HIGHLY STEREOSELECTIVE TRANS ADDITION OF PI-TYPE NUCLEOPHILES TO A BICYCLIC N-ACYLIMINIUM ION - APPLICATION TO THE SYNTHESIS OF INDOLIZIDINE AND PYRROLIZIDINE ALKALOIDS

Enantiopure bicyclic 5-ethoxytetrahydropyrrolo[1,2-c]oxazol-3-one Ib w as prepared in two steps from the known tosylate 4, which is readily a vailable from (S)-pyroglutamic acid. Trapping of the N-acyliminium ion (I), generated in situ from Ib in the presence of Lewis acid, with va rious silylated pi-type nucleophiles gave rise selectively to trans ad ducts 2. The usefulness of this stereoselective access to trans-2,5-di substituted pyrrolidines was illustrated by formal syntheses of 3,5-di substituted indolizidine toxins, starting from 5-allyltetrahydropyrrol o [1,2-c]oxazol-3-one 2a. Moreover, an enantiodivergent synthesis of t he pyrrolizidine alkaloids (+) and (-)-xenovenine was achieved startin g from the same chiral building block 2a.