Seebach’s oxazolidinone is a good catalyst for aldol reactions

Seebach’s proline-derived oxazolidinone 2d overcomes (S)-proline and is at least as efficient as (S)-5-(pyrrolidin-2-yl)tetrazole in several organocatalytic aldol reactions examined. A quick exchange takes place between 2d and carbonyl compounds that gives new bicyclic oxazolidinones, in equilibrium with the very minor active species (enamines). Maximum yields of the aldols (β-hydroxy ketones) were achieved after 1–4 h when, with proline, they are attained after 30–48 h.