Ring contraction during the 6π-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.

Synthesis and Photochromism of Novel Pyridyl-Substituted Naphthopyrans

“…Incorporation of either basic or electron-withdrawing groups at C-3 of the 3H-naphtho [2,1b]pyrans by acid-catalysed condensation between 2-naphthol and the corresponding prop-2yn-1-ol has been previously accomplished only in very poor yields. 33 Inspired by the work of Putala et al, 34 it was envisioned that a Heck cross-coupling reaction could be employed to prepare the 3-pyridyl-3H-naphtho [2,1-b]pyrans 7 from commercially available and easily attainable 1-halo-2-naphthols 8 (Strategy B). Furthermore, it was envisioned an additional strategy (Strategy C) that involved effecting a late-stage 'chromenization' by the acidcatalysed cyclization of the alkene 9, the latter prepared by the Heck cross-coupling reaction between the appropriate prop-2-en-1-ol 10 and the 1-halo-4-pyridyl-2-naphthol 11.…”

Tautomerism in Fuscin - A DFT Treatment

“…It is noteworthy that FP contains an aromatic ring. Degenerate interconversions can be considered a special case of rearrangements called valence isomerization or valence tautomerisation [27][28][29][30][31][32][33][34][35][36][37][38][39]. A general scheme is proposed for the interconversions of six-membered oxygen-containing heterocycles (pyrylium salts, pyrans, and pyrones) and their open-chain forms, having selected pyrylium salts as compounds of the key type [39].…”

Base-Mediated Ring-Contraction of Pyran Systems Promoted by Palladium and Phase-Transfer Catalysis

“…The bis-sulfone 10 underwent an efficient photochemical ring-contraction to the naphtho [2,1-b]furan 11 in 85% yield. 22…”

“…Description of Kinetics Experiments, 1 H and 13 C{ 1 H} NMR Spectra, Mass Spectral Data for compounds 22,29,30,32,34,35,36,38,40,41,42,46,48,50,52,55…”