Regioselectivity in the reaction of 5 - amino - 3 - anilino - 1 H - pyrazole - 4 - carbonitrile with cinnamonitriles and enaminones: Synthesis of functionally substituted pyrazolo [ 1,5 - a ] pyrimidine derivatives

: The development of e ﬃ cient methods for the synthesis of polyfunctional N - heterocycles is an impor tant area of research in organic and medicinal chemistry. Pyrazolo [ 1,5 - a ] pyrimidine derivatives are purine analo gous of biomedical importance and have been extremely studied for their broad spectrum of biological activi -ties. Recently, they have attracted great interest in mate rials science owing to their photophysical properties. 3 ( 5 )- Aminopyrazoles are extensively utilized in the synthesis of condensed heterocyclic systems, particularly pyrazolo [ 1,5 - a ] pyrimidines via the reaction with 1,3 - biselectro philic reagents. However, the information available in the lit erature provides little in the way of reasoning their cyclization, particularly the initial attack either by the exocyclic amino group or endocyclic nitrogen. Unfortunately, the relative nucleophilicity of exo - and endocyclic nitrogen atoms in 1 - unsubstituted 3 ( 5 )- aminopyrazoles is not clear and con -tradicting. It has been found that other factors can modulate the regioselectivity rather than basicity or steric hindrance for both active sites. The reported studies in the structure – activity relationship revealed that pyrazolo [ 1,5 - a ] pyrimidines having a substitution at ﬁ fth, sixth, and seventh positions possess potent biological activities, especially those with an amino group at the seventh position. We here developed a regioselective, high yield synthesis of 7 - amino - 5 - arylpyrazolo [ 1,5 - a ] pyrimidine - 3,6 - dicarbonitriles by the reaction of N -( 5 - amino - 4 - cyano - 1 H - pyrazole - 3 - yl )- benzamide with various cinnamonitriles and enaminones in pyridine at 120°C under controlled microwave heating conditions. All structures of newly synthesized compounds were established by analyt ical and spectral data as well as single - crystal di ﬀ raction and rationalized for their formation.

In this regard, the nature of substituents at fifth, sixth, and seventh positions displayed a crucial role in their biological potency. It is worth mentioning that the presence of an amino group at the seventh position had an additional advantage to form hydrogen bonds with the hinger region of the receptor [20].
The nucleophilicity and reactivity of different active centers in 5-aminopyrazole scaffolds have received considerable interest over the years. Concerning nitrogen centers, several published articles argue that in some synthetic protocols ring nitrogen is the most nucleophilic center while the opposite is observed in others [21,22]. Many different factors such as the nature of the ring substituent, temperature, pressure, solvent, catalyst type as well as the type of reaction controlling either the kinetic or thermodynamic can be utilized to modulate the selectivity of several transformations.
It has been reported that the nature of the solvents plays a crucial role in such regioselective cyclocondensation. Owing to the amphoteric nature of 3(5)-aminopyrazoles, a basic solvent generates a heterocyclic anion resulting from the deprotonation of acidic-pyrole-like ring nitrogen, which enhances its reactivity. However, the acidic medium favors the exocyclic attack via protonating of the ring affording a cationic nucleus [26,27].
Arguably, a thorough investigation to establish the chemistry and regioselectivity of 3(5)-aminopyrazoles leading to complex heterocyclic scaffolds has not been established. Nevertheless, in-depth studies will be beneficial to elucidate their reactivity, their behavior and versatility in different environments, leading to the formation of a diversity of azoloazines in order to develop efficient synthetic methodologies.
Interestingly, a wide range of chemical transformations has now been performed under controlled microwave heating conditions. As compared to conventional heating reactions, microwaves couple directly with the molecules of the entire reaction mixture. Importantly, microwave heating possesses several advantages such as spectacular acceleration of many reactions, higher yields, mild reaction conditions, and shorter reaction times [28,29]. Moreover, several reports postulate the existence of a microwave effect as a specific radiation effect rather than the thermal one to rationalize for rate acceleration, changes in the reactivity as well as selectivity [30,31]. Our aim was to use a variety of factors to control the regioselectivity in such reactions leading to 7-aminopyrazolo[1,5-a]pyrimidines.

Materials and methods
Aldehydes, malononitrile, dimethylformamide dimethylacetal, and ketones were of commercial grade, and Pyridine was of analytically pure grade; all were purchased from Aldrich and Merck companies. 1 H NMR (600 MHz) and 13 C NMR (150 MHz) spectra were recorded using a Bruker DPX instrument (δ ppm). Mass spectra were determined by using a VG Auto spec QMS 30 and MSg (AEI) spectrometer in the EI (70 eV) mode. Melting points were recorded in a Gallen Kamp melting point apparatus and are uncorrected. X-ray crystallography was performed by using a Rigaku Rapid II and Bruker X8 Prospector single crystal X-ray diffractometer. All reactions were monitored by TLC with toluene/acetone 10:5, 10:6 as an eluent and were carried out until the starting materials were completely consumed.

Results and discussion
The reaction of N-(5-amino-4-cyano-1H-pyrazole-3-yl)benzamide 1 with a variety of electrophiles was reported to afford different isomeric pyrazolo[1,5-a]pyrimidine derivatives. Thus, in an early report [32], the reaction of 1 with tetracyano ethylene in dry ethyl acetate or methylene chloride at ambient temperature for 48 h or with 2-(dicyanomethylene) indan-1, 3-dione (CNIND) in dry pyridine and heating under reflux at 100°C for 3 h afforded mainly the corresponding 5-aminopyrazolo [1,5-a]pyrimidine via the initial attachment of ring nitrogen followed by cyclization of the formed 1:1 adduct with an exocyclic amino group, which was subsequently supported by other authors. Soliman et al. [33] have similarly reported that reaction of 1 with various halo reagents, active methylenes, and ketene dithioacetals under phase transfer conditions proceeds via the initial attack of the endocyclic ring nitrogen at the electrophile active site followed by cyclization of the cyclic intermediate with the exocyclic amino group. However, they revealed the initial attack by the exocyclic amino group upon reacting 1 with acetyl chloride, ethoxymethylene, malononitrile, and Lawson's reagent followed by cyclization with the ring nitrogen. A similar pathway was reported by Ahmed et al. [34] and Dozhenko et al. [35]. Elnagdi et al. [36,37] examined the regio-orientation in the reaction of 5-aminopyrazoles with benzylidene malononitrile and enaminones utilizing ( 15 N, 1 H) HMBC to establish a more conclusive structure elucidation as the structure assigned for such reactions was mainly based on 1 H NMR and IR spectra. They concluded that the reaction with benzylidene malononitrile proceeds initially by the nucleophilic attack of the exocyclic amino group; however, the initial attack of the ring nitrogen occurs with enaminones. Moreover, under such basic reaction conditions, the cyano group at the C-4 pyrazole ring remained inactive. In contrast, Hebishy and co-workers [38] recently reported the reaction of N-(5-amino-4-cyano1H-pyrazol-3-yl)-benzamide with arylidinemalononitrile afforded the corresponding 5-aminopyrazolo [1,5-a]pyrimidine derivatives. Recently, the catalyst-free Biginelli-type reaction of 5-amino-3-arylpyrazole-4-carbonitriles with ylidene-1,3-dicarbonyl compounds by refluxing in DMF has been reported by Dotsenko and co-workers [39].
The authors confirmed the formation of the corresponding 4,7-dihydropyrazolo[1,5-a]pyrimidine-3-carbonitriles via the intermediacy of the aza-Michael adduct resulting from the initial attack of the ring nitrogen and subsequent cyclo-condensation with the exocyclic amino group. The reaction of 1 with dimedone catalyzed by tin(II) chloride dehydrate, which acts as a Lewis acid, and a low melting solvent was recently reported to afford the corresponding pyrazolo [5,4-b]quinolones via the nucleophilic attack of the exocyclic amino group on the carbonyl carbon of dimedone followed by cyclization of the formed imino form on to the cyano group and hydrolysis [40]. A similar phenomenon was reported by Schmidtke and co-authors [41].
To shed further light on this unresolved issue and in continuation of our studies in which we utilize microwave heating [42][43][44][45][46][47][48], we report herein the results of our investigation on the reaction of N-(5-amino-4-cyano-1Hpyrazole-3-yl)-benzamide 1 with various electrophiles under controlled microwave heating conditions. Our aim was to modulate several factors controlling regioselectivity in such reactions.
The mass spectrum of 4a showed a molecular ion peak m/z = 351.01 (100%). The IR spectra (KBr) showed an amino group, two cyano groups, and C]N absorption bands at λ max = 3,441, 3,360, 2,179, 2,200, and 1,631 cm −1 . 1 H NMR spectra revealed a broad singlet at δ = 9.02 ppm integrated for two protons, which was assigned to the NH 2 group at C-7. If the reaction product was isomeric 5-amino derivatives, such amino groups will appear at a higher field shift. This low downfield shift could be rationalized for the anisotropic effect of the adjacent pyrazole ring nitrogen. In addition, it reveals signals at δ = 6.97 (1H, t, J = 6.  C-7). Furthermore, the structure of 4a was unambiguously confirmed by single-crystal X-ray diffraction [41] (Figure 2).
With these results in hand, we investigated the scope of such reactions with a variety of substituted cinnamonitrile derivatives. Thus, the reaction of 1 with 2b-f under the same experimental conditions afforded the corresponding 7-aminopyrazolo [1,5-a]pyrimidine derivatives 4b-f, respectively, in excellent yields. The structures of 4b-f were established from their mass spectra, 1 H NMR, and 13 C NMR spectra, which showed a similar pattern as compound 4a as well as elemental analysis. Further confirmation was achieved by single-crystal X-ray diffraction of 4d [42] (Figure 3, Scheme 1). These results were in contrast to that reported for the formation of 5-aminopyrazolo[1,5-a]pyrimidines from the reaction of 1 with arylidenemalononitriles [37] Another example of regioselectivity in the reaction of 5-aminopyrazoles with electrophilic reagents was reported [41] involving its reaction with (E)-3-(dimethylamino)-1-arylprop-2-en-1-one derivatives, which afforded the corresponding 5-arylpyrazolo[1,5-a]pyrimidines formed via the initial attack by the ring nitrogen. It is worth mentioning that such cyclocondensation is not favored in our case. Thus, the reaction of 1 with (E)-3-(dimethylamino)-1-phenylprop-2-en-1-one 5a afforded the corresponding 7-arylpyrazolo[1,5-a]pyrimidine 7a. The possible formation of 5-arylpyrazolo[1,5-a]pyrimidine 6a was ruled out based on analytical and spectral data. The mass spectra showed a molecular ion peak m/z = 311.17 (43%). The 1 H NMR revealed a downfield doublet at δ = 8.65 ppm and a doublet at δ = 8.13 ppm, corresponding to C 6 -H and     the signal assigned for C 6 -H will appear at a higher field shift. The 13 C NMR spectrum revealed C 6  Moreover, the structure of 7a was confirmed by singlecrystal X-ray diffraction (Figure 4) [43]. Similarly, compound 1 reacted with 5b under the same experimental conditions to afford the newly synthesized 7-arylpyrazolo[1,5-a]pyrimidines 7b-d (Scheme 2). The structure proposed for the reaction products was established based on a similar 1 H NMR and 13 NMR pattern as 7a A proposed mechanism to account for the formation of 4a-g and 7a-d was demonstrated in Scheme 3. An initial attack of the exocyclic amino group on the activated double bond system in 2a-g afforded 1:1 adduct 10 followed by cyclization via the addition of the ring NH to the cyano group and subsequent aromatization. The same applies for the formation of 7a, which proceeds via dimethylamine elimination from the reaction of the exocyclic amino group with 5a forming intermediate 13 and subsequent cyclization via the attack of the lone pair at the carbonyl group of ring nitrogen with water loss.

Conclusion
In conclusion, we can reveal that ring nitrogen is the more basic center and the exocyclic amino group is the less hindered. An attack by ring nitrogen is favored with less sterically hindered electrophiles; however, the attack with the exocyclic amino group predominates with bulky electrophiles. However, no firm conclusion on which to determine the preferred tautomer form of the final product has been arrived. Here, we do believe that the nature of the substituents of the aminopyrazole scaffold plays a crucial role in such regioselectivity. The presence of a cyano group at C-4 in compound 1 reduces the donating ability of the ring nitrogen. Moreover, for the titled compound, the chains of dimers formed by pairs of ring N-H····N hydrogen bonds render it less active. We concluded that unambiguous assignment of the regioselectivity in such reactions requires advanced techniques, particularly X-ray diffraction crystallography as insufficient spectral data could be a mess resulting in incorrect conclusions.