37/-Indazoles, ring contraction

As an alternative to the diradical ring closure that occurs with the 3 -indazole ring contraction, the employment of a 1,3-elimination from an ortho, a-disubstituted aromatic has much appeal for cycloproparene formation, not least because of the simplicity of the process and the ready availability of the starting materials. It is not surprising, therefore, that such a report appeared as early as 1974. Radlick and Crawford found that 1-bromo-2-(methoxymethyl)benzene (33) underwent lithium-halogen exchanged and cyclization to 1 upon treatment with butyllithium. The yield was a modest but acceptable 30%. In similar vein, the cyclization methodology provided rocketene (34) albeit in 5% yield (equation 7a). ... 

Pyrido[ 1, 2 1,2]imidazo[4,5-b]quinoxaline nomenclature, 1, 22 2,2 -Pyridoin synthesis, 2, 337 Pyrido[ 1,2-6]indazoles synthesis, 5, 335 E ridoindoles rearrangement, 4, 508 Pyrido[a]indoles synthesis, 4, 233 Pyrido[6]indoles nomenclature, 4, 498 Pyridooxadiazolones ring contraction, 4, 149... 

The synthesis of li7-cyclopropa[/]phenanthrene (142) presented unexpected difficulties and met many failures. Early approaches used a variety of schemes which were not adequate for this highly reactive compound and invariably produced ring-opened products. Thus irradiation of the substituted indazole 138 resulted in nitrogen extrusion and formation of the biradical 139, which reacted with the solvent, benzene, to form 140. The desired cycloproparene 141 was not formed. Ring contraction of 144, in turn, produced derivatives of 9-phenanthroic acid, the formation of which was shown not to involve phenanthrocyclopropenone (143). °° The attempted 1/3/elimination of 145 was similarly unsuccessful and afforded no 142. ... 

The previously unknown benzocyclopropene system was obtained by a similar ring contraction. Photolysis of methyl-3,3-dimethyl-3 -indazole-6-carboxylate (104) in pentane solution gave the moderately stable methyl-1,l-dimethylbenzocyclopropene-3-carboxylate (105).88... 

The unstable fused 1,2,6-oxadiazepine intermediate 299 has been reported as resulting from 1,7-electrocyclisation of the non-stabilised azomethine ylide 298 on to the nitro group subsequent ring contraction afforded the indazole-JV-oxide 300 [01TL5081]. 

The ring contraction of spiro-3//-pyrazoles to cycloproparenes was discovered and developed by Diirr and cowokers in 1969. The derivatives that this methodology has provided are more easily obtained than by other methods and they frequently appear in higher yields . The general process involves the interaction of a diazocyclopentadiene with an alkyne to deliver what was thought to be a spiro-3//-pyrazole, e.g. 29. Upon photolysis [1,5] carbon shifts lead to a 3//-indazole 30 which then ejects nitrogen to give the cycloproparene as discussed above (Scheme 3). In this way one of the few substituted cyclopropa[fl]naphthalenes 31c and the only known spirocycloproparenes 32a-c were obtained. 

In contrast with these ring expansion processes, photoelimination of nitrogen from ethyl o-azidobenzimidate (114) is followed by cyclization with the formation of 3-ethoxy-IH-indazole (115), and irradiation of the 2-azidopyrazines (116) yields the ring-contracted 1-cyanoimidazoles (117), presumably via the nitrenes (118). Products derived by insertion of (p-toluenesulphonyl)nitrene are obtained on irradiation of p-toluenesulphonyl azide in p-xylene or cyclohexane. "... 

Similar ring contractions in which the nitrogen-nitrogen bond is cleaved are found in the reductive ring contractions of pyridazines to pyrroles [148, 149], phthalazines to isoindoles [150], 5,6-diphenyl-1,2,4-triazinones to imidazolones [151], benzo-l,2,4-triazines to benzimidazoles [71], benzo-l,2,3-triazinone to indazolone [la, 152], benzo-l,2,3-triazin-3-oxide to indazole [la], benzo-2,3-diazepines to isoquinolines [153], benzo-l-pyrano-[4,3-e ]-as-triazin-3-one to benzopyranoimidazolone [154], and 2-methyl-4,5-dihydropyridazin-3-ones to pyrrolin-2-ones [155]. 

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