Towards Stereochemical Control: Two Approaches for the Highly anti ‐Diastereoselective Construction of the Spirolactone Moieties of Some Stemona Alkaloids

Some Stemona alkaloids belonging to the tuberostemospironine group possess a spirolactone moiety with anti ‐configuration (C‐9/C‐9a). In this paper, we describe two approaches to this structural unity. By using bromine atom as a traceless directing group, the SmI 2 ‐mediated reductive coupling of ketone 6 and β ‐bromomethacrylate proceeded with complete anti ‐diastereoselectivity. In the absence of an α ‐directing (chelation) group, the one‐pot reaction of the ketone derived from alcohol 15 with the organozinc reagent generated from bromomethacrylate afforded spiro‐ α ‐methylene‐ γ ‐lactone derivative 16 as a single diastereomer. These two highly diastereoselective methods would find application in the synthesis of stemona alkaloids containing anti ‐configured spiro‐lactone/pyrrolidine moieties. In addition, on the basis of our previous work, the total synthesis of (−)‐9‐ epi ‐11‐demethylsessilifoliamide J ( 11 ), and an improved synthesis of (−)‐9,11‐di‐ epi ‐sessilifoliamide J ( 9 ) were accomplished.