TOTAL SYNTHESIS 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 D-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, CCl4, Et3N in DMF at room temperature), without recourse to protecting groups.