Ring contraction during the 6?-electrocyclisation of naphthopyran valence tautomers

Gabbutt, Christopher D., Heron, B. Mark, Kolla, Suresh B., Kilner, Colin, Coles, Simon J., Horton, Peter N. and Hursthouse, Michael B. (2008) Ring contraction during the 6?-electrocyclisation of naphthopyran valence tautomers. Organic & Biomolecular Chemistry, 6 (17), 3096-3104. (doi:10.1039/b807744d).

Abstract

The thermal and photochemical ring-opening of spiro(3H-naphtho[2,1-b]pyran-3,9'-thioxanthene-10, 10-dioxide) 3 results in the facile ring-con traction to 9-(naphtho[2, 1-b]furan-2-yl)-9H-thioxanthene-10, 10-dioxide 6. Similar behaviour is displayed by the isomeric spiro(2H-naphtho[1,2-b]pyran-2,9'-thioxanthene- 10, 1 0-dioxide) 9 affording 9-(naphtho[ 1,2-b]furan-2-yl)-9H-thioxanthene- 10, 1 0-dioxide 12, though more severe reaction conditions were required. The comparative case of this rearrangement for the isomers 3 and 9 was rationalised on the basis of the relative isomer populations of the ring-opened naphthopyrans. The rearrangement of simple mono- and bis-methylsulfonylphenyl substituted photochromic naphthopyrans 18, 20 was examined; the former failed to rearrange whereas the latter could be induced to rearrange only under prolonged UV irradiation. The photochromism of diastereoisomerically pure sulfoxides derived from the oxidation of spiro(3 H-naphtho[2, 1-b]pyran-3,9'-thioxanthene) 2a and spiro(2H-naphtho[1,2-b]pyran-2,9'-thioxanthene) 2b resulted in conversion to the most thermodynamically stable trans-isomer in each case.

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Contributors

Author: Simon J. Coles

Author: Peter N. Horton

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