A compound which (a) has a structure of formula (I"'): or (b) is a pharmaceutically acceptable salt thereof; wherein: R b is H or is a group selected from alkyl, alkenyl, alkynyl, acyl, -C O(alkyl), and-C O(aryl), wherein said group is optionally substituted by one or more substituents selected from hydroxy, halo, haloalkyl, alkoxy, -SH, -S-(alkyl), -SeH, -Se-(alkyl), aryl, heteroaryl, cycloalkyl, heterocycloalkyl, amino, carboxylic acid, ester, guanidino, and amido; X is O, or S; R 1 and R 2 are each independently selected from H or a group selected from alkyl, alkenyl, alkynyl, cycloalkyl, (cycloalkyl)alkyl, heterocycloalkyl, (heterocycloalkyl)alkyl, aryl, heteroaryl, aralkyl, and heteroaralkyl, wherein said group is optionally substituted by one or more substituents selected from hydroxy, halo, haloalkyl, alkoxy, -SH, -S-(alkyl), -SeH, -Se-(alkyl), aryl, heteroaryl, cycloalkyl, heterocycloalkyl, amino, carboxylic acid, ester, guanidino, and amido; or R 1 and R 2 are taken together with the intervening atoms to form a 3- to 7- membered ring, wherein the 3- to 7-membered ring is optionally substituted with one or more substituents selected from hydroxy, halo, haloalkyl, alkoxy, -SH, -S-(alkyl), -SeH, -Se-(alkyl), aryl, heteroaryl, cycloalkyl, heterocycloalkyl, amino, carboxylic acid, ester, guanidino, and amido; and R 3 is H or alkyl optionally substituted with one or more substituents selected from hydroxy, halo, haloalkyl, alkoxy, -SH, -S-(alkyl), -SeH, -Se-(alkyl), aryl, heteroaryl, cycloalkyl, heterocycloalkyl, amino, carboxylic acid, ester, guanidino, and amido; or R 1 and R 3 are taken together with the intervening atoms to form a 5- to 7-membered ring, wherein the 5- to 7-membered ring is optionally substituted with one or more substituents selected from hydroxy, halo, haloalkyl, alkoxy, -SH, -S-(alkyl), -SeH, -Se-(alkyl), aryl, heteroaryl, cycloalkyl, heterocycloalkyl, amino, carboxylic acid, ester, guanidino, and amido; and R 4 is H or (C 1 -C 6)alkyl. R b is H or is a group selected from alkyl, alkenyl, alkynyl, acyl, -C O(alkyl), and-C O(aryl), wherein said group is optionally substituted by one or more substituents selected from hydroxy, halo, haloalkyl, alkoxy, -SH, -S-(alkyl), -SeH, -Se-(alkyl), aryl, heteroaryl, cycloalkyl, heterocycloalkyl, amino, carboxylic acid, ester, guanidino, and amido; X is O, or S; R 1 and R 2 are each independently selected from H or a group selected from alkyl, alkenyl, alkynyl, cycloalkyl, (cycloalkyl)alkyl, heterocycloalkyl, (heterocycloalkyl)alkyl, aryl, heteroaryl, aralkyl, and heteroaralkyl, wherein said group is optionally substituted by one or more substituents selected from hydroxy, halo, haloalkyl, alkoxy, -SH, -S-(alkyl), -SeH, -Se-(alkyl), aryl, heteroaryl, cycloalkyl, heterocycloalkyl, amino, carboxylic acid, ester, guanidino, and amido; or R 1 and R 2 are taken together with the intervening atoms to form a 3- to 7- membered ring, wherein the 3- to 7-membered ring is optionally substituted with one or more substituents selected from hydroxy, halo, haloalkyl, alkoxy, -SH, -S-(alkyl), -SeH, -Se-(alkyl), aryl, heteroaryl, cycloalkyl, heterocycloalkyl, amino, carboxylic acid, ester, guanidino, and amido; and R 3 is H or alkyl optionally substituted with one or more substituents selected from hydroxy, halo, haloalkyl, alkoxy, -SH, -S-(alkyl), -SeH, -Se-(alkyl), aryl, heteroaryl, cycloalkyl, heterocycloalkyl, amino, carboxylic acid, ester, guanidino, and amido; or R 1 and R 3 are taken together with the intervening atoms to form a 5- to 7-membered ring, wherein the 5- to 7-membered ring is optionally substituted with one or more substituents selected from hydroxy, halo, haloalkyl, alkoxy, -SH, -S-(alkyl), -SeH, -Se-(alkyl), aryl, heteroaryl, cycloalkyl, heterocycloalkyl, amino, carboxylic acid, ester, guanidino, and amido; and R 4 is H or (C 1 -C 6)alkyl.