摘要:
A concise practical synthesis (13 steps, ca. 10-12% overall yield) of the unnatural enantiomer of castanospermine ((-)-1) and its 1-epimer 2 from 2,3,4-tri-O-benzyl-D-xylose (3) is described. Key steps in the synthesis are two organometal aldehyde additions, vinylation of 3 to 4 and allylation of 6a to 11, both of which proceed with a considerably high degree of stereocontrol. The fused ring system is generated from the acyclic amino polyol derivative 16a by two successive S(N)2-type cyclizations. Notably, the annulation of the six-membered ring makes use of tetravalent phosphonium reagents (Appel or Mitsunobu type) which cyclize the amino alcohol 22a/b directly to 23a/b without need for N-deprotection and O-activation manipulations.