Synthesis and structure elucidation of novel pyrazolyl-2-pyrazolines obtained by the reaction of 3-(3-aryl-3-oxopropenyl)chromen-4-ones with phenylhydrazine

Novel 3-aryl-5-{4-[5-(2-hydroxyphenyl)-1-phenylpyrazolyl]}-2-pyrazolines 2a-g have been prepared by the treatment of 3-(3-aryl-3-oxopropenyl)chromen-4-ones


Introduction
Pyrazoles are interesting five-membered nitrogen heterocyclic compounds due to their synthetic versatility and broad spectrum of biological properties. 1 Numerous derivatives have been found to act as pharmacodynamic and chemotherapeutic agents and also playing an important role on the central nervous system. 2 On the other hand, the syntheses and selective functionalization of pyrazoles have stimulated the research over the years.Classical methods for the synthesis of substituted pyrazoles involve approaches based either in condensations of hydrazines with 1,3dicarbonyl compounds and in the intermolecular [3+2] cycloaddition reactions of 1,3-dipoles to alkynes.However, other methodologies have been developed for the preparation of novel substituted pyrazole-type compounds. 1-(3-Aryl-3-oxopropenyl)chromen-4-ones 1 are useful ,-unsaturated ketones 3 that can be used as starting materials in the synthesis of a wide variety of nitrogen-containing heterocyclic compounds.4-Aryl-2-(3-chromonyl)-2,3-dihydro-1,5-benzothiazepines were synthesized by the reaction of 1 with 2-aminothiophenol, 4 while the reaction with diazomethane afforded 3-aroyl-4-(3-chromonyl)-2-pyrazolines 5 similarly to that observed with ,-unsaturated ketones.Treating 3-(3-aryl-3-oxopropenyl)chromen-4-ones 1 with hydrazine hydrate provided several pyrazolyl-2pyrazolines, an interesting new pyrazoline-type compounds that were oxidized to bispyrazoles.6 These bis-azole derivatives are important biologically active compounds, since several pharmacological activities, such as anti-allergic, 7 antifungal, 8 anti-inflammatory, 9 antitumor 10 as well as cytotoxic properties 11 have already been described.
In the continuation of an on-going research program devoted to the synthesis and characterization of these bis-heterocyclic ring systems, 6,12 it seemed to be a challenging task to study the reaction of 3-(3-aryl-3-oxopropenyl)chromen-4-ones 1 with substituted hydrazines.Our work in this chemical transformation and a detailed NMR analysis of the structural features of the novel pyrazolyl-2-pyrazolines will be presented and discussed.

Scheme 1
Here, the formation of pyrazolyl-2-pyrazolines 2a-g also implies that both chromone and α,βunsaturated ketone moieties reacted with phenylhydrazine.Contrary to the referred previous work no by-products were isolated or detected even under careful chromatographic study. 13owever, it can be postulated that (3-chromonyl)-2-pyrazoline-type compounds 4 are the primary reaction intermediates (Scheme 2).In a second reaction step these (3-chromonyl)-2pyrazolines 4 react with another molecule of phenylhydrazine to provide the final products 2 (Scheme 2).In this transformation the reaction of the chromone moiety with phenylhydrazine, a non symmetric hydrazine derivative, in acidic medium can proceed in two different ways, due to the equilibrium between the non protonated and protonated form of the more nucleophilic phenylhydrazine amino group (Scheme 2).In the former case, the more nucleophilic amino group attacks the chromone C-2 carbon, in a conjugate-type addition, with consequent pyran ring opening leading to intermediates 5.Then, the intramolecular reaction between the other amino group (NHPh) and the carbonyl unit lead to pyrazolyl-2-pyrazoline derivatives 2. In the protonated hydrazine molecule NHPh becomes the nucleophile attacking the chromone C-2 carbon and after a pyran ring opening give rise to intermediates 6.The pyrazole ring may be formed by an intramolecular reaction between the other amino group of 7 and the carbonyl unit, resulting in the formation of compound 8.Our results indicate that the transformation proceeded by pathway a) of the reaction mechanism (Scheme 2).In fact, NHPh is a very week nucleophile implying that even in acidic medium, it is the more nucleophilic NH2 group that reacts with the chromone moiety.

Structure elucidation
The structures of all new compounds have been elucidated by using elemental analysis, mass spectrometry, infrared spectrophotometry and an exhaustive NMR study.Elemental analysis and mass spectrometric measurements unequivocally prove the elemental composition of compounds 2a-g and 3a-c,e.
The 1 H NMR spectrum of pyrazolyl-2-pyrazoline 2c in CDCl3 at room temperature (22ºC) showed extreme line broadening for almost all signals in the aliphatic and aromatic regions.This line broadening is especially evident in the proton resonances of the 5'-hydroxyphenyl group, H-3' and H-4, while H-5 appears as two broad signals centered at δ 4.99 and 5.10 ppm.Raising the temperature to 50ºC resulted in a clear sharpening of H-3' and both H-4 signals, while those of 5'-hydroxyphenyl group remained broad and there was the coalescence of both H-5 signals.
Decreasing the temperature to 5ºC, two sets of signals for the resonance of each proton were observed being easily identified in the aliphatic region (Figure 1).These data indicate that pyrazolyl-2-pyrazoline 2c exist as a mixture of diastereomers at room temperature in the chloroform solution.This diastereoselectivity arises from the combination of the pyrazoline C-4 stereocenter and two planar chiral subunits due to an internal possible hindered rotation involving the pyrazoline moiety and the 5'-hydroxyphenyl group.Under these conditions there is a slowly rotation between these moieties compared to the NMR time scale, reason why broad signals were observed in the 1 H and 13 C NMR spectra. 14At 5ºC the two diastereomers were frozen and shape signals for the resonances of each one of the isomers were observed.
Since the temperature of 50ºC was not enough to overcome the energy barrier of the referred hindered rotation of compound 2c and the temperature could not be further increased (b.p.CDCl 3 = 61ºC), DMSO-d6 was chosen as solvent to reach higher temperatures.In this case, once again, at temperatures between 22ºC and 50ºC some broad signals still appeared.Only at 60ºC sharp resonances in the aliphatic and aromatic regions of both 1 H and 13 C NMR spectra were observed, indicating that the energy barrier was overcome and there was no more diastereomers, but a mixture of enantiomers.A maximum resolution spectrum of a DMSO-d6 solution of 2c was obtained at 80ºC (Figure 2; vide experimental part).
The complexity of the aromatic region in the 1 H NMR spectra of pyrazolyl-2-pyrazolines 2ag did not allow to unequivocally assign the proton resonances of each one of the diastereomers A and B. However, the 2D NMR experiments (COSY, NOESY, HSQC and HMBC spectra) permitted to assign each proton of both diastereomers.The most easily identified are the proton resonances to the 3-aryl group.In fact, in the case of compounds 2a-e H-2"',6"' appeared at high values of frequency, δ = 7.55-7.66ppm, due to the deshielding anisotropic effect of the C=N pyrazoline bond.These assignments were also confirmed by the strong NOE cross peaks observed in the NOESY spectra between these protons and those of 4-CH2.The remaining proton resonances of this 3-aryl ring were assigned based on the 2D COSY correlations.The proton assignments of the 1-phenyl group were based on the strong NOE cross peaks between the signal of H-5 and those of H-2'',6'', complemented by the correlations observed in the 2D COSY spectra.
It is also worth to mention the OH proton resonance appearing as a broad singlet (δ 5.08-5.20 ppm) in the 1 H NMR spectra of the para-substituted compounds 2b-e.
The OH acetylation of derivatives 2a-c,e promoted a better resolution of the 1 H NMR spectra (mainly in the aromatic region) of compounds 3a-c,e and permitted to assign the individual protons of each one of the diastereomers.Thus, it was also possible to assign the ratio of both diastereomers, based in the area of the methyl signals of the acetyl group.The proportion of both isomers as 52.0-54.5% and 45.5-48.0%led us to designate them as major and minor diastereomers, respectively (vide experimental part).
The main features of the NMR data of these acetylated derivatives 3a-c,e were the resonances of the acetyl (CH3 at δH = 2.04-2.13ppm and δC = 20.8-20.9ppm) and carbonyl at (δC = 168.3-168.6 ppm) groups.Other important characteristics were the resonances of H-4trans 15 (δ = 3.19-3.24ppm) and H-5 (δ = 5.15-5.23 ppm) of the major diastereomers appearing at higher frequency than those of the minor ones (δ = 3.16-3.22and 5.07-5.15ppm, respectively).In the case of H-4cis 15 the signal of the major diastereomer appeared at lower frequency (δ = 3.62-3.66ppm) than that of the minor one (δ = 3.68-3.74ppm).The NOE cross peaks between the 4-CH 2 signals and the doublets at δ = 7.60-7.75ppm and of the H-5 and the doublets at δ = 7.00-7.09ppm allowed to assign these chemical shifts to the resonances of H-2"',6"' and H-2",6", respectively.The COSY correlations permitted to assign the other protons of these two aromatic rings.The assignment of the remaining proton and carbon resonances in the NMR spectra of 3ac,e unit was similar to that describe for compounds 2a-c,e and were based on the 2D NMR spectra (the main HMBC and NOESY correlations are presented in figure 3).

Conclusion
Following our work on pyrazoles we synthesised a new series of 3-aryl-5-{4-[5-(2hydroxyphenyl)-1-phenylpyrazolyl]}-2-pyrazolines 2a-g from the reaction of 3-(3-aryl-3oxopropenyl)chromen-4-ones 1a-g with phenylhydrazine in refluxing acetic acid.A study on the structural characterisation of these compounds by NMR at different temperatures showed a mixture of diastereomers at room temperature.The intramolecular hindered rotation combined with the pyrazoline C-4 stereocenter induces this diastereoselectivity, situation that are to the best of our knowledge reported for the first time.