A compound of the formula or a pharmaceutically acceptable salt thereof, wherein X 1 is a bond and R 6 is hydrogen; X 2 is S 2 and R 4 is optionally substituted aryl; R 1 and R 2 are each hydrogen; R 3 is sulfonyl, acyl, amino, alkyl, heteroalkyl, cycloalkyl, cycloheteroalkyl, aryl, arylalkyl, heteroaryl, or heteroarylalkyl, each of which is optionally substituted; R 5 is arylalkyl, which is optionally substituted; Z is wherein * indicates the point of attachment; m is 0, 1, or 2; W 1 and W 2 are in each instance independently selected from the group consisting of optionally substituted alkylene, alkyleneoxy, alkyleneamino, alkylenethio, alkylenesulfoxyl, and alkylenesulfonyl; W 3 is selected from the group consisting of amino, oxygen, alkylene, alkyleneoxy, alkyleneamino, and heteroalkylene, wherein at least one of W 1 or W 2 is oxygen, and wherein when one of W 1 or W 2 is optionally substituted methylene, W 3 is oxygen or alkyleneoxy, and wherein Z does not include a peroxide bond, a sulfenate bond, or a sulfenamide bond; W 4 is optionally substituted ethylene or propylene; or Z is wherein * indicates the point of attachment; n is 1, 2, or 3; W 1 and W 2 are in each instance independently selected from the group consisting of optionally substituted methylene, oxygen, and amino; W 3 is selected from the group consisting of amino, oxygen, alkylene, and heteroalkylene, wherein at least one of W 1 or W 2 is oxygen, and wherein when one of W 1 or W 2 is optionally substituted methylene, W 3 is oxygen, and wherein Z does not include a peroxide bond, a sulfenate bond, or a sulfenamide bond; W 4 is optionally substituted ethylene or propylene; X 3 is a bond; and Y is hydrogen. X 1 is a bond and R 6 is hydrogen; X 2 is S 2 and R 4 is optionally substituted aryl; R 1 and R 2 are each hydrogen; R 3 is sulfonyl, acyl, amino, alkyl, heteroalkyl, cycloalkyl, cycloheteroalkyl, aryl, arylalkyl, heteroaryl, or heteroarylalkyl, each of which is optionally substituted; R 5 is arylalkyl, which is optionally substituted; Z is wherein * indicates the point of attachment; m is 0, 1, or 2; W 1 and W 2 are in each instance independently selected from the group consisting of optionally substituted alkylene, alkyleneoxy, alkyleneamino, alkylenethio, alkylenesulfoxyl, and alkylenesulfonyl; W 3 is selected from the group consisting of amino, oxygen, alkylene, alkyleneoxy, alkyleneamino, and heteroalkylene, wherein at least one of W 1 or W 2 is oxygen, and wherein when one of W 1 or W 2 is optionally substituted methylene, W 3 is oxygen or alkyleneoxy, and wherein Z does not include a peroxide bond, a sulfenate bond, or a sulfenamide bond; W 4 is optionally substituted ethylene or propylene; or wherein * indicates the point of attachment; m is 0, 1, or 2; W 1 and W 2 are in each instance independently selected from the group consisting of optionally substituted alkylene, alkyleneoxy, alkyleneamino, alkylenethio, alkylenesulfoxyl, and alkylenesulfonyl; W 3 is selected from the group consisting of amino, oxygen, alkylene, alkyleneoxy, alkyleneamino, and heteroalkylene, wherein at least one of W 1 or W 2 is oxygen, and wherein when one of W 1 or W 2 is optionally substituted methylene, W 3 is oxygen or alkyleneoxy, and wherein Z does not include a peroxide bond, a sulfenate bond, or a sulfenamide bond; W 4 is optionally substituted ethylene or propylene; or Z is wherein * indicates the point of attachment; n is 1, 2, or 3; W 1 and W 2 are in each instance independently selected from the group consisting of optionally substituted methylene, oxygen, and amino; W 3 is selected from the group consisting of amino, oxygen, alkylene, and heteroalkylene, wherein at least one of W 1 or W 2 is oxygen, and wherein when one of W 1 or W 2 is optionally substituted methylene, W 3 is oxygen, and wherein Z does not include a peroxide bond, a sulfenate bond, or a sulfenamide bond; W 4 is optionally substituted ethylene or propylene; X 3 is a bond; and Y is hydrogen. wherein * indicates the point of attachment; n is 1, 2, or 3; W 1 and W 2 are in each instance independently selected from the group consisting of optionally substituted methylene, oxygen, and amino; W 3 is selected from the group consisting of amino, oxygen, alkylene, and heteroalkylene, wherein at least one of W 1 or W 2 is oxygen, and wherein when one of W 1 or W 2 is optionally substituted methylene, W 3 is oxygen, and wherein Z does not include a peroxide bond, a sulfenate bond, or a sulfenamide bond; W 4 is optionally substituted ethylene or propylene; X 3 is a bond; and Y is hydrogen.