Synthetic studies on aromatic sesquiterpenoids: synthesis of curcumene ether by olefin cyclisation

Abstract

The acid-catalysed cyclisation of the olefinic β-hydroxy sulphides (15a) and (15b) gave 3-phenylthiocurcumene ether as a diastereoisomeric mixture [(17) and (18)] together with 5,5-dimethyl-4-phenylthio-1-p-tolylcyclohex-1-ene (16) and 2-methyl-3-(1-methyl-1-phenylthioethyl)-2-p-tolyl-cyclobutan-1-ol (19). The diastereoisomers (17) and (18) were converted into curcumene ether (24). Treatment of the silyl enol ether (7) with benzenesulphenyl chloride yielded 5-phenylthiomethyl-2-[phenylthio(p-tolyl)methylene]tetrahydrofuran (8) and 5-chloro-1,6-bis(phenylthio)-1-p-tolylhexan-2-one (9). 1-p-Tolylhexane-2,5-dione (10) was also synthesised from (5) employing a palladium-catalysed oxidation, and this result constitutes a formal total synthesis of cuparene (11).