Synthesis and Characterization of New Naphthoquinonic Derivatives Containing the Pyrazole Ring: Pyrazolylnaphthoquinones

The reaction of 3-aminopyrazole (1) with 1,2-naphthoquinone-4-sulfonic acid sodium salt (2) was studied in different aqueous media. The novel pyrazolylnaphthoquinones synthesized were physical and spectroscopically characterized, including 2D NMR spectroscopy (HETCOR). The possible reaction mechanism is proposed. Introduction Quinones and naphthoquinones are widely distributed in nature and play a vital role in certain celullar functions [1]. On the other hand, pyrazoles are important synthetic intermediates in the construction of many complex molecules with interesting biological activities [2,3]. For these reasons, and in our search of compounds with important bioactivities, we have prepared a new type of naphthoquinonic derivatives containing the pyrazole ring as nitrogenated heterocycle. In this communication we describe the synthesis and characterization of a new type of compounds, the pyrazolylnaphthoquinones 3-5, which were obtained by the reaction of 3-aminopyrazole (1) and 1,2-naphthoquinone-4-sulfonic acid sodium salt (2). Experimental IR (KBr), UV-visible (MeOH), NMR and mass spectra were recorded in a Nicolet 5-SXC FT IR, Shimadzu UV-160A, Bruker AC 200 E and a Finningan 3300 (at 30 y 70ev), respectively. The H and C spectra were run in DMSOd6 (the center of the solvent peak was used as internal standard which was related to TMS) and they were calculated by the ACD program. Compounds 1 and 2 were purchased from Aldrich Co. and Sigma, respectively. Derivatives 3-6 were isolated and purified by radial preparative chromatography, electrophoresis and recrystallization from organic solvents. Results and Discussion Preliminary experiments investigating the reaction between 3-aminopirazole (1) and 1,2Molecules 2000, 5 509 naphthoquinone-4-sulfonic acid sodium salt (2) showed different structures. It was seen that the medium conditions (basic, neutral, acidic and heat) were responsible for the pathway of the reaction. Therefore, the reaction was studied exhaustively and was found that in the pH range 10.4-2.0 and at room temperature, 2-hydroxy-N-(3-pyrazolyl)-1,4-naphthoquinone-4-imine (3) was obtained as unique product (71%). In aqueous HCl 0.5 N and at room temperature, a mixture of 3 (20%), N-(3-pyrazolyl)4-amino-1,2-napththoquinone (4, 5%) and 2(3-pyrazolylamino)-N-(3-pyrazolyl)-1,4-naphthoquinone-4-imine (5, 43%) were isolated. On the other hand, in aqueous HCl 0.5 N at reflux, the reaction afforded a mixture of 4 (7%) and 2-hydroxy-1,4-naphthoquinone (Lawsone, 17%). The spectral data [including the 2D NMR spectroscopy (HETCOR)] were consistent with the proposed structures for 3-5. The possible reaction mechanism is discussed and evidence is presented to discard the existence of isomers arising from the tautomeric equilibrium of the pyrazole ring. References and Notes 1. Shrestha-Dawadi, P. B.; Bittner, S.; Fridkin, M.; Rahimipour, S. Synthesis 1996, 12, 1468 and references therein. 2. Fujita, M.; Egawa, H.; Miyamoto, T.; Nakano, J.; Matsumoto, J. Eur. J. Med. Chem. 1996, 31, 981. 3. El-Shekeil, A.; Babagi, A.; Hassan, M.; Shiba, S. Heterocycles 1988, 27, 2577.