The invention provides novel inhibitors of IAP that are useful as a therapeutic agents for treating malignancies where the compounds have the general formula I: X1 and X2 are independently O or S; L is a bond or —C(X3)—, —C(X3)NR12, —C(X3)O— wherein X3 is O or S and R12 is H or R1; R1 is alkyl, a carbocycle, carbocycle-substituted alkyl, a heterocycle or heterocycle-substituted alkyl, wherein each is optionally substituted with halogen, hydroxyl, mercapto, carboxyl, alkyl, haloalkyl, alkoxy, alkylsulfonyl, amino, nitro, aryl and heteroaryl; R2 is alkyl, cycloalkyl, cycloalkylalkyl, aryl, aralkyl, a heterocycle or heterocyclylalkyl; R3 is H or alkyl; R4 and R4′ are independently H, alkyl, aryl, aralkyl, cycloalkyl, cycloalkylalkyl, heteroaryl, or heteroaralkyl wherein each is optionally substituted with halogen, hydroxyl, mercapto, carboxyl, alkyl, alkoxy, amino and nitro; R5, and R5′ are each independently H or alkyl; R6 is H or alkyl; and salts and solvates thereof.