Novel compounds are disclosed having the structure of Formula I: wherein, n is an integer from 1 to 5; X is —CN, —F, —Cl, —Br, —I, —NO2; W is S, SO, SO2, O, NH, or NR5; R5 is alkyl, aralkyl, alkenyl, or alkynyl; Y is O, S; Z is NR1R2 or heterocycloalkyl; R1, R2 are, independently, hydrogen, alkyl, aralkyl, alkenyl, alkynyl, or cycloalkyl and may be optionally substituted with halo, hydroxy, alkoxy, amino, alkylamino, carboxy, alkoxycarbonyl, or nitrile groups; R3 and R4 are, independently, hydrogen, alkyl, aralkyl, alkenyl, alkynyl, or cycloalkyl and may be optionally substituted with halo, hydroxy, alkoxy, amino, alkylamino, carboxy, alkoxycarbonyl, or nitrile groups, and may together form a cyclic structure; and Ar is mono-, di-, or tri-substituted phenyl or heteroaryl, a pharmaceutically acceptable salts thereof. The compounds are potent bacterial efflux pump inhibitors (EPIs). Such compounds are useful to potentiate the antimicrobial activity of antimicrobial compounds such as beta-lactam antibiotics and quinolone antibiotics against Gram-negative bacteria.