Isoxazolo triazin-4-ones

The preparation of isoxazolo[5,4- [l,2,3]triazine-4-ones is usually achieved via diazotization <1988JP11509, 2003MI147>. For example, 5-amino-3-methylisoxazole-4-ethanoate formed the diazonium salt that was not isolated, and underwent spontaneous cyclizations upon heating to give the isoxazolo[5,4-, [l,2,3]triazine-4-one in modest yield (Scheme 51) <1997FA105>. 

Isoxazolo[5,4-, [l,2,4]triazine-4-ones are prepared by six-membered cyclizations of diazonium salts onto adjacent ester or amide substituents on the isoxazole ring (Section 10.13.9.1.2(i)). [l,2,5]Oxadiazolo[3,4- ]pyrazines (Section 10.13.9.1.2(ii)) and [l,2,5]oxadiazolo[3,4- pyridazines (Section 10.13.9.1.2(iii)) are traditionally accessed by reliable six-membered condensation reactions to give the fused pyrazine and pyridazine rings, respectively. 

One synthesis of isoxazolo[5,4-Z>]pyrimidines from 4-unsubtituted 5-AI has been reported (Section III.C.2). As discussed above for isoxazolo[3,4-Z>]pyridines, the reaction of 3-amino-5-methylisoxazole with carbamate 44 does not produce isoxazolo [3,4-7>]pyrimidines, isoxazolo[2,3-a] 1.3.5-triazines 45 being formed as the only products. Meanwhile, the condensation of 5-amino-3-methylisoxazole with the same carbamate yielded isoxazolo[5,4-Z>]pyrimidine 46 (04KGS496) (Scheme 20). 

The possibility of acetylene formation (Section III.B) may be responsible for the unusually low yields (4-9% after the chromatographic separation) obtained in the synthesis of isoxazolo[5.4-[Pg.187]

---

See also in sourсe #XX -- [ ]

---