A compound having formula: wherein X 1 is -O-, -S-, or -NR c -; wherein R c is hydrogen, (C 1 -C 10)alkyl, or substituted (C 1 -C 10)alkyl, or R c and R 1 taken together with the nitrogen atom can form a heterocyclic ring or a substituted heterocyclic ring, wherein the substituents on the alkyl, aryl or heterocyclic groups are hydroxy, (C 1 -C 6)alkyl, hydroxy(C 1 -C 6)alkylene, (C 1 -C 6)alkoxy, (C 3 -C 6)cycloalkyl, (C 1 -C 6)alkoxy(C 1 -C 6)alkylene, amino, cyano, halogen, or aryl; R 1 is hydrogen, (C 1 -C 10)alkyl, substituted (C 1 -C 10)alkyl, (C 6 -C 10)aryl, or substituted (C 6 -C 10)aryl, (C 5 -C 9)heterocyclic, substituted (C 5 -C 9)heterocyclic; wherein the substituents on the alkyl, aryl or heterocyclic groups are hydroxy, (C 1 -C 6)alkyl, hydroxy(C 1 -C 6)alkylene, (C 1 -C 6)alkoxy, (C 3 -C 6)cycloalkyl, (C 1 -C 6)alkoxy(C 1 -C 6)alkylene, amino, cyano, halogen, or aryl; each R 2 is independently hydrogen, -OH, (C 1 -C 6)alkyl, substituted (C 1 -C 6)alkyl, (C 1 -C 6)alkoxy, substituted (C 1 -C 6)alkoxy, -C -(C 1 -C 6)alkyl (alkanoyl), substituted -C -(C 1 -C 6)alkyl, -C -(C 6 -C 10)aryl (aroyl), substituted -C -(C 6 -C 10)aryl, -C OH (carboxyl), -C O(C 1 -C 6)alkyl (alkoxycarbonyl), substituted -C O(C 1 -C 6)alkyl, -NR a R b, -C NR a R b (carbamoyl), substituted -C NR a R b, halo, nitro, or cyano; wherein the substituents on the alkyl, aryl or heterocyclic groups are hydroxy, (C 1 -C 6)alkyl, hydroxy(C 1 -C 6)alkylene, (C 1 -C 6)alkoxy, (C 3 -C 6)cycloalkyl, (C 1 -C 6)alkoxy(C 1 -C 6)alkylene, amino, cyano, halogen, or aryl; and n is 1, 2, 3 or 4; each R a and R b is independently hydrogen, (C 1 -C 6)alkyl, (C 3 -C 8)cycloalkyl, (C 1 -C 6)alkoxy, halo(C 1 -C 6)alkyl, (C 3 -C 8)cycloalkyl(C 1 -C 6)alkyl, (C 1 -C 6)alkanoyl, hydroxy(C 1 -C 6)alkyl, aryl, aryl(C 1 -C 6)alkyl, Het, Het (C 1 -C 6)alkyl, or (C 1 -C 6)alkoxycarbonyl; X 2 is a bond or a linking group; and R 3 is a protein or a peptide; or a pharmaceutically acceptable salt thereof. wherein X 1 is -O-, -S-, or -NR c -; wherein R c is hydrogen, (C 1 -C 10)alkyl, or substituted (C 1 -C 10)alkyl, or R c and R 1 taken together with the nitrogen atom can form a heterocyclic ring or a substituted heterocyclic ring, wherein the substituents on the alkyl, aryl or heterocyclic groups are hydroxy, (C 1 -C 6)alkyl, hydroxy(C 1 -C 6)alkylene, (C 1 -C 6)alkoxy, (C 3 -C 6)cycloalkyl, (C 1 -C 6)alkoxy(C 1 -C 6)alkylene, amino, cyano, halogen, or aryl; R 1 is hydrogen, (C 1 -C 10)alkyl, substituted (C 1 -C 10)alkyl, (C 6 -C 10)aryl, or substituted (C 6 -C 10)aryl, (C 5 -C 9)heterocyclic, substituted (C 5 -C 9)heterocyclic; wherein the substituents on the alkyl, aryl or heterocyclic groups are hydroxy, (C 1 -C 6)alkyl, hydroxy(C 1 -C 6)alkylene, (C 1 -C 6)alkoxy, (C 3 -C 6)cycloalkyl, (C 1 -C 6)alkoxy(C 1 -C 6)alkylene, amino, cyano, halogen, or aryl; each R 2 is independently hydrogen, -OH, (C 1 -C 6)alkyl, substituted (C 1 -C 6)alkyl, (C 1 -C 6)alkoxy, substituted (C 1 -C 6)alkoxy, -C -(C 1 -C 6)alkyl (alkanoyl), substituted -C -(C 1 -C 6)alkyl, -C -(C 6 -C 10)aryl (aroyl), substituted -C -(C 6 -C 10)aryl, -C OH (carboxyl), -C O(C 1 -C 6)alkyl (alkoxycarbonyl), substituted -C O(C 1 -C 6)alkyl, -NR a R b, -C NR a R b (carbamoyl), substituted -C NR a R b, halo, nitro, or cyano; wherein the substituents on the alkyl, aryl or heterocyclic groups are hydroxy, (C 1 -C 6)alkyl, hydroxy(C 1 -C 6)alkylene, (C 1 -C 6)alkoxy, (C 3 -C 6)cycloalkyl, (C 1 -C 6)alkoxy(C 1 -C 6)alkylene, amino, cyano, halogen, or aryl; and n is 1, 2, 3 or 4; each R a and R b is independently hydrogen, (C 1 -C 6)alkyl, (C 3 -C 8)cycloalkyl, (C 1 -C 6)alkoxy, halo(C 1 -C 6)alkyl, (C 3 -C 8)cycloalkyl(C 1 -C 6)alkyl, (C 1 -C 6)alkanoyl, hydroxy(C 1 -C 6)alkyl, aryl, aryl(C 1 -C 6)alkyl, Het, Het (C 1 -C 6)alkyl, or (C 1 -C 6)alkoxycarbonyl; X 2 is a bond or a linking group; and R 3 is a protein or a peptide; or a pharmaceutically acceptable salt thereof.