Compounds of the formula (I) or salts thereof in which R1 is carboxyl or a derivative of a carboxyl group, preferably a radical of the formula —CN, —C(═X)—Y—R or —C(═X)-Het, in which X is O, S or NRa or N—NRaRb, Y is O, S, NRc or NRc—NRdRe, R is H, a (subst.) hydrocarbon radical (HR) or acyl, and Het is an aliphatic nitrogen heterocycle, where each of the radicals Ra, Rb, Rc, Rd and Re is as defined for R or a radical of the formula —OR*, where R* is as defined for R, R2 is H or (subst.) HR or a heterocyclic radical, or, if Z═O or S, also an acyl radical, (R3)n are n radicals R3, as defined in claim 1, Z is a group of the formula O or S(O)x, where x=0, 1 or 2, and n is an integer 0, 1, 2, 3 or 4, as safeners for crop plants or useful plants against phytotoxic actions of agrochemicals, preferably as safeners against the action of pesticides, such as herbicides, in these plants.