A compound of general formula wherein R 1 is selected from H, aryl and alkyl; R 2 is selected from H, alkyl, aryl and CO-R 6; R 3 is selected from H, halogen, alkyl, alkenyl, alkynyl, aryl, NHR 7, NR 7 R 8, OR 7 and SR 7; R 4 is selected from (C 6 -C 12) alkyl, (C 2 -C 12) alkenyl, (C 2 -C 12) alkynyl and (C 6 -C 10) aryl, wherein aryl is unsubstituted or substituted by 1 to 5 alkyl groups, wherein the alkyl, alkenyl and alkynyl groups are linear, branched or cyclic and wherein the alkynyl group is optionally substituted with one to three OH groups; R 5 is H; R6 is alkyl optionally substituted with halogen, OH, SH, NH2, O-alkyl, S-alkyl, NH-alkyl or NH-di(alkyl); R 7 and R 8 identical or different are H or alkyl optionally substituted with halogen, OH, SH, NH 2, O-alkyl, S-alkyl, NH-alkyl or NH-di (alkyl); their pharmaceutically acceptable salts and/or optical, geometric and tautomeric isomers, tautomers, solvates or isotopic variations thereof; wherein unless otherwise specified: - Alkyl denotes a straight- or branched hydrocarbon chain comprising 1 to 6 carbon atoms - Cycloalkyl denotes a cyclic hydrocarbon ring comprising from 3 to 8 carbon atoms, - Alkenyl denotes a straight- or branched hydrocarbon chain comprising 2 to 6 carbon atoms and one or more double bonds. - Alkynyl denotes a straight- or branched hydrocarbon chain containing 2 to 6 carbon atoms and one or more triple bonds and optionally one or more double bonds. - Aryl denotes a aromatic carbon ring comprising from 6 to 10 carbon atoms. R 1 is selected from H, aryl and alkyl; R 2 is selected from H, alkyl, aryl and CO-R 6; R 3 is selected from H, halogen, alkyl, alkenyl, alkynyl, aryl, NHR 7, NR 7 R 8, OR 7 and SR 7; R 4 is selected from (C 6 -C 12) alkyl, (C 2 -C 12) alkenyl, (C 2 -C 12) alkynyl and (C 6 -C 10) aryl, wherein aryl is unsubstituted or substituted by 1 to 5 alkyl groups, wherein the alkyl, alkenyl and alkynyl groups are linear, branched or cyclic and wherein the alkynyl group is optionally substituted with one to three OH groups; R 5 is H; R6 is alkyl optionally substituted with halogen, OH, SH, NH2, O-alkyl, S-alkyl, NH-alkyl or NH-di(alkyl); R 7 and R 8 identical or different are H or alkyl optionally substituted with halogen, OH, SH, NH 2, O-alkyl, S-alkyl, NH-alkyl or NH-di (alkyl); their pharmaceutically acceptable salts and/or optical, geometric and tautomeric isomers, tautomers, solvates or isotopic variations thereof; wherein unless otherwise specified: - Alkyl denotes a straight- or branched hydrocarbon chain comprising 1 to 6 carbon atoms - Cycloalkyl denotes a cyclic hydrocarbon ring comprising from 3 to 8 carbon atoms, - Alkenyl denotes a straight- or branched hydrocarbon chain comprising 2 to 6 carbon atoms and one or more double bonds. - Alkynyl denotes a straight- or branched hydrocarbon chain containing 2 to 6 carbon atoms and one or more triple bonds and optionally one or more double bonds. - Aryl denotes a aromatic carbon ring comprising from 6 to 10 carbon atoms. - Alkyl denotes a straight- or branched hydrocarbon chain comprising 1 to 6 carbon atoms - Cycloalkyl denotes a cyclic hydrocarbon ring comprising from 3 to 8 carbon atoms, - Alkenyl denotes a straight- or branched hydrocarbon chain comprising 2 to 6 carbon atoms and one or more double bonds. - Alkynyl denotes a straight- or branched hydrocarbon chain containing 2 to 6 carbon atoms and one or more triple bonds and optionally one or more double bonds. - Aryl denotes a aromatic carbon ring comprising from 6 to 10 carbon atoms.