title: Ring contraction during the 6pi-electrocyclisation of naphthopyran valence tautomers.
abstract: The thermal and photochemical ring-opening of spiro(3H-naphtho[2,1-b]pyran-3,9'-thioxanthene-10,10-dioxide) results in the facile ring-contraction to 9-(naphtho[2,1-b]furan-2-yl)-9H-thioxanthene-10,10-dioxide . Similar behaviour is displayed by the isomeric spiro(2H-naphtho[1,2-b]pyran-2,9'-thioxanthene-10,10-dioxide) affording 9-(naphtho[1,2-b]furan-2-yl)-9H-thioxanthene-10,10-dioxide , though more severe reaction conditions were required. The comparative ease of this rearrangement for the isomers and 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 , 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) and spiro(2H-naphtho[1,2-b]pyran-2,9'-thioxanthene) resulted in conversion to the most thermodynamically stable trans-isomer in each case.
authors.affiliation: Department of Colour Science, School of Chemistry, University of Leeds, UK.
authors.forename: Christopher D
authors.initials: CD
authors.lastname: Gabbutt
authors.forename: B Mark
authors.initials: BM
authors.lastname: Heron
authors.forename: Suresh B
authors.initials: SB
authors.lastname: Kolla
authors.forename: Colin
authors.initials: C
authors.lastname: Kilner
authors.forename: Simon J
authors.initials: SJ
authors.lastname: Coles
authors.forename: Peter N
authors.initials: PN
authors.lastname: Horton
authors.forename: Michael B
authors.initials: MB
authors.lastname: Hursthouse