Tandem [4+2]/[3+2]cycloadditions of nitroalkenes .11. The synthesis of (+)-crotanecine

(+)-Crotanecine (1) is the necine base component of a number of pyrrolizidine alkaloids. This necine subunit is an amino triol bearing a primary allylic alcohol characterized by an all-cis relationship of its stereocenters. The synthesis of (+)-crotanecine has been accomplished in 10 steps and 10.2% overall yield. The key step in the asymmetric synthesis is a Lewis acid-promoted, tandem inter[4 + 2]/intra[3 + 2] cycloaddition between a (fumaroyloxy)nitroalkene 14 and chiral beta-silylvinyl ether (-)-26. This synthesis serves to illustrate the synthetic versatility of the tandem cycloaddition to incorporate additional functionality.