The invention relates to the compounds of general formula I: ##STR1## in which: R.sub.1 represents a hydrogen atom, a linear or branched alkyl radical containing from 1 to 6 carbon atoms, a linear or branched alkenyl radical containing from 2 to 6 carbon atoms, an arylalkyl radical having 7 to 10 carbon atoms and which can bear a halogen atom as a substituent on the aromatic ring, a hydroxyl radical or an alkyl or alkoxy radical each containing from 1 to 5 carbon atoms, a 2-indolylmethyl radical, a 4-imidazolylmethyl radical or an alkoxycarbonylmethyl radical containing from 3 to 11 carbon atoms,R.sub.2 and R.sub.3, which may be identical or different, each represents a linear or branched alkyl radical containing from 1 to 6 carbon atoms,n is equal to 1 or 2,R.sub.4 represents a hydrogen atom, a formyl radical or a methyl radical, with the proviso, however, that R.sub.4 is never the methyl radical when n is equal to 2, andeither R.sub.5 and R.sub.6 together form a double bond, or R.sub.5 represents a hydrogen atom and R.sub.6 a hydroxyl radical,in the form of a mixture of diastereoisomers or of pure isomers, their N.sub.b,-oxides and their addition salts with a pharmaceutically acceptable inorganic or organic acid.These compounds are antitumor agents.