — 196°C. Compound 169 is stable up to about — 40°C, where dimerization occurs.2223 In this case, the fused azirine 170 (Scheme 34) is expected to be highly unstable due to complete loss of aromaticity, and no evidence for its formation has been found. 

Many photoreactions of aryl azides can be interpreted as arising via unstable ring-fused azirines. The formation of the 6,7-diaminobenzothiazoles (117) and the 6-amino-8//-thiazolo[5,4-c]azepines (118) on irradiation of the corresponding 6-azidobenzothiazoles (119) in diethylamine can be explained in this way, as shown in Scheme 14. An alternative explanation for the photodecomposition of phenyl azide (120) at low temperatures has recently been advanced and involves the formation of an intermediate l-azacyclohepta-l,2,4,6-tetraene (121). Further evidence for this pathway is to be found in a study of the photodecomposition of phenyl azide in acetic acid the azepin-2-one (122) is believed to arise in this way, as shown in Scheme 15. The formation of the same azepin-2-one on irradiation of phenyl azide in the presence of naked acetate anion has been reported the nature of the intermediate in this transformation is still open to question, but both benzazirine and l-azacyclohepta-l,2,4,6-tetraene intermediates are considered possible. In contrast, irradiation of the azide (123) in an argon matrix at lOK affords, on the basis of spectral evidence, the cyclic carbodi-imide (124). " 2-... 

The a-methylene-/3-lactam 103 is obtained by the carbonylation of the inethyleneaziridine 102 under mild conditions[91]. The azirine 104 undergoes an interesting dimerization-carbonylation to form the fused )3-lactam 105[92]. 

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