Compounds and methods for the treatment of cancer

A compound having a structure of formula (I) or a pharmaceutically acceptable salt thereof

• wherein
• X is S or Se;
• W is (R)(R), where each occurrence of R is independently H or C1-2alkyl;
• n is 0 or 1;
• R1 and R2 are each independently C1-10alkyl;
• R3 is —H, C1-10alkyl, or C0-6alkyl-COOR6;
• R3′ is H, amino, cyano, halogen, aryl, aralkyl, heteroaryl, heteroaralkyl, carboxyl, C1-10alkyl, C1-10alkenyl, or C1-10alkynyl;
• R4 is —OH, —H, —CH3, amino, —OC(O)C1-10aralkyl, —OC(O)C1-10alkyl, or —OC(O)aryl;
• and
• R5 is —OH, cyano, C1-6alkoxy, amino, O—C1-10alkyl, O-aralkyl, —OC(O)C1-10aralkyl, —OC(O)C1-10alkyl, —OC(O)aryl, or a glycine substituent;
• and
• R6 is H or C1-10alkyl.