New isoindolone derivatives of general formula (I) are ##STR1## disclosed in which the radicals R represent hydrogen atoms or, together, form a bond, the symbol R' represents a hydrogen atom or a readily removable radical and the symbols R" are identical and represent phenyl radicals which can be substituted with a halogen atom or a methyl radical at the ortho or meta position, in the (3aR, 7aR) form or in the form of a mixture of the (3aRS, 7aRS) forms, as well as their salts.These derivatives are useful as intermediates for the preparation of therapeutically active products.