. "A compound of claim 13 having the stuctural formula Ib: ##STR136## where at least one of R', R and R' is other than H and each independently represents halo, phenyl, substituted phenyl, hydroxy, mercapto, alkyl of 2 to 6 carbon atoms, alkyl--C(O)--, alkyl substituted with a hydroxy or CF3 ; R3 and R4 are the same or different and are as defined in claim 13; and R represents a heterocyclic group" . . .