Asymmetric [2 + 2] cycloaddition: total synthesis of (-)-swainsonine and (+)-6-epicastanospermine.

[reaction: see text] An asymmetric total synthesis of (-)-swainsonine and (+)-6-epicastanospermine is described from a common intermediate, which is obtained through diastereoselective [2 + 2] cycloaddition of dichloroketene to a chiral enol ether.