Isoxazolo triazines

This ring system is also present in the tricyclic heterocycles isoxazolo-[5, 4 4,5]pyrimido[6, l-c][l, 2,4]triazines. They were prepared (88JIC500)... 

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. 

Neither of these systems has been prepared in the fully conjugated state. The tetrahydro-isoxazolo[4,3-e][l,2,4]triazine (145) is prepared by cyclization of the benzylidene triazinone (144) with hydroxylamine (Equation (18)) <82iJC(B)ii5>. Manganese dioxide oxidation of the isoxazole (146) forms the isoxazolo[4,5-e]triazine derivative (147) (Equation (19)) <86JHC1795>. 

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]

Triazine 4-oxides react with l-ethoxy-7V,7V-dimethylalk-l-enylamines (ketene 0,N-acetals) in two different ways. 1,2,4-Triazine 4-oxides with an unsubstituted 5-position and without a methyl group in the 6-position react with the ketene 0, TV-acetals by 1,3-dipolar cycloaddition to yield isoxazolo[2,3-ci]-l,2,4-triazines as nonisolable intermediates. 5-Methyl-... 

Compound (89), the first representative of the ring system isoxazolo[2,3-a][l,3,5]triazine (13), was obtained by the reaction of (88) with phenoxycarbonylisocyanate (Equation (20)) <87JHC50l >. Other derivatives of this ring system have also been described <90IJC(B)182>. 

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