1.2.3- Triazole ring enamines

The triazole ring can be synthesized by reaction between a suitable pyridine (or piperideine) and an azide. Thus 5-nitro-2-pyridone (or N-substituted pyridones) and sodium azide give a 5-oxotriazolo[4,5-b]pyridine (94)182-184 whereas the enamine 95 reacts with p-nitrophenyl azide to give the hexahydro-l//-triazolo[4,5-c]pyridine 96.185,186... 

Potassium/alcohol 1,2,3-Triazole ring from enamines Diazo group transfer... 

Scheme 33 illustrates the difference in reactivity between triazolines obtained from cyclohexanone and cyclo-pentanone enamines. Thus, the reactions of azidophosphonates 239 with cyclohexanone enamines produce unstable aminotriazolines 240 that cannot be isolated due to their spontaneous elimination of amines to provide triazoles 241. Contrary to that, triazolines 242, derived from cyclopentanone enamines, are isolated in good yield (76-88%) and cannot be converted to the corresponding triazoles even by thermolysis <1995H(40)543>. Probably, introduction of a double bond between two five-membered rings would involve too much molecular strain. 

Junjappa and co-workers (9) reported the cycloaddition of sodium azide to the polarized ketene-(5,5)-acetal 33 to give the tiiazole 35 they also reported an intermolecular cycloaddition of tosyl azide 37 with the enamine 36 to give an unstable triazoline intermediate 38. Ring opening 38 followed by a Dimroth rearrangement afforded the triazole 41 (Scheme 9.9). 

N-Methylindole acts like an enamine with sulfonyl azides and gives a diazoimino compound.443-445 So does dihydropyridine but only in the presence of 5% lithium hydroxide (Scheme 147).242 A triazole is postulated to be formed from the triazoline followed by a ring opening oxidation of diazoamino compounds to triazoles has been known to proceed via a diazoimino compound in equilibrium with the triazole (Scheme 1 19).285-3"-400... 

Cyclization of an enamine function containing the sequence N—N=C—C=C —NMe2 to a ring-carbon of a 1,2,4-triazole offers an efficient route to a tri-azolopyridazine. 

Dipolar cycloadditions can, of course, only produce five-membered rings. Addition of dipolarophiles can generate tetrahydro, dihydro or aromatic oxidation level heterocycles, as illustrated above. Alkene dipolarophiles, with a group that can be eliminated following cycloaddition, give the same result as equivalent alkyne dipolarophiles, for example enamines as the dipolarophile, interact with azides, as the 1,3-dipole, with subsequent elimination of the amine, affording 1,2,3-triazoles. ... 

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