Total synthesis of natural bicyclic lactones (+)-dihydrocanadensolide, (±)-avenociolide, and (±)-isoavenociolide via tungsten-π-allyl complexes

A synthetic method using tungsten-pi-allyl compounds is developed for total syntheses of natural bislactones including (+)-dihydrocanadensaolide (2), (+/-)-avenociolide (3), and (+/-)-isoavenociolide (4). Syntheses of these natural compounds are based on a common intermediate, trans-alpha-methylene butyrolactones bearing an anti-homoallylic alcohol. The kep steps in the syntheses involve an intramolecular alkoxycarbonylation of propargyltungsten complexes to yield tungsten-pi,gamma-lactonyl species, followed by condensation of their CpW(NO)I(pi-allyl) derivatives with suitable aldehydes. This new method is very efficient for the synthesis of(+/-)-avenociolide (3) and (+/-)-isoavenociolide (4). Total syntheses of compounds 3 and 4 require only six and three steps, respectively, on the basis of chloropropargyl derivatives.