3δ2-Chromene (2,3-didehydro-2H-1-benzopyran): Generation and interception

Abstract

The treatment of 3-bromo-2H-chromene (7), dissolved in furan, 2-methylfuran, or 2,5-dimethylfuran, with potassium tert-butoxide, leads to formation of the epoxybenzo[c]chromene derivatives 8-11 in yields of 28-59%. Likewise, in styrene solution, exo-2-phenylcyclobuta[b]chromene 12 is produced (41 % yield). With tetrahydrofuran as the solvent, 2-tertbutoxy-2H-chromene (13) is observed as the only product (79% yield). From these results, it is concluded that 7 is converted by β-elimination into the title compound 5. This reactive intermediate is then intercepted by the furans and styrene in cycloaddition reactions to give compounds 8-12, whereas reaction with KOfBu/HOtBu transforms it to the acetal 13. The epoxybenzochromenes 8, 9, and 11 rearrange on heating at 110°C to give the epoxyxanthene derivatives 16-18.