Pyrazole l-oxide

Oxidation of N -substituted pyrazoles to 2-substituted pyrazole-l-oxides using various peracids (30) facilitates the introduction of halogen at C, followed by selective nitration at C. The halogen atom at or is easily removed by sodium sulfite and acts as a protecting group. Formaldehyde was... 

Hydroxypyrazole 180 was selectively N-alkylated to the corresponding 2-alkyl-pyrazole-l-oxides 181, which could be subsequently deoxygenated and C-halogenated forming 1-alkyl-5-halopyrazoles 182 (Scheme 7) <2001S1053>. 

Sequential functionalization of pyrazole-l-oxides via regioselective metallation led to the synthesis of 3,4,5-trisub-stituted-l-hydroxypyrazoles <2002JOC3904>. 3-Acylated-2-(4-methoxybenzyl)-2//-pyrazole 1-oxides were formed by the reaction between a 3-magnesium 2//-pyrazole-l-oxide and acid chlorides <2002J(P1)428>. 3-Arylated-l-hydroxypyrazoles were synthesized from 3-metallated-pyrazole 1-oxides <2001JOC8654>. The reaction between hexafluorobenzene and the anion of 1-hydroxypyrazole affords a mixture of the products of bis-, tetrakis-, and hexakis-substitution <2004ARK100>. In the case of hexakis(bromomethyl)benzene, its reaction with 1-hydroxy-pyrazole leads to the hexakis-substituted product. 

On irradiation of 4,4-diethyl-3,5-dimethyl-4H-pyrazole-l-oxide (5) at -78°C (15), the valence-tautomeric heterobicyclopentene 6 was detected as the primary photoproduct. At temperatures above -20°C, 6 undergoes thermal isomerization to 5 and 8. Product 8 is obviously formed by walk rearrangement 6 - 7 followed by a fast electrocyclic ring opening 7 - 8. 

Quaternierung und N-Oxid-Bildung fiihrt zu einer weiteren Desaktivierung der 4-Position, so daB die Nitrierung erschwert ist. In gleicher Weise wirken Elektronenacceptor-Substituenten in 1-Position. So erfolgt die Nitrierung von 2-Benzyl-lH-pyrazol-l-oxid nur in der 3-Position zu 2-Benzyl-3-nitro-lH-pyrazol-l-oxid ( 100% Schmp. 106 107°)2478. 

Diphenyl-4-oxo- 706 4H-Pyrazol-l-oxid 409 5-Mercapto-3-subst. 339 3-Methyl-5-methylthio-1,2,4-triphenyl- -iodid 340... 

Pyrazole, 1 -(2-methylaminophenyl)-synthesis, 5, 283 Pyrazole, l-methyl-5-nitro-2-oxide... 

In the other extreme, ethyl 3-oxohexanoate 75 required fusion with 3-pyridyl-hydrazide 73a or 3-hydrazinocarbonyl-pyridine-l-oxide 73b at 120°C in order to give the corresponding pyrazol-3-ones 76a,b (92AF1350) (Scheme 22). Depending on the reaction conditions, the reaction of ethyl 3-oxo-3-phenylpropanoate 77 and cyanomethylhydrazide 78 gave different products. Thus, fusion at 140°C gave pyrazol-3-one 79 and N,A -bis(cyanoacetyl)hydrazine 80, while heating in ethanol... 

Single crystal X-ray analysis definitively assigned different structures to products of oxidative cyclization of 58 as pyrazolo 1,2-/z][ l,2]diazepin-5(3//)-onc 59 <1984ZNB187> and pyrazolo[l,2-tf]pyrazol-l(5//)-onc 60 <1997ZNB994>, depending on the character of the R2 substituent. 

Electroactive 3-(N-phenylpyrazolyl)fullereno[l,2-r/]isoxazolines have been synthesized by using 1,3-dipolar cycloaddition of pyrazole nitrile oxides, generated in situ, to Cgo at elevated temperature or microwave irradiation. The cyclic voltammetry measurements show a strong donor pyrazole ring, and a better acceptor ability of the fullerene moiety than the parent C60 (538). Treating fullerene Cgo with mesitonitrile oxide in toluene gives fullerene-nitrile oxide adduct, which is supposed to be useful for electrical and optical components (539). 

Oxidation of chalcone phenylhydrazone 13 leads to a pyrazole and the expelled proton catalyses formation of a pyrazoline from the chalcone phenyl-hydrazone [43]. The latter undergoes further anodic oxidation (p. 308). In the presence of pyridine as a proton acceptor, the pyrazole becomes the major product. A further example of oxidative cyclization is the conversion of a-oximino phenylhydrazones to 1,2,3-triazole-l-oxides 14 [44]. 

Similar synthesis of l-hydroxypyrazole-2-oxides was realized in the presence of Co(II) ions or HN02 as nitrosating agent. Pyrazole-Af-oxides were successfully obtained from 1,3-dioximes and SOCI2 . 

A general method of potential synthetic usefulness is the preparation of aromatic azapentalenes by oxidation of partially saturated systems. This has been used with success in a few cases (e.g., Sections III,A,3,d,108,109 III,B,3,c,293-295 III,A,284), but it has not been extensively explored. Recently, several workers have reported some possibly significant failures while attempting the dehydrogenation of nonaromatic systems. Various 2,3-dihydroimidazo[l,2-h]pyrazoles resisted oxidation to the corresponding aromatic system with manganese dioxide, chloranil, or dicyanodichloroquinone (DDQ),355 and a... 

Pyrazolo[3,4-c]pyrazole, tetrahydro-rearrangement, 5, 250 PyrazoIo[4,3-c]pyrazole, tetraaryl-electrophilic substitution, 6, 1035 oxidation, 6, 1034-1035 reduction, 6, 1035 vacuum pyrolysis, 6, 1035 Pyrazolo[ 1,2-a]pyrazole-l, 5-diones synthesis, 6, 991 Pyrazolo[ 1,2-a]pyrazoles reactions, 6, 1038 ring opening, 6, 983... 

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