Three-Component Glycolate Michael Reactions of Enolates, Silyl Glyoxylates, and α,β-Enones

Silyl glyoxylates react with enolates and enones to afford
either
glycolate aldol or Michael adducts. Product identity is controlled
by the countercation associated with the enolate. Reformatsky nucleophiles
in the presence of additional Zn­(OTf)2 result in aldol
coupling (A), while lithium enolates provide the Michael
coupling (B). Deprotonation of the aldol product A with LDA induces equilibration to form the minor diastereomer
of Michael product B. This observation suggests that
formation of the major diastereomer of Michael product B does not occur via an aldol/retro-aldol/Michael sequence.