FACILE ASYMMETRIC SYNTHESES OF 1-DEOXYCASTANOSPERMINE AND 1-DEOXY-8A-EPI-CASTANOSPERMINE

The asymmetric syntheses of 1-deoxycastanospermine (6) and 1-deoxy-8a-epi-castanospermine (7) have been completed. A key step in these syntheses was the stereoselective addition of 2-furyllithium to the 4-O-tert-butyldiphenylsilyl-protected threose derivative 13 to give the C(7)-C(8) syn-adduct 14 as the major product. The diastereoselectivity of this reaction is particularly noteworthy since the related addition of 2-furyllithium to the corresponding 4-O-benzyl-protected aldehyde 8 proceeded in the opposite stereochemical sense to give primarily the C(7)-C(8) anti-adduct 10. Oxidation of the furan ring in 14 followed by refunctionalization led to the azido ketone 20, which was then converted into the imine 22 by several procedures. Depending upon the reaction conditions, stereoselective reduction of 22 furnished primarily either 23 or 24. Hydrolysis of the pyranoid acetal moiety in 23 and 24 followed by reductive amination gave 6 or 7, respectively.