Preparation of thiochromans via thermal cyclization

Formation of the thiochroman ring system is achieved by a two step synthesis that involves heating 3-thiophenyl-1-propanols or 4-thiophenyl-2-butanols in toluene with catalytic amounts of p-toluenesulfonic acid. The propanols are made by the addition of sulfur stabilized carbanions to styrene oxide, ethylene oxide, propylene oxide, and isobutylene oxide. The carbanions are generated by treatment of either benzyl phenyl sulfide or thioanisole with butyllithium. The effect of substitution at the 1 and 3 positions of the propanols on the reaction yields is discussed. The mechanism of the reaction apparently involves intramolecular electrophilic aromatic substitution rather than a Claisen or thio-Claisen rearrangement.