Carbenoid induced irreversible ring opening of naphthopyrans

Abstract

Contrary to expectation rhodium carbenoids do not undergo cycloaddition to the 2 H-pyran unit of the isomeric naphthopyrans 3 and 5 . With 3 , a naphtho[2,1- b]pyran-8-ylacetate, 4 is formed and a novel merocyanine dye 6 results from a cycloaddition across the C-5–C-6 double bond of the naphtho[1,2- b]pyran 5 . Tethering the carbenoid to the naphtho[1,2- b]pyran system 5 , as in 10 , results in a similar mode of addition and affords the intensely coloured tetracycle 11 .