Synthesis of some novel pyrazolo[3,4-d ]pyrimidine derivatives

Reaction of ethyl imidates derived from N -aryl-5-amino-4-cyanopyrazoles with amines or arylhydrazines gave only 4-substituted pyrazolo[3,4-d ]pyrimidines, resulting from cyclization followed by Dimroth rearrangement. From the reaction with arylhydrazines, a mixture of the hydrazines and their oxidized forms, the azo products, was obtained. This was proven by an independent synthesis starting from the corresponding 4-chloropyrazolo[3,4-d ]pyrimidines as starting material. The structures of the compounds obtained were confirmed by mass spectrometry, 1 H and 13 C NMR.


Introduction
In recent years, pyrazolopyrimidines and related fused heterocycles have been identified as bioactive molecules.They are known to function as CNS (Central Nervous System) depressants, 1 neuroleptic agents, 2 and as tuberculostatic 3 .Pyrazolo [3,4-d]pyrimidines were identified as a general class of adenosine receptors. 4,5Moreover, fluorinated compounds find much importance in the pharmaceutical field. 6The introduction of a CF 3 group provides compounds with increased lipophylicity and activity when compared to their non-fluorinated analogues.The trifluoromethyl substituted compounds have been reported to possess biological activity as herbicides, fungicides, 7 analgesic, antipyretic, and inhibitors for platelet aggregation. 8e have concentrated our attention on ortho-aminocyanopyrazoles and their derivatives as inhibitors of xanthine oxidase. 9With the objective of synthesizing members of this class of compounds we started from the key intermediates 2a-d (ethyl N-4-cyano-1-(4-substituted)-1Hpyrazol-5-ylformimidate).These compounds are readily prepared from the N 1 -substituted-5amino-4-cyanopyrazoles 1 (Scheme 1) and react with arylhydrazine derivatives to produce pyrazolo [3,4-d]pyrimidines.The antifungal activity of these new heterocycles is currently being evaluated.

ISSN 1551-7012
Page 94 © ARKAT USA, Inc. Zn/AcOH   Moreover, compound 5a could be converted to compound 6a by long reflux in ethanolic triethylamine solution and compounds 6 were reduced by zinc in acetic acid to afford compounds 5.These results also prove the correct structures of compounds 5 and 6 as suggested in scheme 1.

Scheme 2
It seemed of interest to study the analogous reactions of ethoxymethylene amino derivatives 2 with amines.In this case, we considered that the presence of the amidine moiety may ensure the possibility of closure of the pyrimidine ring, resulting in novel derivatives of pyrazolo [3,4d]pyrimidine of significant interest for biological study, since such compounds are substituted analogues of the well-known drug allopurinol. 13s the amines for studying their reactions with the amidine 2b, we selected the relatively low-basicity m-anisidine and the more basic ammonia.
Heating ethoxymethylene amino derivatives 2b with amines led to the formation of pyrazolo [3,4-d]pyrimidines 9 and 10.The rates of these reactions differ quite significantly.In order for the process to go to completion (TLC), it was necessary to stir the reaction mixture for 3 h in methanol at RT.A reaction time of 14 h in refluxing methanol was required for the least basic m-anisidine to react to completion.Again the reaction of 2b (X = Cl) with m-anisidine afforded the Dimroth rearrangement product 9 via the intermediate 8 (Scheme 3).The reaction of pyrazole 2b with ammonia led exclusively to the 4-aminopyrazolo [3,4-d]pyrimidine derivative 10 with mp 280-282 ºC [Lit. 14

mp 284 ºC].
The alternative structure 8 was excluded based on NMR data.The one-dimensional 1 H NMR spectrum, showed all the expected signals with protons 3 and 6 overlapping in a sharp singlet at 8.54 ppm (integrating for two protons) and a sharp NH signal at 10.19 ppm.In the HMBC spectrum, we observe an intense correlation peak for the NH proton with the peaks at 107.2 (C-2``) and 113.5 ppm (C-6``) which is characteristic only of structure 9 but not for 8, where the indicated proton and carbon atoms are separated by five bonds.
The structure of compounds 9 could also be confirmed by reaction of 4chloropyrazolopyrimidine 7 with m-anisidine (Scheme 3).

Scheme 3
In conclusion reaction of various monosubstituted hydrazines with ethyl N-4-cyano-1-(4substituted)-1H-pyrazol-5-ylformimidate 2 yields a mixture of the Dimroth rearrangement products, 5, together with their oxidized forms 6.The proportion of 5 and 6 depends on the reaction time; longer heating yielding mainly compounds 6. Reduction of compounds 6 afforded the corresponding hydrazinyl derivatives 5. Reaction of compound 2 with m-anisidine afforded only the Dimroth rearrangement product 9.

Experimental Section
General Procedures.Melting points were determined on a Gallenkamp melting point apparatus and are uncorrected.IR spectra were registered on a Perkin Elmer FTIR-1600. 1 H NMR (300 MHz) and 13 C NMR (75.4 MHz) spectra were recorded on a Varian Unity Plus Spectrometer.Double resonance, HMQC and HMBC experiments were carried out for complete assignment of proton and carbon signals in the NMR spectra, whenever possible.High resolution mass spectra were obtained on a AutoSpec E spectrometer.Elemental analyses were obtained on a Leco CHNS-932 instrument.Compounds 3, 7 and 10 were prepared by known methods. 10,14neral procedure for preparation of 2a-e A mixture of 5-amino-4-cyano-1-substituted pyrazole (0.2 mol), triethylorthoformate (20 ml) and acetic anhydride (20 ml) was heated under reflux for 7 h and then evaporated under reduced pressure.The residue was treated with ethanol and the solid product so formed was collected by filtration, washed with ethanol and crystallized from EtOH-H 2 O.

Reaction with phenylhydrazine derivatives Method A. Reaction of ethyl N-4-cyano-1-(4-substitutedphenyl)-1H-pyrazol-5-yl formimidate
To a solution of pyrazol-5-ylformimidate (0.1 mol) in ethanol (20 ml), phenylhydrazine derivative (0.1 mol) was added (a catalytic amount of triethylamine was added in the case of phenylhydrazine HCl).The reaction mixture was heated under reflux for 7-10 h.The orange precipitate formed after cooling was filtered off, dried and recrystallized from ethanol to afford the oxidized product as an orange solid (except for entries 3 and 4, table 1, where no orange solid precipitated on cooling).The mother liquor was poured onto ice and the solid formed was filtered off and crystallized from EtOH to produce the hydrazinyl product as a white powder.

Method B. Reaction of 1-(4-chlorophenyl)-4-chloropyrazolo[3,4-d]pyrimidine with phenyl hydrazine derivatives
To a solution of 4-chloropyrazolo [3,4-d]pyrimidine (0.1 mol) in ethanol (20 ml) was added the phenylhydrazine derivative (0.1 mol) and a catalytic amount of triethylamine.The reaction mixture was heated under reflux for 7 h.It was followed the same work up as mentioned above.

Reduction of diazenyl derivatives to hydrazinyl derivatives, with Zn/AcOH (conversion of compounds 6 to 5)
To a solution of diazenyl derivatives 6 (0.1 mol) in glacial acetic acid (25 ml), Zn dust (2 g) was added.The reaction mixture was refluxed for 2 h during which time the color turned to pale yellow.The reaction mixture was filtered while hot, left to cool to RT and then poured onto crushed ice (25 g).The precipitated solid was collected by filtration and recrystalized from CHCl 3 to afford the corresponding hydrazinyl derivative which was found identical in all respects with that obtained from the above reaction (TLC, mp, NMR).(5a, 74 % yield, 5b, 58 %yield, 5e, 65 % yield).

Reactions with m-anisidine
Method A. To a solution of pyrazol-5-ylformimidate (0.1 mol) in methanol (20 ml) was added m-anisidine (0.1 mol).The reaction mixture was heated under reflux for 14 h.The precipitate formed after cooling overnight was filtered off and dried and recrystallized from ethanol (77% yield).Method B. To a solution of 4-chloropyrazolo [3,4-d]pyrimidine (0.1 mol) in methanol (20 ml) was added m-anisidine (0.1 mol).The reaction mixture was refluxed for 5 h.The precipitate formed during reflux was filtered off and found identical in all respect with that obtained from method A (85 %).