are each independently of one another H, unsubstituted C1 -C4 alkyl or C1 -C4 alkyl which is substituted by 1 to 3 OH groups, C3 -C20 alkyl which is interrupted by one or more than one oxygen atom, or are allyl, cyclopentyl, cyclohexyl, phenyl, benzyl or tolyl; L is a direct bond, --C(R11)(R12)-- or --S--;wherein R11 and R12 are each independently of the other H or C1 -C8 alkyl; or R11 and R12, to