Total Syntheses of (+)-2,8,8a-Tri-epi-swainsonine and (-)-1-epi-Swainsonine

Parallel syntheses of the title swainsonine-related enantiomers 9 and 15 were achieved in five steps and in 55% overall yield via enantiomeric threose N-benzylimines 4 and 10, derived from L- and t tartaric acid, respectively. A stereospecific 4 + 4 homologative procedure using 2-(trimethylsiloxy)- furan (TMSOF), obtained from 2-furaldehyde, was employed to form the eight-carbon skeleton of the indolizidine triols and to install the proper chirality. Remarkably, the syntheses were completed by cyclizations of tetrahydroxypiperidine intermediates 8 and 14 to 9 and 15, respectively (ca. 90 % ; PPh3, CCld, Et3N in DMF at room temperature), without recourse to protecting groups.