Synthesis of 7-substituted pyrazolo[1,5-a ]pyrimidine-3-carboxamides as potential non benzodiazepine hypnotics

A variety of new pyrazolo[1,5-a ]pyrimidines has been prepared as potential drugs for the treatment of insomnia. The general synthetic route used for this purpose involves the condensation of substituted cyanoacetamides 5a-c , prepared by reaction of cyanoacetic acid with amines in presence of acetic anhydride, with dimethylformamide dimethylacetal and subsequent treatment of the formed enamines 6 with hydrazine hydrate. This process affords the corresponding aminopyrazole carboxamides 9 that react with the enaminonitrile 12 to generate the targets. Structures of the substance prepared in these sequences were established by using spectroscopic methods, including 15 N HMBC and NOE difference experiments, as well as X-raycrystallo-graphic analysis.


Introduction
Almost 40% of adults between ages 40-70 suffer from insomnia at least one time during their lives.The drug Zaleplon 1 [1][2][3][4] has been found to be efficient in the treatment of sleep disorder where difficulty in falling asleep is the primary issue.Unlike many other hypnotic drugs, this substance does not interfere with sleep architecture and can be administered for up to five weeks without the risk of dependence or rebound insomnia upon discontinuation.Indiplon 2 1,5 has recently been released for use for the same purpose while the developments of ocinaplon 3, 6 which is an anxiolytic drug in the pyrazolopyrimidine family of drugs, has been discontinued owing to liver complications observed in clinical trials.As a result, a need exists for the development of analogs of 1-3.In the studies described in this publication, a new, simple and efficient route to 7-substituted pyrazolo [1,5-a]pyrimidine-3-carboxamides [7][8][9][10] that have structures related to 1-3 has been developed.

Results and Discussion
Preparation of the target compounds was initiated by reactions of amines 4a-c with mixtures of acetic anhydride and cyanoacetic acid, following the recently published procedure. 7,11,12This process affords the cyanoacetamides 5a-c in 85-93% yields (Scheme 1).The products undergo reaction with dimethylformamide/dimethylacetal (DMF/DMA) to yield the corresponding enamines 6a-c.Although this process has the potential of producing mixtures of the stereoisomeric enamines 6 and 7, the fact that only the E-isomers 6 were generated has been demonstrated by using NOE experiments that showed that the vinyl protons were spatially proximate to the amide NH.

Scheme 1
An ethanol solution of the cyano-enamine 6a and hydrazine hydrate at room temperature undergoes reaction to form the acyclic hydrazine derivative 8 (Scheme 2).When 8 was either stirred at 50 ºC or heated in pyridine solution, cyclization gradually took place via addition of hydrazine to the cyano group.This process generated pyrazole 9a, which was shown by using 1 H NMR spectroscopy (eg., ring CH at δ 8.1 ppm, singlet) to be the tautomeric form shown in Scheme 2. Stirring a solution of 6 in DMSO at reflux produces a mixture of 9a and the tautomer 10 in a 2 to 1 ratio.Unexpectedly, stirring a solution of 8 in ethanol afforded the cyanopyrazole 11.Consequently, it appeared that 9 was the kinetic product and 11 was the thermodynamic product.In contrast to 6a, an ethanol solution of enamine 6b reacted with hydrazine hydrate at reflux to give the amino pyrazole 9b as a single product (Scheme 2).

Scheme 2
Pyrazoles 9a,b reacted smoothly with enaminonitrile 12 in pyridine to yield the aminopyrazolo[1,5-a]pyrimidines 13 and not the regioisomeric compounds 14.Attempts to isolate the acyclic intermediate in these processes were not successful. 15N HMBC analysis was used to demonstrate that the structures of 13 were those of 7-aminopyrazolo [1,5-a]pyrimidines.
Specifically, the position of the amino group on C-7 was evidenced by 4 J coupling of the protons at δ 8.4 ppm with the sp 3 bridged head nitrogen atom at δ 210.4 ppm.It is expected that if the regioisomeric structure 14 had been formed, protons of the amino group at δ 8.4 ppm would have been coupled only with N-4 at δ 224.2 ppm.In addition, 13 was observed to react with benzoyl chloride in pyridine to afford the benzoyl derivative 15.
The reaction of pyrazoles 9 with the enaminones 16 led to formation of the acyclic products 17 that underwent cyclization to generate 18 upon stirring in DMF containing sodium acetate at reflux.It should be noted that the pyrazolo[1,5-a]pyrimidines 18 could be prepared directly from 9 and 16 by stirring in refluxing pyridine overnight (Scheme 3).
ARKAT USA, Inc. Attempts to synthesize pyrazolo [3,4-d]pyrimidines 20 by the reactions of 9a,b with dimethylformamide/dimethylacetal (DMF/DMA) failed and only the acyclic amidines 19 were formed.These substances did not cyclize to generate the corresponding pyrazolopyrimidines.Although, 19 may exist in 1H-3-amino forms, their existence in 1H-5-amino forms was established by analysis of 1 H NMR spectra that showed that H-3 is a singlet at δ 9.27.Reaction of 9b with excess dimethylformamide/dimethylacetal affords the hydrazone 21 as a result of methylation of the pyrazole NH (Scheme 4).
ARKAT USA, Inc.Further investigations showed that 5a-c reacted with aryldiazonium chloride to yield arylhydrazones 22a-d whose structures were established by using X-ray crystallography (Figure 1).The previous finding that 2-arylhydrazono-3-oxoalkanenitriles prefer to exist in anti conformations (eg., 22) rather than syn.Hydrogen bonded forms provides further support for the conclusion that stereoelectronic factors are more important than hydrogen bonding interactions in governing conformations in these systems. 13,14he results of previous studies 7,15,16 demonstrated that the arylhydrazonitriles could be utilized as precursors for 1,2,3-triazoles, isoxazoles and pyrazoles.In the current effort, the possible utility of the arylhydrazones 22a-d as precursors for the above mentioned heterocycles was explored.Thus, reaction of 22c with hydroxylamine hydrochloride in refluxing DMF containing sodium acetate was observed to produce 24 via the amidoxime 23.The structure of 24 was established on the basis of the results of NOE difference experiments.Specifically, irradiation of the amine hydrogen resonance at δ 6.21 ppm did not enhance the aryl hydrogen signals at δ 7.41-8.91ppm.On the other hand, reaction of 22a with hydroxylamine hydrochloride in refluxing DMF in the presence of sodium acetate yielded the isoxazolone 25 via the amidoxime 23 through loss of an amine and not water.This same isooxazolone can be obtained by cyclization of the amidoxime 26, which was previously prepared by Elnagdi et al. 17 by treatment with sodium ethoxide (Scheme 5).

Conclusions
The results of the study described above have led to the development of a simple approach for synthesis of 5-aminopyrazole-4-carboxamides and methodology for the conversion of these compounds into pyrazolo[1,5-a]pyrimidines, substances that potentially interesting biological and medicinal properties.Furthermore, the observations made during this work showed that the reaction of hydrazine hydrate with enamines can afford cyanopyrazoles or aminopyrazoles, depending on the reaction conditions used.

Experimental Section
General.All melting points are reported uncorrected.IR spectra were recorded using KBr disks using a Perkin-Elmer System 2000 FT-IR spectrophotometer. 1 H NMR (400 MHz) and 13 C NMR (100 MHz) spectra were recorded on a Bruker DPX 400, 400 MHz super-conducting NMR spectrometer on CDCl3 or DMSO-d6 solutions with TMS as internal standard.Chemical shifts are reported in ppm.Mass spectra were measured using a VG Autospec Q MS 3 and MS 9 (AEI) spectrometers with EI (70 EV).Microanalyses were performed on a LECO CHNS-932 Elemental Analyzer.The crystal structure was determined by using a X-ray instrument at the National Research Center, Dokki, Cairo.

General procedure for the preparation of 5a-c
A solution of cyanoacetic acid (10 mmol) in Ac2O (10 mL) was heated on a water bath at 85 o C for 10 min.Then, the appropriate starting materials 4a-c were added to the reaction mixture and heating was continued at reflux for a further 15 min.The reaction mixture was cooled and poured onto cold water.The solid products 5a-c were collected by filtration and crystallized from the appropriate solvent .

General procedure for the preparation of 9b and 11
A mixture of the enaminonitrile 6a (2.45 g, 10 mmol) and hydrazine hydrate (80%, 0.65 ml) in EtOH (40 mL) was stirred at reflux for 2h, concentrated in vacuo to one third of its volume, and cooled to room temperature.The crude solid which formed was collected by filtration, washed with water, and crystallized from the appropriate solvent.

General procedure for the preparation of 19 and 21
A mixture of 9b (1.4 g, 5 mmol) and DMFDMA (0.6 g, 5 mmol) in case of 19 or (1.3 g, 11 mmol) in case of 21, in DMF (20 mL) was stirred at reflux for 6h, cooled to room temperature and poured onto ice cold water.The formed crude solid was collected by filtration, washed with water and crystallized from the appropriate solvent.

General procedure for the preparation of 22a-d
A cold solution of the aryldiazonium salts (10 mmol) was prepared by adding a solution of sodium nitrite (1.4 g dissolved in 10 mL water) to a cold solution of arylamine hydrochloride (10 mmol in 6 mL, 6M HCl) with stirring.The resulting solution of aryldiazonium salts were then added to a cold solution of 3-oxoalkanonitriles 5a-c in ethanol (50 mL) in the presence of sodium acetate trihydrate (4.2 g, 30 mmol).The mixture was stirred at room temperature for 1 h and the solid was collected by filtration, washed with water and crystallized from the appropriate solvent.

Crystallographic analysis for 22c
Crystals were mounted on a glass fiber.All measurements were performed on an ENRAF NONIUS FR 590.The data were collected at 25 o C using the ω scanning technique to a maximum of a 2θ of 24.108 o .The structure was solved by direct methods using SIR 92 and refined by full-matrix least squares.Non-hydrogen atoms were refined anisotropically.Hydrogen atoms were located geometrically and were refined isotropically.

General method for the preparation of 24 and 25
A mixture of arylhydrazononitriles 22a,c (5 mmol), and hydroxylamine hydrochloride (0.5 g, 7.5 mmol) was stirred at reflux in DMF (20 mL) in presence of anhydrous sodium acetate (1 g) for 5 h.Then, the reaction mixture was a cooled to room temperature and poured onto ice cold water.The formed solid was collected by filtration washed with water and crystallized from EtOH/DMF mixture as pale brown crystals.

Note:
The biological activities of the new zaleplon analogues prepared in this study are now under investigation and the results will be the subject of a further communication.

General procedure for the preparation of 6a-c
A mixture of 5a-c (10 mmol) and DMFDMA (1.2 g, 10 mmol) in EtOH (25 mL) was stirred at room temperature for 4h (for6a) or at reflux for 2 h (for 6b and 6c).The separated solid products formed on standing at room temperature were collected by filtration, washed by EtOH and rcrystallized from the appropriate solvent.