2- Substituted pyrazole 1-oxides reactions

A similar use of maleimide was seen in Yu s hetero-Diels-Alder reactions. The Ar-ethyl pyrazole reacted under aqueous conditions at room temperature, with air oxidation to furnish the fully aromatized product <2001TL8931>. Other 1,3-substituted pyrazoles required heating in acetic acid at 50°C for 24h, with product yields of 42-67%. 

Reaction of 3-bromo-l,5-di-(4-tolyl)-l,5-diazapenta-l,3-diene 99 with hydroxyamine affords 2-substituted pyrazole 1-oxides 100 (1995JCS(P1)2773) (Scheme 29). 

The regioselectivity by electrophilic aromatic substitution is conserved when switching to acidic reaction conditions. Thus 2-substituted pyrazole 1-oxide 123 was nitrated regio and monoselectively at C3 by HN03-H2S04 to give 124 in quantitative yield (1992ACSA972). Further nitration takes place first at the benzyl 4-position, and then at the pyrazole 5-position (Scheme 39). 

Except for the brief statement that 2-methyl-3-nitropyrazole 1-oxide reacts with acetyl chloride to give l-methyl-5-chloro-4-nitropyrazole apparently no reports on the reaction between 2-substituted pyrazole 1-oxides 86 and acetylchloride or acetic anhydride exist (1977JCS(P1)672). 

In addition to the aromatic 2-substituted pyrazole 1-oxides 74, a series of nonaromatic diazapentalene 4,4 -dioxides 75-78 and diazapental-ene 4-oxides 79-84 (Scheme 22) have been described and reviewed (1989H1615). The properties and chemistry of these compounds are related to those of the 2-substituted pyrazole 1-oxides 74. Reactions that connect the aromatic and nonaromatic pyrazole derived N-oxides have been described in Section 2.I.9.I. 

The orientation of the substituent gtoups in 1,2,4-oxadiazole substituted pyrazoles 39, formed by reaction of benzonittile oxides with an unsymmetrically substituted hydrazine, has been determined by C NMR assignments <1998JHC161>. The scope and limitations in the regioselective synthesis of 1,3,5-trisubstituted pyrazoles from / -amino enones and hydrazine derivatives were investigated by C chemical-shift prediction mles for 1,3,5-trisubstituted pyrazoles <2001H(55)331>. 

Intramolecular palladium-catalyzed cyclization reactions have also been used to synthesize pyrazole derivatives. iV-Aryl-iV-(o-bromobenzyl)hydrazines 494 participated in a palladium-catalyzed intramolecular amination reaction to give 2-aryl-2//-indazoles 495 (Equation 101) <20000L519>. Palladium-catalyzed intramolecular C-N bond formation of iV-acetamino-2-(2-bromo)arylindolines 496, followed by hydrolysis and air oxidation in the presence of aluminium oxide, allowed the preparation of indolo[l,2-3]indazoles 498 via intermediate 497 (Scheme 58) <2002TL3577>. 3-Substituted pyrazoles have been prepared from the intramolecular cyclization of A -tosyl-iV-(l-aryl/ vinyl-1-propyn-3-yl)hydrazine and then exposme of the reaction mixture of the cyclization to potassium /i //-butoxide <1997SL959>. iV-Aryl-iV -(o-bromobenzyl)hydrazines 499 or [A -aryl-A -(t>-bromobenzyl)hydrazinato-A ]-triphenyl-phosphonium bromides 501 participated in a palladium-catalyzed intramolecular amination reaction to give 1-phenyl-l//-indazoles 500 (Scheme 59) <2001TL2937>. 

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

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