STUDIES DIRECTED TOWARD THE TOTAL SYNTHESIS OF CERORUBENIC ACID-III .1. EXPEDIENT CONSTRUCTION OF THE TETRACYCLIC CORE BY OXYANIONIC SIGMATROPY

A synthesis of the ABCD ring framework of cerorubenic acid-III is desc ribed. Diketone 5 was first prepared by intramolecular oxidative coupl ing of the dienolate 10 and then suitably desymmetrized to deliver 15b . Anionic oxy-Cope rearrangement of this intermediate resulted in cons truction of 18, a ketone not only having all three contiguous stereoge nic centers properly established but also equipped with adequate funct ionality for the further elaboration of ring D. In the present effort, this thrust took the form of homologation to 21, conversion to the ac tivated diene 22, and Diels-Alder cycloaddition to methyl acrylate at high pressure. Once it became obvious that first-formed ketone 26 grea tly preferred adoption of trans stereochemistry at the ring juncture, attempts to skirt this issue were made by preparing both 29 and 32. Ho wever, these advanced intermediates proved unresponsive to conjugate r eduction, and attention was therefore redirected to alternative possib le means for elaboration of the eastern sector.