1,4-Diphenylcyclopent-2-ene 1,4-endoperoxide (1) on catalysis with trimethylsilyl trifluoromethanesulfonate reacted with 1,4-diphenyl-1,3-cyclopentadiene in a diastereospecific manner to give cis,cis-3a,5a,8a,8b-tetrahydro-2,5a,7,8b-tetraphenyl-1H,6H-dicyclopenta[c,e]-1,2-dioxine (3), the structure of which was determined by X-ray, 2,5-Diphenylfuran, 1,1-diphenylethylene and styrene also reacted with 1 under the same conditions to yield the cis-fused analogues of 3 in high yield.