4- -1,2-dihydropyrazol-3-ones

From unsubstituted or 4,4-dimethylpyrazolidin-3-ones. Pyrazoli-din-3-ones 335a,b were oxidized to 4,5-dihydropyrazol-3-ones 336a,b with lead(IV) acetate at low temperature 336a isomerizes readily to... 

Dihydroazoles can exist in at least three forms (cf. Section 4.01.1.3), which in the absence of substituents are tautomeric with each other. The forms in which there is no hydrogen on at least one ring nitrogen normally predominate because imines are generally more stable than vinylamines in aliphatic chemistry. Thus for dihydropyrazoles the stability order is A" (hydrazone) (288) > A (azo) (289) >A (enehydrazine) (290). 

There are four possible pyrazol-3-(Mie structures I-IV. The nomenclature most frequently used in the literature has been taken from Chemical Abstracts, wlwre for example, structure I is named 2,3-dihydropyrazol-3(lW)-(Mie and structure II is named 4,5-dihydropyrazol-5(l//)-one. Compounds I and II have also been named as pyrazolinones. Compounds with structure IV have been referred to as 2,3-diaza-2,4-cyclopentadienones. To avoid this needless confusion, the nomenclature used throughout this review is in accord with the recommendations set forth by lUPAC. 

Hydrazones that are formed by heating the y3-keto ester and the hydrazine in an alcohol usually require more vigorous conditions in order to cyclize to pyrazol-3-ones. Thus, hydrazone 52, obtained by heating oxobutanoate 51 and phenylhy-drazine in ethanol, required heating under reflux in benzene ccMitaining phosphorus pentoxide in order to cyclize into l,2-dihydropyrazol-3-one 53 (66JOU1103) (Scheme 16). 

On the other hand, hydrazone 54 cyclized by heating in an ethanolic solution of aqueous sodium hydroxide to give 1,2-dihydropyrazol-3-one 55 in 92% yield (88JHC543) (Scheme 17). 

Methyl-5-oxo-l-phenyl-4,5-dihydropyrazol-4-ylidene)-2,3-dihydro-l//-azepine (3), formed in 62% yield by the condensation of 2-amino- or 2-butoxy-37/-azepine with 3-methyl-l-phenylpyrazol-5(4//)-one, with Meerwein s reagent yields the tetrafluoroborate salt 4 which, on treatment with sodium dihydrogen phosphate, liberates the free base 5.64... 

The (diazoacetyl)dihydropyrazole 4a is converted into the 1 //-l, 2-diazepin-6(7//)-one 5a in hot acetic acid 73 a somewhat similar reaction affords the diphenyl analog 5b.74... 

The standard route to sydnones is the cyclodehydration of iV-nitroso-P-amino acids. 2-Chloro-4,5-dihydro-13-dimethylimidazolium chloride 97 has been shown to be an excellent reagent for this conversion, as, for example, in the synthesis of 3-phenylsydnone 98 <99JOC6989>. Functional group manipulations of fused sydnones have led to the isolation of several new compounds including 99 and 100 <99H(51)1433>. 4-Acetylsydnones react with hydrazine at room temperature to give 2,4-dihydropyrazol-3-ones 101 <99H(51)95>. ... 

Diphenyl-1,2,4-oxadiazolin-5-one undergoes a similar cleavage with evolution of carbon dioxide on photolysis in dioxane [Eq. (18)].66 The reactive intermediate formed on photolysis of 2,4-diphenyl-l,3,4-oxadiazolin-5-one forms dihydropyrazole, pyrazole, and 1,2,4-triazole derivatives by cycloaddition to conjugated alkenes, acetylenes, and nitriles [Eq. (19)], respectively.66... 

The silver acetate-promoted 1,3-dipolar cycloaddition of nitrilimines with 3(/f )-pheny]-4(A )-cinnamoyl-2-azetidinone produced the major adduct, 4-(4,5-dihydro- (g) pyrazol-5-yl)carbonyl-2-azetidinones, with high stereoselectivity.70 The 1,3-dipolar cycloadditions of substituted 2,7-dime(liyl-3-thioxo-3,4,5,6-ici.rahydro-2//- 1,2,41 triazepin-5-one with iV-aryl-C-ethoxycarbonylnitrilimines are highly chemoselective, where the sulfur atom of the dipolarophile interacts with the carbon atom of the dipole.71 The enantioselective 1,3-dipolar cycloaddition of nitrile imines with electron deficient acceptors produces dihydropyrazoles in the presence of 10 mol% of chiral Lewis acid catalyst.72... 

Reduction of one or two double bonds in the azole ring gives nonaromatic systems. Historically, these derivatives have been described as azolines and azolidines, as illustrated by the pyrazole derivatives 25 and 26. In 3-pyrazoline 25 the 3 indicates the first ring atom associated with the double bond . In the older literature the position of the double bond is indicated by An, where n is the number of the first ring atom associated with the double bond, e.g., A3-pyrazoline 25. Alternatively, they are named as dihydro or tetrahydro derivatives of the parent azole (e.g., 2,5-dihydropyrazole 25) and this nomenclature is IUPAC-preferred nomenclature for rings with more than two heteroatoms . 

Copper(II) acetate in refluxing methanol has been used to prepare 3-alkyl-5-methyl-6-phenyl-3,4-dihydro-l,2,3-triazin-4-ones 568 from 2-(alkylamino)-4-methyl-5-phenyl-l,2-dihydropyrazol-3-ones 567 (Scheme 256). Alternatively, this conversion may be achieved, albeit in a slow reaction, using airoxygen in the presence of sodium hydro-gencarbonate <2006EJ03021>. 

Attansai et al have reported the first general protocol for the preparation of polymer-bound 1,2-diaza-1,3-butadienes (93) and demonstrated their utility in the solid state synthesis of 4-triphenylphosphoranylidene-4,5-dihydropyrazol-5-ones (94) (Scheme 17). ... 

Addition of diazomethane to 2-substituted 3-alkylsulfanyl-17/-inden-l-ones 8 afforded dihydropyrazoles 9 which rearranged to mixtures of 2-substituted 4-alkylsulfanyl-l-naphthols 11 and 3-alkylsulfanylmethyl-l//-inden-l-ones 12 on pyrolysis. 

In a second German patent (70GEP1935272) (Scheme 28) the preparation of 5-methylsulfonyl-aminopyrazol-3-one 99 took place simply by reacting 5-amino-l,2-dihydropyrazol-3-one 98 with methylsulfonyl chloride at room temperature. 

Diazopyrazol-3-one salts have been prepared in situ and kept for a short period at low temperature by either diazotization of 4-aminopyrazol-3-ones or by diazo group transfer onto 4-unsubstituted 2,4-dihydropyrazol-3-ones from 4-toluenesulfo-nylazide or aziridinium salts. Coupling these diazonium salts with carbon nucleophiles is the most common reaction (04AHC(87)141). 

The condensation of 4-acyl-l,2-dihydropyrazol-3-ones 145a c with ethylenedia-... 

Catalytic hydrogenation of l,2-dihydropyrazol-3-one 547 with 10% palladium on carbon in ethanol selectively reduced the a,jS-unsaturated bond of the... 

Substituents on the pyrazol-3-one ring have taken part in both Diels-Alder and 1,3-dipolar cycloaddition reactions. Stabilized dihydropyrazol-3-one enolate 569 generated by deprotonation of pyrazol-3-one 113 with LDA in tetrahydrofuran has undergone facile cycloaddition with l,l-Z z5methylsulfanyl-2-nitroethene and yielded directly the indazol-3-one 571 (99JCS(Pl)3001) (Scheme 160). It is proposed that the... 

---