Pyrazolo quinoxalines

Pyrazolo[3,4-d]pyridazines (555) can be prepared readily from hydrazines and pyrazoles substituted in positions 4 and 5 with an acyl and an ester group, or with two ester groups. 4,5-Pyrazolinediones have been used as starting materials for the synthesis of the quinoxaline derivatives (548) (see above) and of pyrazolo[3,4-e][l,2,4]triazines (556)... 

Pyrazolopyrimidinones C NMR, 5, 308 methylation, 5, 310 synthesis, 5, 329 Pyrazolopyrimidinones, thioxo-synthesis, 5, 324 Pyrazoloquinazolines synthesis, 5, 317, 322 Pyrazolo[ 1,5-n]quinazolines synthesis, 5, 273 Pyrazolo[ 1,5-c]quinazolines synthesis, 5, 324 Pyrazolo[2,3-n]quinazolines synthesis, 3, 378 Pyrazoloquinazolinones synthesis, 5, 342 Pyrazolo[4,3-/]quinazolinones synthesis, 5, 273 Pyrazolo[4,3-g]quinazolinones synthesis, 5, 273 Pyrazolo[3,4-/]quinolines synthesis, 5, 273 Pyrazolo[4,3-/]quinolines synthesis, 5, 273 Pyrazolo[l,5-n]quinoxalines synthesis, 5, 339 Pyrazolo[3,4-6]quinoxalines synthesis, 5, 272... 

Pyrazolo[3,4-/>]quinoxalines can undergo ring fission under reductive or hydrolytic conditions to give different types of quinoxalme. The following examples illustrate these possibilities. 

Chloro-3-(o-chlorophenylhydrazonomethyl)quinoxaline (204, R = H) gave 1-o-chlorophenyl-l//-pyrazolo[3,4- 7]quinoxaline (205, R = H) (diazabicy-cloundecene, Me2NCHO, reflux, 3h 83%) likewise 2-chloro-3-[a-(o-chlorophenylhydrazono-a-methoxycarbonylmethyl]quinoxaline (204, R = CO2-Me) gave methyl l-o-chlorophenyl-l//-pyrazolo[3,4- ]quinoxaline-3-carbox-ylate (205, R = C02Me) (diazabicycloundecene, Me2NCHO-dioxane, reflux, 2h 97%). ... 

Chloro-2-quinoxalinecarbonitrile (208) gave l//-pyrazolo[3,4-/ ]quinoxalin-3-amine (209) (excess neat H2NNH2 H20, reflux, 1 h 90% " H2NNH2 H2O,... 

Note Quinoxalinecarbohydrazides, their derivatives, and quinoxalinecarbonyl azides are especially useful for cyclizations typical examples are given here. 4-Oxo-Af -phenyl-3,4-dihydro-2-quinoxalinecarbohydrazide (127) gave 1-phe-nyl-l//-pyrazolo[3,4-h]quinoxaline-3(2//)-one (128) (TsOH, AcOH, 95°C,... 

Phenyl-l//-pyrazolo[3,4-h quinoxaline (579) gave 3-anilino-2-quinoxalinecar-boxamide (581) [NaBH4, Pr OH, reflux, 80 h 58% the proposed mechanism via the nitrile (580) is unconvincing, but the fact remains] 430 the analogous 3-p-chloroanilino-2-quinoxalinecarboxamide was made similarly in 30% yield.440... 

The ketoxime, 2-[l-(hydroxyimino)ethyl]quinoxaline (216), gave 3-methyl- 1H-pyrazolo[3,4-b]quinoxaline (217) (H2NNH2H20, HC1, H20, EtOH, reflux, 4 h 86%).995... 

In a different type of cyclization, the pyrazolo[3, 4 5,6]pyridazino[3,4-A]quinoxalin-3-one 83 results from the oxidation of the n-hydroxyhydrazide 82 (Scheme 26) <2005JHC551>. 

Spatz JH, Umkehrer M, Kalinski C, Ross G, Burdack C, Kolb J, Bach T (2007) Combinatorial synthesis of 4-oxo-4H-imidazo[l, 5-a]quinoxalines and 4-oxo-4H-pyrazolo[ 1, 5-a]qui-noxalines. Tetrahedron Lett 48 8060-8064... 

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