. "A compound according to claim 1, wherein A is 3a group of the formula 1_, wherein R is selected from the group' consisting of C.C8 alkyl, C.Cg alkoxy, halogen, hydroxy, nitro, NHCOR , NR R or 5 6 7 trifluoromethyl, wherein R ,R and R are as defined in claim 1; and B is a group of the formula ^^) wherein r is 2,3 or 4." . . .