Structure of the acylation products from 3,6-disubstituted and 3,6,7-trisubstituted 1 H -pyrazolo[5,1-c ][1,2,4]triazoles

The monoacyl derivatives from 6-methyl-3-phenyl-1 H -pyrazolo[5,1-c ][1,2,4]-triazole 1 may exist in three isomeric forms. Spectroscopic and chemical arguments are presented for the confirmation of the identities of the resulting products


Results and Discussion
Because the pyrazolo[5,1-c][1,2,4]triazole system can exist as three tautomeric forms, 1a, 1b, 1c, monoacylation can produce three isomers: the keto form, 7-acyl-6-methyl-3-phenyl-1Hpyrazolo [5,1-c]  17 The pyrazole and triazole cycles, if considered to be independent, show different reactivity.In electrophilic substitution reactions, pyrazole is more reactive than the other azoles (with the exception of imidazoles).Additionally, its higher level of basicity, suggests pyrazole is more nucleophilic and more reactive in electrophilic substitution reactions than is 1,2,4-triazole. 16n order to verify these suppositions, we have carried out the acetylation of 6-methyl-3phenyl-1H-pyrazolo[5,1-c][1,2,4]triazole 1 under the following conditions: a) with acetic anhydride, b) with acetic anhydride in the presence of concentrated H 2 SO 4 , c) with acetyl chloride in pyridine, d) with acetyl chloride in tetrahydrofuran and e) with acetyl chloride in tetrahydrofuran in the presence of tributylamine.The reactions were monitored by TLC and the most appropriate conditions were then used for the synthesis of other acyl derivatives.
All the acetylation reactions of 6-methyl-3-phenyl-1H-pyrazolo [5,1-c] [1,2,4]triazole 1a led to the same N-monoacetylated compound, as the main product, showing a molecular ion peak at 240 a.m.u., together with unreacted 1a in experiments (c) and (d) and C-monoacetylated compound 2c and the diacetylated compound 3a in experiment (b).The monoacetylated compound (from e) does not react with 2,4-dinitrophenylhydrazine in alcoholic solution in the presence of concentrated H 2 SO 4 (Brady's reagent for ketone) and the IR spectrum shows a valence vibration ν C=O at 1722 cm -1 .The 1 H-NMR spectrum shows δ = 6.17 ppm for H7 8 while the δ = 12.5 ppm peak (from 6-methyl-3-phenyl-1H-pyrazolo[5,1-c] [1,2,4]triazole 1) assigned to H in -NH-, disappears upon acetylation.We conclude that the obtained monoacetylated compound has either structure 2a or 2b.The evidence for N-acetylation was supported by the mass spectrum of the obtained compound, which indicated the loss of mass 42 a.m.u.(CH 2 =C=O), a typical fragmentation for N-acetyl derivatives.In order to elucidate the position of acetylation, we performed the following 2-D spectra: 1 H-1 H COSY, 1 H-13 C COSY: HETCOR (HMQC) and 1 H- 13 C HMBC for the compound formed by the monoacetylation of 6methyl-3-phenyl-1H-pyrazolo [5,1- The 1 H-1 H COSY spectrum shows the presence of a very low intensity coupling between the protons of the methyl 6-C-CH 3 and the 7-H proton.This observed 4 J HH coupling is much more likely to arise in the case of isomer 2b (R = CH 3 ), monoacetylated on the 5-N nitrogen atom, in which there is a double bond between the 6-C and 7-C atoms, which can be considered an allylic type coupling.Because the coupling is not as intense as theory would predict for the monoacetylated compound 2a (R=CH 3 ), we conclude that the isomer 2b (R = CH 3 ) is present to only a small extent.
The 1 H- 13 C HMBC spectrum shows the following proton-carbon distance-couplings: 2 J C=O, The acetylation with acetic anhydride in the presence of concd.H 2 SO 4 has led to a mixture (according to 1 The mass spectrum of this mixture indicates the molecular ions for the monoacetylated compounds and for the diacetylated products at 240 a.m.u. and 282 a.m.u., respectively.It also shows the fragmentation of the (CH 3 ) from the molecular ion, (240 a. m.u.) with the formation of the 225 a.m.u.ion ( which is specific to ketones) and which can only take place in the case of compounds acetylated at carbon 2c or 3a (Scheme 3).

Conclusions
The acetylation of 6-methyl-3-phenyl-1H-pyrazolo[5,1-c] [1,2,4]triazole 1 proceeds in different ways according to the reaction conditions.Thus, in the presence or absence of a tertiary amine the acetylation takes place at the 1-N nitrogen atom and in a very small proportion at the 5-N nitrogen atom.In contrast, under acidic conditions, the acetylation takes place at the 1-N nitrogen atom and / or the 7-C carbon atom, with the formation of compounds monoacetylated at atoms 1-N or 7-C and diacetylated at the atoms 1-N and 7-C.

Aknowledgements
The authors thank Prof. Dr. Valentin Zaharia (Department of Pharmacy, University of Medicine and Pharmacy, Cluj-Napoca, Romania) for performing the microanalyses.