Doctoral thesis is devoted to the studies of structure-antiradical/antioxidant activity relationships of the analogues of natural antioxidants avenanthramides. The literature review focuses on one of the most recent and during the last years widely studied antiradical mechanisms – SPLET (sequential proton loss and electron transfer) mechanism. Various extracts of antioxidants have been prepared from grains of oat and barley, seeds of buckwheat and Japanese quince, seeds, pomace and buds of black currant, as well as hulls of hop; applicability of these extracts for improvement of the oxidative stability of vegetable oils (and their methyl esters) has been explored in the experimental part of thesis. Synthesis of various analogues of natural antioxidants avenanthramides has been carried out: both small modifications (variation of the substituents in the aromatic rings and hydrogenation of the double bond) and construction of heterocyclic analogues (derivatives of quinolin-2(1H)-ones, quinazolin-4-ones, coumarins and Meldrum`s acid) have been realized. The new methodologies are elaborated and known synthetic methods are optimized to reach the target compounds. All synthesized compounds have been tested for their antiradical activity using free radicals 1,1-diphenyl-2-picrylhydrazyl and galvinoxyl. It has been found out that compounds containing moiety of the vanillin or syringaldehyde possess the highest antiradical activity; the 2-arylmethyl-1,3-dicarbonyl residue has appeared essential for antiradical activity, too. The new class of effective antioxidants – 5-arylmethyl-1,3-dioxane-4,6-diones – has been discovered. The plausible mechanisms of the reaction between free radical DPPH and each group of synthesized analogues of avenanthramides are discussed. The representatives of each group of obtained antioxidants have been tested to clear up their impact on the oxidative stability of vegetable oils and their methyl esters. The proton affinity and bond dissociation enthalpy of the most reactive OH and CH bonds of synthesized compounds have been calculated by HF-3-21g method; the values of these parameters are compared with the experimentally measured antiradical/antioxidant activity.