trans-Hydroindanes possessing an angular methyl group were stereoselectively synthesised via treatment of the four geometrical isomers 1a-1d of 9-methoxycarbonyl-4-methyl-1-phenylthionona-1,8-dien-3-one with tert-butyldimethylsilyl trifluoromethanesulphonate in the presence of triethylamine.