kyl group), a group of the formula ##STR21## (wherein R15 represents a hydrogen atom or a lower alkyl group; and R14 represents a lower alkyl group), a group of the formula ##STR22## (wherein R16 represents a hydrogen atom or a lower alkyl group; and q is an integer of 1 to 3) or a group of the formula --(CH2)p --Z (wherein Z represents a group of the formula --SH, a group of the formula --SCN or