Aldol Reaction - Simple Diastereoselectivity

Cis Lithium enolates (Z) generally give Syn Aldol products


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In an aldol reaction, cis enolates prefer to form syn aldols. The reaction proceeds via a chair-like transition state where the aldehyde chooses to react with the methyl group equatorial. As the enolate has no choice over its orientation, only a syn product is possible.