TANDEM [4+2] [3+2]CYCLOADDITIONS OF NITROALKENES .11. THE SYNTHESIS OF (+)-CROTANECINE/

(+)-Crotanecine (1) is the necine base component of a number of pyrrol izidine alkaloids. This necine subunit is an amino triol bearing a pri mary allylic alcohol characterized by an all-cis relationship of its s tereocenters. The synthesis of (+)-crotanecine has been accomplished i n 10 steps and 10.2% overall yield. The key step in the asymmetric syn thesis is a Lewis acid-promoted, tandem inter[4 + 2]/intra[3 + 2] cycl oaddition between a (fumaroyloxy)nitroalkene 14 and chiral beta-silylv inyl ether (-)-26. This synthesis serves to illustrate the synthetic v ersatility of the tandem cycloaddition to incorporate additional funct ionality.