Knoevenagel-Stobbe synthesis

In 1996, Ciufoiini and Roschanger reported a total synthesis of phenanthroindolizidine alkaloids (tylophorine and antofine) from the sterically congested 2-substituted 4,5-diarylpyridine 61, which was prepared by a modified Knoevenagel-Stobbe synthesis [61]. Scheme (6) outlines the steps in their approach. Central to the success of this effort was the ability of a-dicarbonyl enone 58 to combine in a formal [4+2]-cycloaddition with the sterically demanding vinyl ether 56, The resultant pyran 59 was treated with DIBAL and protected with benzyl chloride giving 60, which was then treated with hydroxylamine hydrochloride to give diarylpyridine 61, After a two-step conversion of 61 to 63, a cyano... 

---