Asymmetric Total Synthesis of Sarpagine and Koumine Alkaloids

Abstract

We report here a concise, collective, and asymmetric total synthesis of sarpagine alkaloids and biogenetically related koumine alkaloids, which structurally feature a rigid cage scaffold, with L-tryptophan as the starting material. Two key bridged skeleton-forming reactions, namely tandem sequential oxidative cyclopropanol ring-opening cyclization and ketone α-allenylation, ensure concurrent assembly of the caged sarpagine scaffold and installation of requisite derivative handles. With a common caged intermediate as the branch point, by taking advantage of ketone and allene groups therein, total synthesis of five sarpagine alkaloids (affinisine, normacusine B, trinervine, N_a -methyl-16-epipericyclivine, and vellosimine) with various substituents and three koumine alkaloids (koumine, koumimine, and N-demethylkoumine) with more complex cage scaffolds has been accomplished.

Keywords: allene; cyclopropanol; koumine; sarpagine; total synthesis.