Microwave-Assisted Improved Regioselective Synthesis of 12 H -Benzopyrano[3,2- c ][1]benzopyran-5-ones by Radical Cyclisation

• Published in: Organic chemistry International Vol. 2011; pp. 1 - 6
• Main Authors: Basu, Pradipta Kumar, Ghosh, Amrita
• Format: Journal Article
• Language: English
• Published: 15.09.2011
• ISSN: 2090-200X; 2090-2018
• DOI: 10.1155/2011/394619

Two new effective methodologies have been adopted for the preparation of 4-(2 ′ -bromobenzyloxy)benzopyran-7-ones 3(a–h) . In the first methodology, 4-hydroxy[1]benzopyran-2-ones 1(a–d) was alkylated with 2-bromobenzyl bromide 2a or 2-bromo-5-methoxy benzyl bromide 2b under phase transfer catalysis condition using lithium hydroxide/tetrabutyl ammonium bromide in N,N -dimethylformamide at 40–50°C and in the second method the microwave irradiation protocol has been exploited by simply mixing of 4-hydroxy[1]benzopyran-2-ones 1(a–d) with 25% excess of 2-bromobenzyl bromide 2a or 2-bromo-5-methoxy benzyl bromide 2b . A catalytic amount of TBAB and potassium carbonate were added and irradiated in an open Erlenmeyer flask in a microwave oven for 4–10 min. The tributyltin-hydride-mediated radical cyclisation of 3(a–h) was carried out under microwave irradiation to generate 12 H -benzopyrano[3,2- c ][1]benzopyran-5-ones 4(a–h) in 78–88% yield and in this technique yields were significantly improved and reaction time was shortened compared to the previously reported conventional radical cyclisation method.