The invention is directed to the use of compounds of formula (I) - R3 and R4 are independently a hydrogen atom a C1-C6 alkyl group or a C2-C6 alkenyl group - R5 is a CI-C6 alkyl group a C2-C6 alkenyl group or a (CH2)o.z-aryl group - R6 is a C1-C6 alkyl group a C2-C6 alkenyl group a (CH2)o-2-aryl group or a C5-C6 cycloalkyl or cycloalkenyl group and - R7 is a hydrogen atom a C1-C6 alkyl group or a C2-C6 alkenyl group or - R3 R4 and R5 are as above defined and - R6 and R7 together with the carbon atom to which they are attached form a C5-C6 cycloalkyl or cycloalkenyl group.
