= 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. \b[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. \b[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.<Recent results toward the stereoselective synthesis of biologically active natural products by Luiz C. Dias and others>[2007, synthesis section and Scheme 2, epoxidation opposite OTBS]{ref}
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