Uso de ácidos orgânicos e irradiação de micro-ondas na síntese de xantenonas com potencial atividade antirradicalar

Abstract

Organocatalysis is considered a foundation pillar of Green Chemistry. The biodegradability and low or none toxicity make natural organic acids eco-friendly options of catalysts. In this work, it was evaluated the potential of oxalic, succinic, malonic and acetic acids as catalysts for the synthesis of xanthenones under microwave irradiation and solvent-free conditions. The reaction studied involved the coupling of an aldehyde, a phenol and an 1,3-dicarbonilic compound. The best yield for the synthesis of xanthenone 4 was achieved when using 20 mol% oxalic acid under 5 min of microwave irradiation. This approach allowed the synthesis of 30 xanthenones, in which 11 of them are novel. The -naphtol- and sesamol-derived xanthenones were obtained with 35-93% of yield and 30-91% of yield, respectively. A preliminary screening revealed that the ability of xanthenones 15, 16, 26, 30 e 31 to scavenge the reactive nitrogen species DPPH was comparable to that of the known antioxidant resveratrol (77%). The concentration necessary to scavenge free radicals by 50% (SC50) for xanthenones 16, 30 e 31 was two-fold lower than that of resveratrol (42.4 M) indicating the lower potency of the latter. The synthesized xanthenones were found to be poor scavengers of the reactive oxygen species O2-. The SC50 value for Xanthenone 26 against O2- was similar to that of resveratrol (691.3 M). Overall, the results indicate that an efficient approach based on the use of oxalic acid as a catalyst was developed to synthesize a series of xanthenones under microwave irradiation. Also, the great ability of xanthenones to scavenge reactive nitrogen species makes this class of compound eligible for further studies of antioxidant activity.