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