Synthesis of benzylideneacetophenone under microwave irradiation; green chemistry

Short report

Gabriela Ibieta Jiménez¹, José A. Bravo², José Luis Vila^1,*
*Corresponding author: joselu62@hotmail.com

Abstract

The synthetic equivalent of the natural chalcone known as benzylideneacetophenone was obtained under microwave irradiation conditions. The synthetic reaction included benzaldehyde and acetophenone as precursors. The reaction was done under alkaline conditions (NaOH). The puré compound was obtained after crystallization and was characterized by NMR techniques.

Keywords: Benzylideneacetophenone, trans-Chalcone, Microwave Synthesis, Irradiation, Green Chemistry, NMR.

Resumen

Spanish title: Síntesis de bencilidenacetofenona por irradiación de microondas; química verde. El equivalente sintético de la chalcona natural bencilidenacetofenona, fue sintetizada a partir de la reacción de benzaldehido y acetofenona en medio básico asistido por microondas y purificado por cristalización. La caracterización del compuesto fue realizada por RMN de ¹H y ¹³C.

INTRODUCTION

Phenolics like chalcones of natural origin are known as benzylideneacetophenones. These compounds are natural precursors of open chain flavonoids. In this category are classified chalcones whose structure consists of two benzene rings linked by a , -unsaturated carbonyl system bridge. The two benzene rings are named as ring A and B [1]. The range of the biological activities reported for chalcones is wide and varied including antibacterial, anti-inflammatory, antimalarial, antifungal, antiviral, antioxidant, mutagenic and cytotoxic, among the most significant [2]. Besides classical methods for synthesizing chalcones, the green chemistry approach proposes as well a variety of examples. There are current different methods, these differ in conditions implying catalysts, solvents and times of irradiation [3].

The advantages with respect to conventional methods (no MW using) are well known and include shortening of reaction times and cleaner conditions in the obtaining of the producís. The product was purified by crystallization and it was identified by NMR techniques.

The present short report is a novelty regarding the microwave method used to synthesize the simplest existing chalcone (benzylideneacetophenone).

RESULTS AND DISCUSIÓN

The condensation reaction between acetophenone and benzaldehyde under microwave irradiation (170 W, 10 min.) and alkaline conditions (40%) in C2H5OH afforded yellowish crystals of compound 1 [6]. See Fig. 1. The product was re-crystallized in CI2CH2 and CH3OH (5: 95 v/v) and subsequently submitted to ID and 2D NMR analyses. Comparison of spectral data of 1 with those published [7] permitted the identification of 1 as benzylideneacetophenone or íra/js-chalcone (IUPAC name: (E)-1,3-diphenylprop-2-en-1-one). Figures 2 and 3 are the ¹H and ¹³C NMR spectra of 1 which are coincident in chemical shift valúes and shape with the published spectra [7].

Protons A are the more deshielded due to the carbonyl's anisotropy cone. The E alkene presents two protons (B and O with a wide common J valué of 15.5 Hz, protón B is the more deshielded as consequence of its position respect to carbonyl. The rest of protons (D) present non-analyzable signáis (300 MHz) corresponding to 8 aromatic protons distributed in both benzene rings.

EXPERIMENTAL

General

The NMR spectra were run in a Bruker DRX300, (300 MHz ¹H, 75 MHz ¹³C) equipment at the Department of Chemistry of UMSA, TMS was used as internal standard. Microwave device was DAEWOO DC electronics, model KOC-1B0K, power: 170 W. Synthesis monitoring by Silica gel 60 F254 TLC plates by Merck.

Chemicals

Benzaldehyde (p.a.), acetophenone (p.a.) from Sigma-Aldrich. Ethanol and NaOH commercial.

Synthesis procedure

In a 50 mL Erlenmeyer flask 1 mL of NaOH (40%) was added together with 1 mL of ethanol and stirred until dissolution. After dissolution, 0.9 mL of benzaldehyde and 0.927 g of acetophenone were added to the reaction mixture and stirred until dissolution at room temperature. Microwave radiation (170 Watts) was applied to the flask in a microwave oven during 10 minutes with cooling of the flask in ice bath each 10 seconds. After 10 minutes of reaction 25 mL of water were added to the flask. After 24 hours of repose at room temperature, an orange precipítate was filtered and separately dissolved in ethanol 95° GL (25 mL). This solution was evaporated at reduced pressure to afford a solid which was subsequently crystallized in dichloromethane - ethanol (5 : 95 v/v) to give yellow crystals. The yield was 43%.

CONCLUSIÓN

We achieved the so far unreported synthesis assisted by microwave irradiation of the known compound benzylideneacetophenone (trans-chalcone). Characterization was done by using NMR techniques. Yield was 43%. Even though the green chemistry conditions as: reduced reaction times and reduced use of non aqueous solvents were achieved, the yield of the principal product 1, was not so encouraging. The amounts of reagents should be revised in future assays in order to increase the yield of 1.

ACKNOWLEDGMENTS

Authors wish to thank Dr. Yonny Flores Segura, Laboratory of NMR, Department of Chemistry, UMSA for the recording of NMR spectra. The Research Institute of Natural Products (IIPN) is acknowledged for financial support.

NOTE

² Department of Chemistry, Research Center of Natural Products CIPN, Laboratory of Phytochemistry, Universidad Mayor de San Andrés UMSA, P.O. Box 303, Calle Andrés Bello s/n, Ciudad Universitaria, Cota Cota, Phone 59122792238, La Paz, Bolivia, jabravo@umsa.bo

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