Cationic Cyclization of a Substrate Having an Internal Acetylenic Bond. Synthesis of Euphol and Tirucallol

Article abstract of DOI:10.1021/jo00294a043

New methodological studies directed toward the synthesis of tetracyclic triterpenoids bearing the 8,9-olefinic bond involve the acid-catalyzed cyclization of the dienediynol 9 as the key step.Surprisingly the ring closure, giving mainly 10, leads to the euphane rather than the lanostane ring system ( see Scheme I).This discovery has made possible the first synthesis of euphol as well as its C-20 epimer tirucallol.New methodology was developed for the highly stereoselective production of tetrasubstituted olefinic bonds during the synthesis of the pro-C-13,14 olefinic bond of cyclization substrate 9 ( Scheme II).The constitution of ketone 10 was established by conversion into the dione 30 as shown in Scheme III.This racemic material was rigorously identified with the natural enantiomer, prepared by degradation of euphol.Dione l-30, obtained by degradation of the natural product was used for reconstruction of the C-17 side chaine to give euphol (1a) and tirucallol (1c); thus a formal totally synthesic pathway to these products has been established.