A unified strategy for the synthesis of phenanthroizidine alkaloids: Preparation of sterically congested pyridines

Total syntheses of the representative phenanthroizidine alkaloids, tylophorine and antofine, and of their seco congeners, septicine and julandine, have been completed from sterically congested 3,4-diarylpyridines prepared by a modified Knoevenagel-Stobbe synthesis. Central to the success of this effort was the ability of alpha-dicarbonyl enones to combine in a formal [4 + 2]-cycloaddition with sterically demanding vinyl ethers.