Indoles azirines, aryl

The thermal chemistry of a number of aryl-substituted azirines often results in the formation of indole derivatives (68TL3499). Thus, heating a sample of azirine (152) gave 3-phenylindole (155) and dihydropyrazine (156). The formation of (155) was suggested to... 

Taber and Tian have employed the Neber protocol to prepare a-aryl azirines that underwent thermal rearrangement to afford substituted indoles via a unique Ie type ring closure <06JACS1058>. A variety of substituted N-alkyl and N-aryl indoles have been prepared by Zhao and co-workers who observed Ie cyclization of 2-aryl-3-arylamino-2-... 

Isomerization. 3-Aryl-3 t-azirines undergo isomerization to furnish indoles by heating with Rh2(OCOCF3)4 in 1,2-dichloroethane. ... 

After copper and palladium, rhodium is the third most important transition metal for the synthesis of the indole ring. For a 2007 review on this reaction, see Patil and Paiil [1], Some early examples (Scheme 1) are Alper s rhodium reaction of 2-aryl-2/7-azirines to give 2-styiylindoles (equation 1) [2], Watanabe s Rh-catalyzed Fischer indole synthesis (equation 2) [3], Ucciani s 3-methylindole synthesis via the hydroformylation of o-nitrostyrene (equation 3) [4], and Burst s preparation of 3-acetyl-2-hydrox-yindoles from the Rh-catalyzed decomposition and carbenoid aromatic C-H bond insertion (equation 4) [5]. Narasaka extended Alper s 2-aryl-2//-azirine reaction to a Rh(II)-catalyzed synthesis of 2,3-disubstituted indoles [6], and both Cenini [7] and Alper [8] stretched the deoxygenation of o-nitrostyrenes to give indoles. Burst s Rh-catalyzed decomposition of a-diazo carbonyl compounds was used by Bauban [9] and Jha [10] in the synthesis of substituted oxindoles. 

The thermal chemistry of a number of related aryl-substituted 2/f-azirines were also studied. The 2-naphthyl-substituted azirines 299 were found to rearrange to benz[g]indole 300. The selectivity encountered here was attributed to the higher energy content of the intermediate leading to the alternative benz[/]indole system 302. Heteroaromatic systems were also found to undergo thermal rearrangement via highly selective reaction paths ... 

A study of the preparation of ocimene by isomerization of a-pinene has been reported. a-Azidoacetophenones have been converted to 2-aryl-l,3-oxazole-4-carbaldehydes by rearrangement of the carbon framework on exposure to DMF/POCI3. The rearrangement occurs via alkenyl azides and 2//-azirines. Enamine rearrangement of pyridinium salts to indoles has been studied.Kinetic stabilization and reactivity of a series of singlet 2,2-dialkoxy-l,3-diphenyloctahydropentalene-l,3-diyls enable cis-selective formation of the ring-closed compounds (Scheme 105). ... 

Reaction of 2-aryl-azirines (79) with either Co2(CO)8 or [Rha(CO)2]2 gives the styryl indoles (80) in up to 90% yield. These products could be useful intermediates in alkaloid synthesis. 

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