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: ##STR00001## X.sub.1 and X.sub.2 are independently O or S; L is a bond or --C(X.sub.3)--, --C(X.sub.3)NR.sub.12, --C(X.sub.3)O-- wherein X.sub.3 is O or S and R.sub.12 is H or R.sub.1; R.sub.1 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; R.sub.2 is alkyl, cycloalkyl, cycloalkylalkyl, aryl, aralkyl, a heterocycle or heterocyclylalkyl; R.sub.3 is H or alkyl; R.sub.4 and R.sub.4' 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; R.sub.5, and R.sub.5' are each independently H or alkyl; R.sub.6 is H or alkyl; and salts and solvates thereof.