One-Pot and Chemoselective Synthesis of 6-Bromo-2-Oxo-2H-Chromene-4-Carboxylate Catalyzed by Nano Silica Powder

Coumarin derivatives are important chemicals in the perfume, cosmetic, agricultural and pharmaceutical industries [1]. However, the conventional methods for coumarin synthesis require drastic conditions. For example, 4-methyl-7-hydroxycoumarin has been prepared by stirring a mixture of resorcinol and ethyl acetoacetate in concd H2SO4 for 12–24 h. The development of alternative environmentally friendly synthetic methods of coumarins is strongly requested [2]. Here, we report a simple and efficient synthesis of coumarins via catalytic action of nano silica powder in the conversion of dimethyl2-(5-bromo-2-hydroxyphenyl)3-(triphenylphosphoranylidene) butanedioate to methyl 6-bromo-2-oxo-2H-chromene-4-carboxylate in solvent-free conditions at 60◦C in fairly high yield Protonation of the highly reactive 1:1 intermediates, produced in the reaction between triphenylphosphine and dimethyl acetylenedicarboxylate, by 4-bromophenol leads to vinyltriphenylphosphonium salt, which undergoes aromatic electrophilic substitution reaction with conjugate base to produce dimethyl2-(5-bromo-2-hydroxyphenyl)3-(triphenylphosphoranylidene) butanedioate. Nano Silica Powder was found to catalyze conversion of dimethyl 2-(5-bromo-2-hydroxyphenyl)-3(triphenylphosphoranylidene)butanedioate to methyl 6-bromo-2-oxo2H-chromene-4-carboxylate in solvent-free conditions at 60◦C in fairly high yield. In summary, we have found that nano silica powder is able to catalyze conversion of ylide 5 to compound 6 in solvent-free conditions.