Liquid-crystalline medium, method of its stabilization and liquid crystal display

The invention relates to compounds of formula I, and a liquid-crystalline medium, preferably having a nematic phase and a negative dielectric anisotropy, which a) one or more compounds of the formula I and b) one or more compounds of the formula II includes wherein the parameters have the respective meanings given in claim 1, containing, its use in an electro-optical display, especially in an active matrix display based on the VA, ECB, PALC, FFS or IPS effect, such displays which contain such a liquid-crystalline medium, and the use of the compounds of formula I for the stabilization of a liquid-crystalline medium containing one or more compounds of formula II.