1.3.5- Triazines, pyrazolo-fused

The pyrazolo-fused pyrimido[4,5-< ]-l,2,4-triazine 35 was synthesized using the 5-amino-6-cyano triazine 131 (Scheme 21) as the precursor. Reaction with phosgeniminium chloride gave the amide halide intermediate 132 which in turn produced the pyrimido[4,5-< ]-l,2,4-triazine 35 upon treatment with gaseous hydrogen chloride <1996T3037>. 

In Scheme 7, ring-opening reactions of some fused pyrazoles are shown. Thus, Baraldi et al. <1999S453> described that the pyrazolo[l,5-<7][l,2,4]triazine-dione derivative 72 is sensitive toward nucleophiles its reaction with benzylamine at room temperature gives rise to the acyl semicarbazide 73 in high yield (78%). 

Pyrazolo[4,3- ][l,3,4]oxadiazine 78, whose preparation is given in Section 10.13.9.1.2(iv), was condensed with various amines to give pyrazolo[3,4- ][l,2,4]triazines 101 (Scheme 78 Table 9) <2002IJB664>. This ring system was earlier reported by synthesis of the fused six-membered ring, through condensation onto pyrazolediones <1996CHEC-II(7)489>. 

The removal of the iV-etheno group from the fused imidazole ring of tricycle 102 was achieved using rV-bromosuccin-imide (NBS) in 1 1 DMF-water to yield the 2-azanucleoside (pyrazolo[3,4-4][l,2,3]triazine) 103 (Scheme 79) <2004JOC4695, 20050BC1714>. An analogous tricycle transformation to imidazo[4,5-r7 [l,2,3]triazines has been reported earlier <1996CHEC-II(7)489>. 

When the synthesis of a heterocyclic system containing a diazole fused onto l,2,3-triazin-4-one (e.g., imidazo[4,5-i [l,2,3]tnazin-4-one (Section 10.13.9.1.l(i)) and pyrazolo[3,4-rf [l,2,3]triazin-4-one (Section 10.13.9.1.1(ii))) or 1,2,3-triazole fused onto pyrimidin-4(7)-(di)one (e.g., [l,2,3]triazolo[4,5-rf pyrimidin-4(7)-(di)one (Section 10.13.9.1.l(iv))) is required, the synthesis of the six-membered ring is the most reliable method. There are several recent accounts of six-membered annulations onto 1,2,3-triazoles to give [l,2,3]triazolo[4,5-r/ pyrimidines (Section 10.13.9.1.l(iv)), which are valuable alternatives to 1,2,3-triazole annulation using nitrosation techniques <1996CHEC-II(7)489>. 

The removal of the W-etheno group from the fused imidazole ring of tricycles, diimidazo[l,2-rt4, 5 -r ][l,2,3]tria-zines and imidazo[l,2-f]pyrazolo[4,3-i ][l,2,3]triazines, is a reliable method for making imidazo[4,5-rf [l,2,3]triazines <1996CHEC-II(7)489> and pyrazolo[3,4-r/ [l,2,3]triazines, respectively (Section 10.13.10.1.2). 

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