The present invention is directed to compounds represented by the following structural formula, Formula I: ##STR00001## wherein: (a) X is selected from the group consisting of a single bond, O, S, S(O).sub.2 and N; (b) U is an aliphatic linker; (c) Y is selected from the group consisting of C, O, S, NH and a single bond; (d) E is C(R3)(R4)A or A and wherein (i) A is selected from the group consisting of carboxyl, tetrazole, C.sub.1-C.sub.6 alkylnitrile, carboxamide, sulfonamide and acylsulfonamide; (e) B is selected from the group consisting of S, O, C, and N; (f) Z is selected from the group consisting of N and C; with the proviso that when B is C then Z is N.