Pyrazole 4-nitroso

Nitro groups are easily reduced, catalytically or chemically, to give amino compounds, e.g. 4-nitroisothiazoles give the corresponding 4-amino derivatives (72AHC(i4)i). In the pyrazole series, intermediate nitroso compounds can be isolated. Nitrosoimidazoles are also relatively stable. 

From UV studies of 4-phenyl-, 4-nitro- and 4-nitroso-pyrazoles, Habraken et al. (67RTC1249,72JHC939) conclude that the 4-pyrazolyl group acts as an electron-donating group. UV spectra of pairs of 1-aryl- and 2-aryl-indazoles and their utility in the determination of isomeric structures are discussed in (67BSF2619) many other UV data on indazole derivatives can be found in (71PMH(3)67). 

The behaviour of pyrazoles towards nitrosation is similar to their behaviour described above towards diazo coupling, i.e. aminopyrazoles and pyrazolones readily react with nitrosation agents, like alkyl nitrites (81FES1019), to afford stable nitroso derivatives. Some simple nitrosopyrazoles have been isolated, for example the blue-green 3,5-dimethyl-4-nitrosopyrazole, and many others have been proposed as reactive intermediates in the direct conversion of pyrazoles into diazonium or diazo derivatives (Scheme 25) (B-76MI40402). 

In Nitro-pyrazol kann die Nitro-Gruppe durch Zinn(II)-chlorid in verdunnter Kalilauge zur Nitroso-Gruppe ([Nitroso-pyrazol) reduziert werden (s. Bd. X/l, S. 1064). 

Aryl-substituted 6-methyl-4//-pyrazolo[3,4-c]-l,2,5-oxadiazoles 275 are easily obtained in 88-97% yields by the oxidation of 2-aryl-5-methyl-4-nitroso-2//-pyrazol-3-amines 274 with Pb(OAc)4 (Equation 52) <2000S72>. 

Since the early review on sydnones by Stewart (192) and the subsequent coverage by Potts (1), several new applications of these remarkably stable mesoionic heterocycles have been described. In particular, the synthesis of pyrazoles from sydnones has been pursued by several groups. Badachikar et al. (193) prepared several new potential antibacterials (297) from the appropriate sydnones 296, which were synthesized in the standard fashion by the cyclodehydration of the corresponding A-nitroso-A-arylglycine. 

Pyrazole, 3,5-dimethyl-4-nitroso-pK., 5, 225 (B-76MI40402, 72JHC939) Pyrazole, l,3-dimethyl-5-phenyl- H NMR, 5, 186 (B-73NMR165, B-76MI40402) Pyrazole, l,5-dimethyl-3-phenyl- H NMR, 5, 186 (B-73NMR165, B-76MI40402 Pyrazole, 3,5-dimethyl-1-phenyl-,5N NMR, 5, 196 <80OMR(13)97)... 

The tautomerism of other pyrazoles has also been studied. The examination of the tautomerism, protonation, and E/Z isomerism of a 4-nitroso-5-aminopyrazole and its salts shows that this compound exists as a mixture of two amino/nitroso isomers 50a and 50b the imino/oxime tautomer 50c was not present [66], The salt is a mixture of isomers 51a and 51b resulting from the protonation on the oxygen of the nitroso tautomers. The functional tautomerism of l,5,6,7-tetrahydro-4//-inda-zol-4-ones (a pyrazole derivative) has been studied theoretically and experimentally [67], In the case of imidazoles 2-substituted by Se, there is experimental evidence that the selenone is more stable than the selenol tautomer [68],... 

The acid-base properties of 4-nitroso-5-aminopyrazoles 50 have been studied. In particular, protonation sites have been determined where salts 51 are formed [66], The effects produced on II, 13C, and 15N chemical shifts by protonation and by hydrogen-bonding solvents on five azoles (imidazole, 4,5-dimethylimidazole, pyra-zole, 3,5-dimethyl-pyrazole, and 4,5-dihydro-3-methyl-2//-benz[g]indazole) have been determined experimentally. Phase effects on the 13C chemical shifts of the C-4 atom of pyrazole were discussed, based both on empirical models and on GIAO calculations of absolute shieldings in different complexes. The special case of the chemical shifts of pyrazoles in the solid state, where they form multiple N-H N hydrogen bonds, was also studied theoretically [88],... 

The second equation for 15N includes nitroso derivatives (very far removed from other 15N chemical shifts). The usefulness of 15N chemical shifts (combined with GIAO calculations and some X-ray structure determinations) was demonstrated in the case of C-bromo pyrazoles 152-158 [151],... 

A-Methyl-/V-nitroso-a-(trimethylsilyl)benzylamine reacts thermally with DMAD to give 3,4-dicarbomethoxy-l-methyl-lH-pyrazole. The reaction is highly sensitive to the temperature upon increasing the temperature from 25 to 110 °C, the reaction time drops from 168 h to 3 min and yields increase from 30 to 100%. No reaction occurs from non-silylated /V-methyl-/V-nitrosobenzylamine. Other acetylenic esters have been successfully tested. In contrast, phenylacetylene gives a poor yield of the adduct and diphenylace-tylene does not react.473... 

Direct nitrosation of pyrazoles cannot be accomplished to obtain 4-nitrosopyrazoles, j9-diketones are nitrosated and then treated with hydrazine or a hydrazine derivative.594 Nitrosation of 3- and 5-hydroxypyrazoles (pyrazolinones) takes place readily in the 4-position,595 but 4-hydroxypyrazoles yield 5-nitroso derivatives.598 The following compounds are also nitrosated at the 4-position 1,3-dimethyl-5-methylthiopyrazole,542 3-methyl-5-ethoxypyrazole,597... 

The activation of the benzyl CH2 by the flanking sulfamoyl group in the pyrazole (180) induces cyclization on to the nitroso group in the presence of base to give the 2,2-dioxopyrazolo[3,4-c]-[l,2,6]triazines (181) (60-70%) (Equation (26)) <89JHC797>. 

Azidouracils. photolysis, 55, 197, 200 Azimines, pyrazole-derived, formation from azomethine imines and nitroso compounds, 60, 34 Azinium azolates, definitions, 60, 200 Aziridine... 

H-Purin 2,6-Bis-[l-methyl-hydrazino]- E16a, 742 (Cl - NR-NH2) lH-Pyrazol l,3-Dimethyl-5-(3-guanidino-ureido)-4-nitroso- -(Hydro-4-methyl-benzolsulfo-nat) E8b, 562 l,3-(CH3)2-6-oxo — 1,6-H2 — <(pyrazolo[4,3-c]-1,2,5-oxadiazin) + R2NH ... 

Propan l-(2-Methoxy-5-methyl-phenyl)-2-nitro-l-nitroso-(dimer) X/l, 74 Propansaure 2-Amino-3-(4-hydroxy-phenyl)- -(carboxy-methylamid) XV/2, 195, 617 3H-Pyrazol 3-Cyclopropyl-4,5-dimethoxycarbonyl-3-methyl-E14, 994 (R2C = N2 4- In) Pyrimidin 2,6-Dimethoxy-5-(2-methoxycarbonyl-ethenyl)-4-methyl- E9b/2, 224f. 

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