Gabbutt, Christopher D., Heron, B. Mark, Thomas, David A, Light, Mark E and Hursthouse, Michael B (2004) Carbenoid induced irreversible ring opening of naphthopyrans. Tetrahedron Letters, 45 (32). pp. 6151-6154. ISSN 0040-4039

Contrary to expectation rhodium carbenoids do not undergo cycloaddition to the 2H-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.

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