A compound of formula: wherein ---- is a single bond or a double bond; n is 0, 1, or 2; R 1 is SO 2 R a, in which R a is aryl, or heteroaryl; wherein the aryl is selected from a monovalent 6-carbon monocyclic, 10-carbon bicyclic, 14-carbon tricyclic aromatic ring system; and wherein the heteroaryl is selected from a monovalent aromatic 5-8 membered monocyclic, 8-12 membered bicyclic, or 11-14 membered tricyclic ring system having one or more heteroatoms, wherein the one or more heteroatoms are independently selected from O, N, S, and Se; wherein the aryl or heteroaryl may be optionally substituted by substituents selected from C 1 -C 10 alkyl, C 2 -C 10 alkenyl, C 2 -C 10 alkynyl, C 3 -C 20 cycloalkyl, C 3 -C 20 cycloalkenyl, C 1 -C 20 heterocycloalkyl, C 1 -C 20 heterocycloalkenyl, C 1 -C 10 alkoxy, aryl, aryloxy, heteroaryl, heteroaryloxy, amino, C 1 -C 10 alkylamino, arylamino, hydroxy, halo, oxo (0=), thioxo (S=), thio, silyl, C 1 -C 10 alkylthio, arylthio, C 1 -C 10 alkylsulfonyl, arylsulfonyl, acylamino, aminoacyl, aminothioacyl, amidino, mercapto, amido, thioureido, thiocyanato, sulfonamido, guanidine, ureido, cyano, nitro, acyl, thioacyl, acyloxy, carbamido, carbamyl (-C NH 2), carboxyl (-COOH), and carboxylic ester, wherein aryl and heteroaryl can also be fused with each other; and R2 is H, alkyl, NHC -CH=CH-C NR c R d, or C NR c R d, in which each of R c, and R d, independently, is H, or hydroxy; each of R 3, R 4, R 5, and R 6, independently is H, alkyl, CH=CH-C NR c R d, NHC -CH=CH-C NR c R d, or C NR c R d, in which one of R c and R d is H and the other is hydroxy; provided that one or more of R 2, R 3, R 4, R 5, and R 6 has the following definitions: R 2 is NHC -CH=CH-C NR c R d; or R 3, R 5, and R 6 is independently CH=CH-C NR c R d, or NHC -CH=CH-C NR c R d, or R 4 is C NHOH, CH=CH-C NR c R d, or NHC -CH=CH-C NR c R d. ---- is a single bond or a double bond; n is 0, 1, or 2; R 1 is SO 2 R a, in which R a is aryl, or heteroaryl; wherein the aryl is selected from a monovalent 6-carbon monocyclic, 10-carbon bicyclic, 14-carbon tricyclic aromatic ring system; and wherein the heteroaryl is selected from a monovalent aromatic 5-8 membered monocyclic, 8-12 membered bicyclic, or 11-14 membered tricyclic ring system having one or more heteroatoms, wherein the one or more heteroatoms are independently selected from O, N, S, and Se; wherein the aryl or heteroaryl may be optionally substituted by substituents selected from C 1 -C 10 alkyl, C 2 -C 10 alkenyl, C 2 -C 10 alkynyl, C 3 -C 20 cycloalkyl, C 3 -C 20 cycloalkenyl, C 1 -C 20 heterocycloalkyl, C 1 -C 20 heterocycloalkenyl, C 1 -C 10 alkoxy, aryl, aryloxy, heteroaryl, heteroaryloxy, amino, C 1 -C 10 alkylamino, arylamino, hydroxy, halo, oxo (0=), thioxo (S=), thio, silyl, C 1 -C 10 alkylthio, arylthio, C 1 -C 10 alkylsulfonyl, arylsulfonyl, acylamino, aminoacyl, aminothioacyl, amidino, mercapto, amido, thioureido, thiocyanato, sulfonamido, guanidine, ureido, cyano, nitro, acyl, thioacyl, acyloxy, carbamido, carbamyl (-C NH 2), carboxyl (-COOH), and carboxylic ester, wherein aryl and heteroaryl can also be fused with each other; and R2 is H, alkyl, NHC -CH=CH-C NR c R d, or C NR c R d, in which each of R c, and R d, independently, is H, or hydroxy; each of R 3, R 4, R 5, and R 6, independently is H, alkyl, CH=CH-C NR c R d, NHC -CH=CH-C NR c R d, or C NR c R d, in which one of R c and R d is H and the other is hydroxy; provided that one or more of R 2, R 3, R 4, R 5, and R 6 has the following definitions: R 2 is NHC -CH=CH-C NR c R d; or R 3, R 5, and R 6 is independently CH=CH-C NR c R d, or NHC -CH=CH-C NR c R d, or R 4 is C NHOH, CH=CH-C NR c R d, or NHC -CH=CH-C NR c R d.