Patent ID: 8361988

Claim:
A compound of the formula or pharmaceutically acceptable salts thereof, wherein: X is O or S; A is —(CH 2 ) n M; n is 1 to 3; M is —OR 1 , —C(O)OR 2 , —CONR 4 R 4a , —SOR 5 , —SO 2 NR 5 R 5a , or —N 3 ; R 1 is —H, —COR 2 , —CONR 4 R 4a , —CH 2 —P(O)(OH) 2 , or —P(O)(OET) 2 ; R 2 is —H, aryl, or C 1 -C 6 alkyl; R 3 is aryl or C 1 -C 3 alkylaryl; R 4 and R 4a are independently together with the nitrogen to which they are attached form —(AA) x , wherein x is 1 to 5, and AA is a natural, non-natural, D- or L- amino acid, wherein —(AA) x comprises a terminal —COOR 3 group wherein the carbonyl is protected or unprotected; R 5 and R 5a are independently aryl or C 1 -C 3 alkylaryl; and Base is a purine derivative or a pyrimidine derivative selected from the group consisting of