Aldol Reaction - Simple Diastereoselectivity

Cis Lithium enolates (Z) from bulky ketones give Syn Aldol products

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The bulky t-butyl group attached to the carbonyl controls the enolate geometry. Only a single geometrical isomer is possible as the cis conformation reduces the steric interactions between the methyl and t-butyl groups. The cis enolate then reacts with the aldehyde to give only the syn aldol product via a chair-like transition state and the aldehyde phenyl equatorial.

C. H. Heathcock, C. T. Buse, W. A. Kleschick, M. C. Pirrung, J. E. Sohn and J. Lampe, J. Org. Chem., 1980, 45, 1066–1081.