Pyrazolo -1,2,4-triazine 6-oxides

Imidazo[4,5-c]pyrazole 4-oxides (762 R = p-N02CeH4) are obtained by base-catalysed cyclisation of the nitro-pyrazoles (761).Cyclisation of the pyrazolylhydrazone (763) gives either the pyrazolo-triazole (764) or the pyrazolo-triazine (765), depending on conditions. ... 

Reactions of 3-hydrazino-l,2,4-triazine 1-oxide 31 or 3-hydrazinopyrido [2,3-c]-l,2,4-triazine 1-oxide 32 with diethoxymethyl acetate or triethyl orthoformate proceed as cyclization reactions at the N(4) atom and the amino group to form the corresponding pyrazolo[3,4-c]-l,2,4-triazine 6-oxides 33 and 34 (74MI, 80JOC5421, 80MI). 

Phenyl-l,2,4-triazine 4-oxide 91 reacts with l,3-dimethyluracil-6-hydrazones 92 in DMF in the presence of triethylamine to give pyrazolo[3,4-. 

Costanzo etal. <1994JHC1369> published the synthesis of a series of pyrazolo[5, l-c]benzo[l,2,4]triazine-5-oxides 48 and compared the 13C NMR chemical shifts of these products. These values for a set of four derivatives are shown in Table 1. 

Table 1 13C NMR chemical shifts (300 MHz) of some pyrazolo[5,1 -c]benzo[1,2,4]triazine-5-oxides 48 measured...

The JV-amino derivatives 269 underwent oxidative ring expansion to tri-azine 270. Analogous pyrazolo[3,4-c]pyridines, however, gave only intractable products. The latter system lacked the peri interaction necessary for repulsion of nucleophilic attack at the 4-position of the triazine ring.157... 

The presence or absence of the dioxolane protecting group in dienes dictates whether they participate in normal or inverse-electron-demand Diels-Alder reactions.257 The intramolecular inverse-electron-demand Diels-Alder cycloaddition of 1,2,4-triazines tethered with imidazoles produce tetrahydro-l,5-naphthyridines following the loss of N2 and CH3CN.258 The inverse-electron-demand Diels-Alder reaction of 4,6-dinitrobenzofuroxan (137) with ethyl vinyl ether yields two diastereoisomeric dihydrooxazine /V-oxide adducts (138) and (139) together with a bis(dihydrooxazine A -oxide) product (140) in die presence of excess ethyl vinyl ether (Scheme 52).259 The inverse-electron-demand Diels-Alder reaction of 2,4,6-tris(ethoxycarbonyl)-l,3,5-triazine with 5-aminopyrazoles provides a one-step synthesis of pyrazolo[3,4-djpyrimidines.260 The intermolecular inverse-electron-demand Diels-Alder reactions of trialkyl l,2,4-triazine-4,5,6-tricarboxylates with protected 2-aminoimidazole produced li/-imidazo[4,5-c]pyridines and die rearranged 3//-pyrido[3,2-[Pg.460]

A few papers report on use of the N1-N2 bond formation for closure of the 1,2,4-triazine ring. For instance, pyrazolo[5,l-c]-l,2,4-triazine 5-oxides have been obtained through the intramolecular cyclization of l-(2 -nitroaryl)-5-aminopyrazoles under alkaline conditions (Scheme 181) <1996EJM259, 1997S556, 1999JME2218,... 

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