Azines of formula (I): ##STR1## in which R and R' either are identical and represent an alkyl radical containing 1 to 4 carbon atoms or an alkenyl radical containing 3 to 5 carbon atoms, or together form a radical of formula (II):--CHR.sub.1 --(CR.sub.2 R.sub.3).sub.n --CHR.sub.4 -- (II)in which R.sub.1, R.sub.2, R.sub.3 and R.sub.4, identical or different, represent a hydrogen atom or an alkyl radical containing 1 to 4 carbon atoms and n represents 0 or 1, in their different stereoisomer forms, their preparation process and their use.