yl, 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, together with the linking carbon atom, form a cyclopentylene, cyclohexylene or cycloheptylene ring, each unsubstituted or substituted by 1 to 3 C1 -C4 alkyl groups; R4 and R5 are are each independently of the other H or C1 -C8 alkyl; R