The invention relates to compounds of the formula I, and to a liquid-crystalline medium, preferably having a nematic phase and negative dielectric anisotropy, which comprises one or more compounds of formula I, defined herein, for use in an electro-optical display, particularly in an active-matrix display based on the VA, ECB, PALO, FFS or IPS effect, and to the use of the compounds of the formula I for the stabilization of a liquid-crystalline medium which comprises one or more compounds of the formula II and one or more compounds of the formulae III-1 to III-4, defined herein.