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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.