provided that m>k; wherein R6 is a hydrogen atom or a lower alkyl group; R7 is a hydrogen atom, a lower alkyl group, a lower alkoxy group, an acyloxy group, a saturated heterocyclic oxy group, or a group represented by R8O???CO???(CH2)z???O??? wherein R8 is an alkyl group and z is zero or an integer; R21 is a hydrogen atom or a lower alkyl group; R22 is a cyano group, an esterified or non-esterifi