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Issue 17, 2008
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Ring contraction during the 6π-electrocyclisation of naphthopyran valence tautomers

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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-contraction 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-10,10-dioxide) 9 affording 9-(naphtho[1,2-b]furan-2-yl)-9H-thioxanthene-10,10-dioxide 12, though more severe reaction conditions were required. The comparative ease 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(3H-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.

Graphical abstract: Ring contraction during the 6π-electrocyclisation of naphthopyran valence tautomers

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Publication details

The article was received on 07 May 2008, accepted on 09 Jun 2008 and first published on 09 Jul 2008


Article type: Paper
DOI: 10.1039/B807744D
Org. Biomol. Chem., 2008,6, 3096-3104

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

    C. D. Gabbutt, B. M. Heron, S. B. Kolla, C. Kilner, S. J. Coles, P. N. Horton and M. B. Hursthouse, Org. Biomol. Chem., 2008, 6, 3096
    DOI: 10.1039/B807744D

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