Abstract Two pairs of enantiomerically pure cis-fused cyclopenteno-1, 2, 4-trioxanes (7, ent- 7 and 8, ent-8) are prepared (Schemes 1–3). Their identities are established by dye- sensitized photo-oxygenation of ent-7 and 8, ent-8 to the allylichydroperxides, reduction to the corresponding alcohols, and conversion to the (1S)-camphanates (Scheme 4), the structures of which are determined by X-ray analysis. The dynamic properties of ent-7 are ...