Total synthesis of the monoterpenoid alkaloid (±)-tangutorine.

A novel approach to a formal total synthesis of the monoterpenoid indole alkaloid (±)-tangutorine has been developed starting from an α,β-unsaturated cyclic dehydroamino ester. Synthesis of the rather unusual trans-substituted 2,3-indoloquinolizidine substructure was accomplished via Cu(II)-mediated conjugate addition and organozinc/copper coupling as the key steps, thereby setting the stage for ring-closing metathesis to produce the quinolone substructure. Finally, Bischler-Napieralski cyclization gave rise to the pentacyclic system of (±)-tangutorine thereby realizing a formal synthesis in an overall yield of 5.2% in eight consecutive steps.