REVISED STRUCTURES OF PLENIRADIN AND BAILEYIN AND THEIR BEARING ON THE BIOGENESIS OF HELENANOLIDES

One of us has proposed2 a biogenetic scheme for the class of sesquiterpene lactones known as helenanolides which is depicted in Scheme I. It was speculated that the formation of these compounds is initiated by enzyme-mediated anti-Markownikoff cyclization of a cis-1(10),irans-4, 5-germacradienolide3 or the equivalent 4, 5-epoxide similar to the acidcatalyzed process shown. The resulting trans-fused guaianolideion could stabilize itself by proton expulsion or reaction with a nucleophile; alternatively, it could undergo a series ofrearrangements (arrows) to a helenanolide.6. © 1979, American Chemical Society. All rights reserved.