The cyclization of unsaturated epoxides to tricyclic systems is shown to be a stereospecific process. Boron fluoride etherate treatment of an epoxy olefin with a central trans-substituted double bond, trans-7, leads to two tricyclic alcohols, 12 and 14, both of which have trans-fused A/B rings. The corresponding cis compound, cis-7, produces an A/B cis-fused alcohol, 20. These reactions also yield monocyclized products of specific geometry depending on the stereochemistry of the original double bond and the conformational folding of the epoxy olefin chain. The stereochemistry of these products suggests that cyclization occurs via intermediate cations of fixed geometry rather than as a “nonstop” process. © 1969, American Chemical Society. All rights reserved.