Gabbutt, C. D., Heron, B. M., Thomas, D. A., Light, M. E. and Hursthouse, M. B.
Carbenoid induced irreversible ring opening of naphthopyrans
Tetrahedron Letters, 45, (32), . (doi:10.1016/j.tetlet.2004.06.064).
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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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