Abstract: The invention relates to novel compounds of formula I ##STR1## wherein X is a direct bond, sulfur or --CHR.sub.5 --, R.sub.1 is hydrogen, C.sub.1 -C.sub.25 alkyl, unsubstituted or C.sub.1 -C.sub.4 alkyl-substituted C.sub.5 -C.sub.8 cycloalkyl; unsubstituted or C.sub.1 -C.sub.4 alkyl-substituted C.sub.5 -C.sub.8 cycloalkenyl; unsubstituted or C.sub.1 -C.sub.4 alkyl-substituted phenyl; C.sub.7 -C.sub.9 phenylalkyl or --CH.sub.2 --S--R.sub.6 ; R.sub.2 is hydrogen, C.sub.1 -C.sub.25 alkyl, C.sub.2 -C.sub.24 alkenyl, unsubstituted or C.sub.1 -C.sub.4 alkyl-substituted C.sub.5 -C.sub.8 cycloalkyl; unsubstituted or C.sub.1 -C.sub.4 alkyl-substituted C.sub.5 -C.sub.8 cycloalkenyl; unsubstituted or C.sub.1 -C.sub.4 alkyl-substituted phenyl; C.sub.7 -C.sub.9 -phenylalkyl or --CH.sub.2 S--R.sub.6 ; R.sub.3 is hydrogen or methyl; R.sub.4 is C.sub.1 -C.sub.8 alkyl, O.sup..cndot., OH, NO, --CH.sub.2 CN, C.sub.1 -C.sub.18 alkoxy, C.sub.5 -C.sub.12 cycloalkoxy, C.sub.3 -C.sub.6 alkenyl, C.sub.3 -C.sub.6 alkynyl, C.sub.1 -C.