Total Synthesis of (−)-Tuberostemonine

The first total synthesis of the complex pentacyclic Stemona alkaloid tuberostemonine was accomplished in 24 steps and in 1.4% overall yield from a hydroindole intermediate which is readily obtained in three steps from Cbz-l-tyrosine. An innovative synthetic strategy was applied that relays the single stereocenter of the amino acid precursor into nine of the ten stereogenic carbons of the target molecule. Among the highlights of the synthetic methodology are the 3-fold use of ruthenium catalysis, first in an azepine ring-closing metathesis and then in an alkene isomerization followed by a cross-metathesis propenyl-vinyl exchange, as well as the stereoselective attachment of the gamma-butyrolactone moiety to the core tetracycle by use of a lithiated ortho ester.