= Solution
Apply the <Felkin-Anh model> to the stereocentre alpha to the <ketone>. Its large group is the chain continuing through the phenyl-bearing <carbon>, its medium group is methyl, and its small group is <hydrogen>. In the preferred transition-state arrangement, put the large group approximately perpendicular to C=O and approach from the opposite face, passing the small <hydrogen> along a <Bürgi-Dunitz trajectory>. The remote <ether> cannot form the usual small alpha-alkoxy chelate, and the coordinating <solvent> further supports using an open-addition prediction.<Felkin-Anh and Cram Chelate by University of Windsor>[pp. 1–2: perpendicular large group and nucleophilic approach past the small group]{ref}
The predicted new stereocentre is \b[$S$]. In the question's zigzag projection, this corresponds to the new OH bond directed behind the page and the newly introduced methyl toward the reader, with the original stereocentres unchanged. The new-centre priority order is OH $>$ phenyl $>$ the substituted alpha-carbon chain $>$ methyl. This is a model-based major-product prediction, rather than a quantitative diastereomeric ratio.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2025/paper-1-felkin-addition.png]
{title=Felkin-Anh approach past the small alpha hydrogen and the predicted S-configured tertiary alcohol}
Back to article page
