Patent ID: 8835393

Claim:
A compound of formula I: wherein X 1 , X 2 and X 3 are each independently O or S; Y is CH 2 ; A is a 6-member aromatic ring or a 6-member heteroaromatic ring comprising 1 to 4 heteroatoms, wherein the aromatic ring and heteroaromatic ring are each substituted with hydroxyl, halogen, alkoxy, aryl, or a heterocycle; wherein each aryl and heterocycle substituent is optionally substituted with hydroxyl, halogen, mercapto, carboxyl, alkyl, alkoxy, haloalkyl, amino, nitro, cyano, cycloalkyl, aryl or a heterocycle; R 1 is H; R 2 is alkyl or cycloalkyl, wherein each alkyl and cycloalkyl is optionally substituted with hydroxyl, mercapto, halogen, amino, carboxyl, alkyl, haloalkyl, alkoxy or alkylthio; R 3 is H or alkyl; R 4 and R 4 ′ are each independently H or alkyl, with the proviso that when one of R 4 and R 4 ′ is H, the other is alkyl; R 5 and R 5 ′ are each independently H or alkyl; and R 6 and R 6 ′ are each independently H or alkyl; or a pharmaceutically acceptable salt thereof.