Benzopyrones. Part X. Bromination of chromones and coumarins with dibromoisocyanuric acid. Nitrations of chromones

Abstract

The bromination of benzopyrones with dibromoisocyanuric acid (DBI)(12) has been investigated. Chromone, ethyl 4-oxochromen-2-carboxylate, and ethyl 2-oxochromen-3-carboxylate each gave the 6-bromo-derivative on monobromination. Treatment of chromone with 2 mol. equiv. of DBI gave 5,6,8-tribromochromone (9) and a little 3,5,6,8-tetrabromochromone (10); similar treatment of ethyl 4-oxochromen-2-carboxylate gave the 5,6,8-tribromo-product (9); ethyl 5,6,7,8-tetrabromo-2-oxochromen-3-carboxylate (17) was the product from ethyl 2-oxochromen-3-carboxylate under the same conditions. These products were identified by n.m.r. spectroscopy and by chemical means. Attempted monobromination of coumarin failed to give a single product, but with 3 mol. equiv. of DBI 3,5,6,7,8-pentabromocoumarin (15) was obtained. Some ambiguity concerning the identity of the products from the nitration of chromone and 3-methylchromone has been resolved. In both cases nitration occurred at C-6.