Process for preparing a compound of the general formula I wherein R 1, R 2 and R 3 independently of each other are selected from hydrogen and methyl, R 4 is selected from C 1 -C 6 -alkyl, and X is selected from C 1 -C 20 -alkyl and C 2 -C 20 -alkenyl, comprising the following steps: a) providing a hydroquinone compound of the general formula II, wherein R 1, R 2 and R 3 independently of each other are selected from hydrogen and methyl, b) reacting the hydroquinone compound II provided in step a) with an unsaturated compound of the general formula III.a or III.b wherein X is selected from C 1 -C 20 -alkyl and C 2 -C 20 -alkenyl, Y is selected from OH, halogen, -O-R 11, -S-R 12 and -SO 2 -R 12, R 11 is selected from C 1 -C 4 -alkyl, C 1 -C 4 -alkanoyl and trifluoroacetyl, and R 12 is selected from C 1 -C 6 -alkyl, trifluoromethyl and phenyl, where phenyl is unsubstituted or substituted with 1, 2, 3, 4 or 5 radicals selected from halogen and methyl, in the presence of a bentonite catalyst, and c) reacting the condensation product obtained in step b) with a C 2 -C 7 -carboxylic acid or with a C 2 -C 7 -carboxylic acid anhydride in the presence of a bentonite catalyst, where the bentonite catalyst used in step b) has a BET surface area in the range of from 100 to 600 m 2 /g and a residual acidity, measured as mg KOH / g bentonite by titration with potentiometric indication, in the range of from 5 to 50, and where steps b) and c) are conducted in the presence of a polar aprotic solvent. R 1, R 2 and R 3 independently of each other are selected from hydrogen and methyl, R 4 is selected from C 1 -C 6 -alkyl, and X is selected from C 1 -C 20 -alkyl and C 2 -C 20 -alkenyl, comprising the following steps: a) providing a hydroquinone compound of the general formula II, wherein R 1, R 2 and R 3 independently of each other are selected from hydrogen and methyl, b) reacting the hydroquinone compound II provided in step a) with an unsaturated compound of the general formula III.a or III.b wherein X is selected from C 1 -C 20 -alkyl and C 2 -C 20 -alkenyl, Y is selected from OH, halogen, -O-R 11, -S-R 12 and -SO 2 -R 12, R 11 is selected from C 1 -C 4 -alkyl, C 1 -C 4 -alkanoyl and trifluoroacetyl, and R 12 is selected from C 1 -C 6 -alkyl, trifluoromethyl and phenyl, where phenyl is unsubstituted or substituted with 1, 2, 3, 4 or 5 radicals selected from halogen and methyl, in the presence of a bentonite catalyst, and c) reacting the condensation product obtained in step b) with a C 2 -C 7 -carboxylic acid or with a C 2 -C 7 -carboxylic acid anhydride in the presence of a bentonite catalyst, a) providing a hydroquinone compound of the general formula II, wherein R 1, R 2 and R 3 independently of each other are selected from hydrogen and methyl, b) reacting the hydroquinone compound II provided in step a) with an unsaturated compound of the general formula III.a or III.b wherein X is selected from C 1 -C 20 -alkyl and C 2 -C 20 -alkenyl, Y is selected from OH, halogen, -O-R 11, -S-R 12 and -SO 2 -R 12, R 11 is selected from C 1 -C 4 -alkyl, C 1 -C 4 -alkanoyl and trifluoroacetyl, and R 12 is selected from C 1 -C 6 -alkyl, trifluoromethyl and phenyl, where phenyl is unsubstituted or substituted with 1, 2, 3, 4 or 5 radicals selected from halogen and methyl, in the presence of a bentonite catalyst, and X is selected from C 1 -C 20 -alkyl and C 2 -C 20 -alkenyl, Y is selected from OH, halogen, -O-R 11, -S-R 12 and -SO 2 -R 12, R 11 is selected from C 1 -C 4 -alkyl, C 1 -C 4 -alkanoyl and trifluoroacetyl, and R 12 is selected from C 1 -C 6 -alkyl, trifluoromethyl and phenyl, where phenyl is unsubstituted or substituted with 1, 2, 3, 4 or 5 radicals selected from halogen and methyl, c) reacting the condensation product obtained in step b) with a C 2 -C 7 -carboxylic acid or with a C 2 -C 7 -carboxylic acid anhydride in the presence of a bentonite catalyst, where the bentonite catalyst used in step b) has a BET surface area in the range of from 100 to 600 m 2 /g and a residual acidity, measured as mg KOH / g bentonite by titration with potentiometric indication, in the range of from 5 to 50, and where steps b) and c) are conducted in the presence of a polar aprotic solvent.