 Solution

The free allylic hydroxyl can hydrogen-bond to the peracid and accelerate directed oxygen transfer to the nearby double bond. The remote styryl double bond lacks this directing contact. Epoxidation of the hydroxyl-adjacent alkene is therefore favoured, even though both alkenes are in principle oxidizable. Peracid epoxidation is a concerted oxygen transfer: alkene $\pi$ donation to terminal peroxide oxygen, O–O cleavage, proton transfer and carbonyl re-formation occur through a cyclic transition-state arrangement. The alkene's E geometry is retained as a trans relationship between its substituents in the epoxide.

The two alkene faces are diastereotopic because the molecule already contains stereocentres. A plausible substrate-controlled conformation minimizes allylic strain and presents the face opposite the bulky OTBS-bearing part of the chain to the oxidant. Predict epoxide oxygen on the face opposite OTBS, retaining the pre-existing methyl and OTBS configurations. Hydrogen-bond direction, steric approach and conformational population compete; one cannot deduce an exact diastereomeric ratio from the flat drawing alone. The same facial preference has been discussed for this synthetic series.^[ref]