Stereocontrolled total syntheses of (±)-pisiferic acid and (±)-O-methylpisiferic acid

A stereocontrolled approach to the construction of the angularly ester substituted trans-octahydrophenanthrene ring system related to diterpenes involving an aryl participated diazoketone cyclisation strategy is delineated. The tetrahydrophenanthrenone 7 was prepared from the tetralone 6 through the intermediates 13, 14, 16 and 17. The formyl derivative of 7 was treated with alkaline H2O2 to give the diacid 18 which was converted into the diazomethyl ketone 9. Aryl participated cyclisation of 9 afforded the enedione 10 which was stereoselectively converted into the keto-ester 11. The transformation of 11 into the diterpenes 1 and 4 has been efficiently accomplished.