Triazolopyrimidines

Four isomeric structures are possible. All of them have a bridgehead nitrogen atom. A characteristic feature in these triazolopyrimidines is the ease of a Dimroth rearrangement (99AHC) in two systems this results in the 

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Synthesis of this ring may be achieved by the construction of one of the heterocycles followed by using it as a basis to build the other ring onto it or by the Dimroth rearrangement of l,2,4-triazolo[4,3-a]pyrimidines. 1,2-Diaminopyrimidines are general precursors, and they can be generated from 1-amino or 2-aminopyrimidines. The 3- and 5-amino-l,2,4-triazoles are alternative precursors that can act as a source of three carbons to complete the pyrimidine ring. 

Amination of 2-aminopyrimidine (8) with O-mesitylenesulfonylhydrox-ylamine (NH2OMes) gave the iV-aminopyrimidinium salt 9, which can be 

Treatment of 2-(aroylamino)pyrimidines (18), prepared from 15 and aroyl chlorides with PC15 followed by azidolysis in aqueous acetone, gave the tetrazoles 19, whose subsequent pyrolysis afforded 2-aryl-l,2,4- 

The 5-amino-l,2,3-triazoles 8 react with ethyl acetoacetate and ethyl ethoxymethylenemalonate to give 9 and 10, respectively [71JCS(C)2156]. Cyclocondensation of the aminotriazole 8a with isopropyl 2-acetyl-o-fluorocinnamate (11) gave 4,7-dihydro-l,2,3-triazolo[l,5-a]pyrimidine 12 [88JAP(K)63107983]. Condensation of 8b and 8c with acetylacetone in ethanolic piperidine afforded triazolol[l,5-a]pyrimidine derivatives 13a and 13b, respectively. When the reaction with 8b was carried out in glacial ace- 

Scission of the triazole ring in the triazolopyrimidine 13a occurred in tri-fluoroacetic acid or in hot glacial acetic acid alone or in the presence of acetyl chloride, yielding the 2-substituted pyrimidine derivatives 14a-c. However, 13b-d were stable to prolonged treatments with acids [71JCS (C)2156]. 

Compounds 12 caused increases in blood circulation in coronary and kidney vessels and also showed cardiotonic effects [88JAP(K)63107983]. 

The aldehyde from the hydrolysis of 25 proved to be difficult to isolate but could be converted directly to the hydrazone 26, whose oxidation with lead tetraacetate gave the l,2,3-triazolo[l,5-c]pyrimidine 27 (78JHC1041). 

The electron impact on 8-azaxanthine 57 and its five methyl derivatives 58-62 did not show any exclusively common trend of fragmentation. However, the fragmentation process was characterized by the loss of HNCO or MeNCO followed by consecutive elimination of HCN, CO, and N2. Fragmentation of MeNCO was observed only in the case of compounds whose N-l is replaced by a methyl group [80IJC(B)394]. 

Timothy C. Johnson, Richard K. Mann, Paul R. Schmitzer, 

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Tlie chemistry of one of the four possible classes of 1,2,4-triazolopyrim-idiiies, namely l,2,4-triazolo[l,5- ]pyrimidines, was reviewed in 1993 by G. Fischer [93AHC(57)81]. In two previous chapters published in this series we reviewed the chemistry of l,2,4-triazolo[4,3- ]pyrimidines [99AHC(73)131] and their [4,3-c] congeners [99AHC(75)243]. In this chapter we survey the fourth and last class of 1,2,4-triazolopyrimidines. Tlie literature has been inspected to issue number 26, Volume 129(1998) of Chemical Abstracts. 

Tire final chapter in the present volume is the third of a triad of reviews of 1,2,4-triazolopyrimidines, and it covers l,2,4-triazolo[l,5-c]pyrimidines. As with the first and second part of this series (which appeared in Volumes 73 and 75, respectively), it is authored by Professor M. A. E. Shaban and Dr. A. E. A. Morgaan (Alexandria University, Egypt). 

Table 17 [1,2,4]Triazolopyrimidines and benzologues with a fusion site of the triazole ring that have been formed via transformation of the isomeric [1,2,4]triazolo[4,3-c]pyrimidine compounds...

Fusion of a 1,2,4-triazolo ring onto a pyrimidine nucleus to form 1,2,4-triazolopyrimidine systems may take place in four different modes that lead to the following isomeric structures, all of which possess a nitrogen bridgehead (alternative non-IUPAC nomenclatures are also given) ... 

Libermann and Jacquier isolated two of the four possible isomeric 1,2,4-triazolopyrimidines, namely, 5-oxo-7-substituted-l,2,4-triazolo[4,3-a]pyrim-... 

Triazolopyridines and 1,2,4-triazolopyrimidines were synthesized using a modified Mitsunobu reaction starting from acylated 2-hydrazinopyridines and acylated-hydrazinopyrimidines under mild conditions <07ARK132>. A series of 3-alkylthio-4,5-diaryl-4//-1,2,4-triazoles were prepared from benzoic acid hydrazides <07JHC1323>. 

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