= Solution
\b[A is the predicted major <diastereomer>.] At the <carbon> adjacent to the <ketone>, take phenyl as the large group, vinyl as medium, and <hydrogen> as small. There is no adjacent heteroatom donor that would force a five-membered chelate. The appropriate first model is therefore the non-chelating <Felkin-Anh model>.
View along the <carbonyl>-<carbon>–stereocentre bond. Rotate to place the large alpha <phenyl group> approximately perpendicular to the <carbonyl> plane. Of the two resulting approaches opposite that large group, the favourable <Bürgi-Dunitz trajectory> passes nearer <hydrogen> rather than vinyl. Propyl transfer to the <carbonyl> and <protonation> of its <oxygen> give the configuration labelled A: in the printed projection the new OH projects forward and the propyl bond backward, while the original vinyl bond remains backward. Rotation used to construct a <Newman projection> must not be mistaken for inversion of the original stereocentre.
This is a relative-<selectivity> prediction, not a numerical product ratio. Simple attack on the face that happens to look less crowded in the initial flat drawing is unreliable because the relevant C–C single bond can rotate. B is the competing product from the less favourable approach beside vinyl. The model and its preference for the trajectory beside the small group are described in the institutional treatment.<Felkin-Anh and Cram Chelate by University of Windsor>[pp. 1–2, section 1, nonpolar alpha substituents and the small-group trajectory]{ref}
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2022/paper-1-felkin-model.png]
{title=Original Newman construction of the preferred Felkin–Anh approach and the local stereochemistry corresponding to product A}
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