A compound having the structure of Formula , or a pharmaceutically acceptable salt thereof, wherein R 1a is selected from the group consisting of H, C 1 -C 4 alkyl, -NH 2 , -NHAc, -COOH, -SO 2 CH 3 , -SCH 3 , -OCH 3 , and A, wherein the alkyl is optionally substituted with -OH, -NH 2 , -NHAc, -COOH, -SO 2 CH 3 , -SCH 3 , -OCH 3 , , or A; R 1b is C 2 -C 5 alkyl; X is selected from the group consisting of H and C 1 -C 4 alkyl, wherein the alkyl is optionally substituted with A, -OH, or -C(CH 3 ) 2 OH; L 1 is selected from the group consisting of a bond, -CH 2 -, -CF 2 -, -O-, -S-, -SO 2 -, -NH-, and -CH 2 CH 2 -; Y is selected from the group consisting of C 1 -C 3 alkyl, aryl, and heteroaryl, wherein the alkyl, aryl, and heteroaryl are optionally substituted with 1-5 substituents that are independently selected from A, C 1 -C 3 alkyl, and C 1 -C 3 alkoxy; A is selected from the group consisting of L 2 is selected from the group consisting of a bond, -(CH 2 ) n -, -C(O)NH(CH 2 ) n -, -[O(CH 2 CH 2 )] n -, -[O(C 1 -C 4 alkylene)]-, -[O(CH 2 CH 2 )] n -OCH 2 CH 2 CF 2 -; -C(O)NHCH 2 CH 2 -[O(CH 2 CH 2 )] m -; and -C(O)NHCH 2 CH 2 -[O(CH 2 CH 2 )] m -OCH 2 CH 2 CF 2 -; m is an integer from zero to four; n is an integer from one to four; wherein the compound is substituted with at least one A; and when X is -CH 3 ; L 1 is -CH 2 -; Y is aryl substituted with A; and L 2 is -CH 2 -; then A is not -L 2 -COOH, except when R 1a comprises -COOH or -SO 2 CH 3 ; and when X is -CH 3 ; L 1 is -CH 2 -; Y is aryl substituted with A; L 2 is -CH 2 -, -O-(CH 2 ) 2 -O(CH 2 ) 2 -, or ―O-(CH 2 ) 2 -O(CH 2 ) 2 (CF 2 )-; and A is then A and L 1 are not in a para position with respect to each other. R 1a is selected from the group consisting of H, C 1 -C 4 alkyl, -NH 2 , -NHAc, -COOH, -SO 2 CH 3 , -SCH 3 , -OCH 3 , and A, wherein the alkyl is optionally substituted with -OH, -NH 2 , -NHAc, -COOH, -SO 2 CH 3 , -SCH 3 , -OCH 3 , , or A; R 1b is C 2 -C 5 alkyl; X is selected from the group consisting of H and C 1 -C 4 alkyl, wherein the alkyl is optionally substituted with A, -OH, or -C(CH 3 ) 2 OH; L 1 is selected from the group consisting of a bond, -CH 2 -, -CF 2 -, -O-, -S-, -SO 2 -, -NH-, and -CH 2 CH 2 -; Y is selected from the group consisting of C 1 -C 3 alkyl, aryl, and heteroaryl, wherein the alkyl, aryl, and heteroaryl are optionally substituted with 1-5 substituents that are independently selected from A, C 1 -C 3 alkyl, and C 1 -C 3 alkoxy; A is selected from the group consisting of L 2 is selected from the group consisting of a bond, -(CH 2 ) n -, -C(O)NH(CH 2 ) n -, -[O(CH 2 CH 2 )] n -, -[O(C 1 -C 4 alkylene)]-, -[O(CH 2 CH 2 )] n -OCH 2 CH 2 CF 2 -; -C(O)NHCH 2 CH 2 -[O(CH 2 CH 2 )] m -; and -C(O)NHCH 2 CH 2 -[O(CH 2 CH 2 )] m -OCH 2 CH 2 CF 2 -; m is an integer from zero to four; n is an integer from one to four; wherein the compound is substituted with at least one A; and when X is -CH 3 ; L 1 is -CH 2 -; Y is aryl substituted with A; and L 2 is -CH 2 -; then A is not -L 2 -COOH, except when R 1a comprises -COOH or -SO 2 CH 3 ; and when X is -CH 3 ; L 1 is -CH 2 -; Y is aryl substituted with A; L 2 is -CH 2 -, -O-(CH 2 ) 2 -O(CH 2 ) 2 -, or ―O-(CH 2 ) 2 -O(CH 2 ) 2 (CF 2 )-; and A is then A and L 1 are not in a para position with respect to each other. <EOS>