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.

A quick exchange takes place between proline-derived oxazolidinone 2d and acetone (carbonyl compounds in general). The active though very minor species (enamine 1b) appear more rapidly than mixing proline and acetone. Reaction times of aldol reactions can be shortened from 30-48 h to 1-4 h (and, as an unexpected bonus, yields increase by 11-23%).