Enantioselective total syntheses of the pentacyclic5,11-methanomorphanthridine
Amaryllidaceaealkaloids(-)-montanine (
1), (-)-coccinine (
2), and(-)-pancracine (
3) were accomplished using anintramolecular concertedpericyclic allenylsilane imino ene cycloaddition as a key step.These complex natural products were constructedstarting from readily available enantiomerically pure epoxy alcohol
15 which was converted to allenylsilanealdehyde
28 via an efficient nine-step sequence. Theimine generated from aldehyde
28 and iminophosphorane
47 underwenta stereospecific thermal imino ene reaction to afford key intermediate
cis aminoalkyne
49. It was possible totransformthis compound
via Lindlar hydrogenation followed by anintramolecular Heck reaction to seven-membered ringtetracycle
51. This olefinic intermediate could befunctionalized through its epoxide to yield
![](/images/gifchars/alpha.gif)
-hydroxymethylintermediate
54, and then pentacyclic alcohol
64.Procedures were then developed to convert this material totheenantiomerically pure alkaloids
1-
3. Aformal enantioselective total synthesis of (-)-brunsvigine(
4) was alsoachieved
via triol
72.