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

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.