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.