1.2- Dioxo compound pyrazoles

The isoxazoles (585) were formed regioselectively from the (dioxoalkyl)phosphonium salts (584) with hydroxylamine hydrochloride, the direction of cyclization being different from that of the nonphosphorus-containing 1,3-dioxo compound (see Chapter 4.16). Aqueous sodium hydroxide converted (585) into the isoxazole (586) and triphenylphosphine oxide. Treatment of (585) with n-butyllithium and an aldehyde gave the alkene (587). With hydrazine or phenylhydrazine analogous pyrazoles were formed (80CB2852). 

Methyl 3-acyl-l-diphenylmethyleneamino-4,5-dioxo-4,5-dihydro-l//-pyrrole-2-carboxylates 489 are formed from 488 and oxalyl chloride in good yields. Preparative thermolysis of these compounds at 130-140°C gives mixtures of dipyrazolo[l,2- l,2- [l,2,4,5]tetrazines 491 as major products and pyrazoles 492 as minor hydrolytic by-products. The intermediacy of mesoionic compound 490 is expected (Scheme 83) <2004T5319>. 

The 2,3-dioxo-6-thioxo-2,3,5,6-tetrahydro-l/7-imidazo[l,2-A]pyrazole 393 and 5,6-dioxo-2,3-dihydro-l//-imidazo[l,2- ]-imidazole 395 were synthesized by condensation of the respective 5-amino-3-thioxo-2,3-dihydro-pyrazole 392 and 2-aminoimidazoline 394 compounds with either oxalyl dichloride or diethyl oxalate in moderate to poor yields (Equations 178 and 179) <1995JPR472, 2002EJM845>. These cyclizations can suffer from various side reactions such as expulsion of CO, polymerization, or formation of open-chain products. To solve these problems, reagents such as oxalic acid bis-imidoyl- and bis-hydrazoylchlorides 397 and 400 as well as 2,3-dichloroquinoxalines 403... 

A modified procedure for the preparation of pyrazoles includes the solventless condensation of diketones and hydrazines in the presence of a catalytic amount of sulfuric acid at ambient temperature <2004GC90>. 1,3-Diketones can be synthesized directly from ketones and acid chlorides and then converted in situ into pyrazoles by the addition of hydrazine <2006OL2675>. The possible modifications to the 1,3-dioxo component in the reaction with hydrazines include the use of -ketoesters, -ketonitriles, -halocarbonyl compounds, -hydroxyketones, -sulfonylketones, and 1,3-dihaloalkanes . 

S,S-Dioxo-2,3-dihydrobenzo[d]isothiazol-5-yl-pyrazole (215) [67], an antiinflammatory agent, was synthesised from compound 214 by treatment with CBr4, PhsP and aqueous NaHCOs. Preparation of the reagent 214 was ac-... 

Proton NMR spectra of the corresponding 5> n-(l,7-dioxo) and a //-(l,5-dioxo) isomers of pyra-zolo[l, 2-a]pyrazoles are quite similar and the positions are similar to those of related a,) -unsaturated carbonyl compounds (see Scheme 2) <80JA4983,82JOC207,82JOC214,84CPB930,86JPR648). 

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