Vinylogous Esters and Amides: Useful Synthons for Diversity-Oriented and Natural Product Synthesis

The first total synthesis of boehmeriasin A was achieved in seven steps from readily available materials with an overall yield of 29%. The absolute stereochemistry of the natural product was determined to be of the (R)-configuration. (-)-(R)-Boehmeriasin A demonstrated potent cytotoxicity in several cancer cell lines including drug resistant cancer cells where paclitaxel is inactive. An efficient and mild method for the construction if chromones was developed that requires only a single purification step at the end of the sequence and results in moderate to good yields of the isolated chromones. This method was applied toward the synthesis of 5-hydroxy-2-(2-phenylethyl)chromone, a neuroprotective chromone. The reduction of tertiary amides using Cp2Zr(H)Cl to the corresponding aldehydes on a preparatory scale results in good isolated yields of the products. Through a modified workup procedure, issues not previously observed were overcome to afford good isolated yields of the corresponding aldehydes.