= Solution
The <alkene> undergoes concerted <oxygen> transfer from the terminal peroxide <oxygen> of <mCPBA>. In the cyclic <electron>-flow description, the <alkene> pi bond begins one C–O bond, the peroxide <oxygen> supplies the second, O–O breaks, and <proton> transfer/carboxyl rearrangement gives m-chlorobenzoic acid. Both C–O bonds are installed from the same <alkene> face, so the <alkene>'s relative substituent geometry is retained in the <epoxide>. There is no freely rotating carbocation intermediate.
The neighboring free OH <hydrogen>-bonds to the approaching peracid and favors a cyclic, organized transition-state geometry. \b[<oxygen> is delivered from the OH-directed face, giving the indicated syn relationship between <epoxide> <oxygen> and the allylic hydroxyl.] In the supplied projection, the <epoxide> C–O bonds and the original C–OH bond are on the rear/hashed face. Steric interactions still influence the preferred <hydrogen>-bonded conformer, but “attack from the least hindered face” alone misses the positive directing role of OH. The distant benzyl <ether> is not the directing free hydroxyl.
The classical experimental comparison of allylic alcohols and protected derivatives established this directing principle; it supports the mechanistic interpretation without itself measuring the selectivity of this acyclic substrate.<376. Aspects of stereochemistry. Part I. Stereospecificity in formation of epoxides from cyclic allylic alcohols by H. B. Henbest and R. A. L. Wilson>[1957, pp. 1958–1965, hydroxyl-directed peracid epoxidation]{ref}
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{title=Hydroxyl-directed oxygen transfer and stereoretentive acetonide formation}
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