Synthesis of pyrazole-4-carbohydrazide derivatives of pharmaceutical interest

New 1-phenyl-or 1-methyl-5-benzamidopyrazole-4-carbohydrazide derivatives were prepared in 70–90% yields from 1-methyl-or 1-phenyl-6-phenylpyrazolo[3,4-d ]1,3-oxazin-4(1 H )-one derivatives and hydrazine hydrate. Small quantities of the isomeric 5-aminopyrazole-4-( N - benzoyl)hydrazides were detected in some reaction mixtures, proving that intramolecular benzoyl migration can take place in the 5-benzamidopyrazole-4-carbohydrazide molecule. The direct formation of pyrazole-4-carbohydrazides from 5-benzamidopyrazole-4-carboxylic acid ethyl esters and hydrazine hydrate was unsuccessful.


Introduction
In the course of our medicinal chemistry researches, we needed to prepare pyrazole-4carbohydrazide derivatives 2a-h (Schemes 1 and 2) in view of their potential pharmacological activities.In the literature it is reported that some benzohydrazides are used to inhibit fibrosis and to treat fibrosing disorder. 1Several aryl-and heteroaryl-hydrazides produce inhibitory effects on glutamic acid decarboxylase (GAD), GABA-α-oxoglutarate amino transferase (GABA-T) and monoamine oxidase. 2,3Moreover, Isoniazid ® , namely pyridine-4-carbohydrazide, is the drug of choice in the treatment of tubercolosis. 4 review of the literature 5 revealed that 1-phenyl-5-benzamidopyrazole-4-carboxylic acid ethyl ester 1 (Scheme 1) when refluxed for 5 h in hydrazine hydrate afforded the 5-amino derivative 3, by losing the benzoyl moiety, instead of the hydrazide derivative 2a.Previously we have reported 6 that the reaction of pyrazole [3,4-d]-1,3-oxazin-4-one derivatives of type 4 with anilines afforded, in high yields, a number of phenylamides of 5benzamidopyrazole-4-carboxylic acids.In analogy with this reaction we reacted the pyrazole [3,4-d]-1,3-oxazinones 4a-h with hydrazine hydrate, hoping to obtain the pyrazole-4carbohydrazides 2a-h.When the pyrazolo-oxazinone 4a was reacted with hydrazine hydrate, a product was obtained in 75% yield, together with a small amount of a second one.On the basis of elemental analysis and molecular weight (MS), they proved to be isomers.We assigned structure 2a to the high-yield product and structure 5a to the isomer (Scheme 2).
The attribution of the structures was based on NMR data.The 1 H-NMR spectrum of the major product showed a signal for two hydrogens at δ4.39, exchangeable with D 2 O, and the minor isomer produced the same signal at δ6.40; the low-field signal is attributable to the amino group bonded to the pyrazole nucleus, and the signal at δ4.39 to the more shielded hydrazide amino group.The latter value is in accordance with the signal reported in the literature for the hydrazide amino group of 2-pyridinecarbohydrazide. 7 The two NH signals of 5a (9.96 and 10.36 δ) were closer than those of 2a (9.43 and 10.40 δ) owing to a more similar environment of the NH groups in 5a.In order to confirm the structure 5a, the 1-phenyl-5-aminopyrazole-4carbohydrazide 3a and benzoyl chloride 6a were reacted (Scheme 3), affording a compound identical in all respects to 5a.
Moreover, reaction of this product 5a with ethyl orthoformate gave the 1-phenyl-5benzamido-1H-pyrazolo [3,4-d]pyrimidine-4-one 7a which was also obtained by benzoylation of the 5-amino-1H-pyrazolo [3,4-d]pyrimidine-4-one 8. 8 At this point, the other pyrazolo [3,4d]oxazinones 4b-h were reacted with hydrazine hydrate affording the related pyrazolo-4carbohydrazides 2b-h in 70-90% yields.The NMR spectra of these compounds showed the signal for the hydrazide amino group in the δ 4.32-4.39range and those for the two NH groups at δ 9.20-10.79.The reaction mixtures of 2b-h were monitored for the presence of 5b-h by 1 H-NMR.On the basis of the 5-amino signal, the spectra revealed the presence of small amounts of 5b,f (signals at 6.44 and 6.25 δ respectively).These results were confirmed by TLC using authentic specimens of 5b,f as reference compounds.Compounds 5b,f were synthesized as for 5a (Scheme 3) and identified by elemental and spectroscopic data.The 1 H-NMR spectra of 5b,f showed the 5-amino signal at 6.45 and 6.22 δ, respectively.The formation of 2a-h can easily be explained by a nucleophilic attack on the carbonyl group of the oxazinone ring of 4a-h.It appears that this process can be followed by intramolecular migration of a benzoyl group in compounds 2a,b,f to give the isomers 5a,b,f, whose formation is rationalized in Scheme 4.
Lastly, the hitherto unknown pyrazolo-oxazinones 4b,d,f were synthesized following Scheme 5.The structures of the above compounds, as well as those of the intermediate derivatives 10 and 11 were assigned on the basis of satisfactory elemental and spectroscopic data.

General procedure for 1-R-5-aminopyrazole-4-(N-benzoyl)carbohydrazides 5a,b,f
A solution of 0.01 moles of a pyrazole-4-carbohydrazide 3a-f in acetonitrile (50 ml) was heated under reflux with 0.01 mole of the appropriate benzoyl chloride 6a,b,f for 7h.The solid which separated was collected then recrystallized from ethanol to give pure 5; yields 78-85%.5a (80%) identical in all respect to the low-yield product obtained by reacting 4a with hydrazine hydrate.5b  8 and the benzoyl chloride 6a in pyridine/dioxane (1:1) mixture were heated under reflux for 1 h, then cooled and poured on cold diluted HCl.The solid which separated was collected, and recrystallized from ethanol to give pure 7a which was identical to that synthesized by method A (Rf, mixed m.p., MS); yields 53%.

Scheme 5 Experimental Section General Procedures
. Melting points were determined on a Büchi 530 capillary melting point apparatus and are uncorrected; IR spectra were recorded in Nujol mulls with a Jasco IR-810 spectrophotometer; 1 H-NMR spectra were obtained in DMSO-d 6 on a Bruker AC-E 250 MHz spectrometer (TMS as internal standard).Mass measurements at low resolution were obtained on an Autospect Ultima Orthogonal T.O.F.T. mass spectrometer operating at 75 eV.Elemental analyses (C, H, N), performed by Dipartimento di Scienze Farmaceutiche-Università di Catania, were within ±0.4% of theoretical values.