A compound of Formula II: or a pharmaceutically acceptable salt, a tautomeric or polymorphic form thereof, wherein; Base is a substituted or unsubstituted purine or pyrimidine; W is alkyl, cycloalkyl, alkenyl, cycloalkenyl or alkynyl; X is O or S; Y is hydrogen or OR 9; Z is hydrogen, OR 10, SR 10, NR 5 R 6, F, Cl, Br or I; Q is OR 4, SR 4 or NR 5 R 6; each E is independently CR 7 R 8; each n is 1 or 2; R 1 and R 2 are selected as follows: i) R 1 and R 2 are each independently hydrogen, alkyl, alkenyl, alkynyl, aryl, aralkyl, cycloalkyl, cycloalkenyl, alkylheterocyclyl or alkylheteroaryl; ii) R 1 and R 2 together with the nitrogen atom on which they are substituted form a 3-7 membered heterocyclic or heteroaryl ring; or iii) R 1 is hydrogen, alkyl or cycloalkyl, and R 2 is m C Q 1, wherein Q 1 is OR 4, SR 4 or NR 5 R 6; each G is independently CR 7 R 8; and each m is 1 or 2; R 3 and R 4 are each independently hydrogen, alkyl, alkenyl, alkynyl, aryl, aralkyl, cycloalkyl or cycloalkenyl; R 5 and R 6 are selected as follows: i) R 5 and R 6 are each independently hydrogen, alkyl, alkenyl, alkynyl, aryl, aralkyl, cycloalkyl or cycloalkenyl; or ii) R 5 and R 6 together with the nitrogen atom on which they are substituted form a 3-7 membered heterocyclic or heteroaryl ring; R 7 and R 8 are selected as follows: i) R 7 and R 8 are each independently hydrogen, alkyl, alkenyl, alkynyl, aryl, aralkyl, cycloalkyl, cycloalkenyl, alkylheterocyclyl or alkylheteroaryl, wherein alkyl is optionally substituted by alkoxy; or ii) R 7 and R 8 together with the carbon atom to which they are attached form a 3-7 membered cycloalkyl ring; R 9 and R 10 are selected as follows: i) R 9 and R 10 are each independently hydrogen, alkyl, alkenyl, alkynyl, aryl, aralkyl, cycloalkyl or cycloalkenyl; or ii) R 9 and R 10 together with C and the oxygen or sufur atoms on which they are substituted form a 5 membered ring. Base is a substituted or unsubstituted purine or pyrimidine; W is alkyl, cycloalkyl, alkenyl, cycloalkenyl or alkynyl; X is O or S; Y is hydrogen or OR 9; Z is hydrogen, OR 10, SR 10, NR 5 R 6, F, Cl, Br or I; Q is OR 4, SR 4 or NR 5 R 6; each E is independently CR 7 R 8; each n is 1 or 2; R 1 and R 2 are selected as follows: i) R 1 and R 2 are each independently hydrogen, alkyl, alkenyl, alkynyl, aryl, aralkyl, cycloalkyl, cycloalkenyl, alkylheterocyclyl or alkylheteroaryl; ii) R 1 and R 2 together with the nitrogen atom on which they are substituted form a 3-7 membered heterocyclic or heteroaryl ring; or iii) R 1 is hydrogen, alkyl or cycloalkyl, and R 2 is m C Q 1, wherein Q 1 is OR 4, SR 4 or NR 5 R 6; each G is independently CR 7 R 8; and each m is 1 or 2; i) R 1 and R 2 are each independently hydrogen, alkyl, alkenyl, alkynyl, aryl, aralkyl, cycloalkyl, cycloalkenyl, alkylheterocyclyl or alkylheteroaryl; ii) R 1 and R 2 together with the nitrogen atom on which they are substituted form a 3-7 membered heterocyclic or heteroaryl ring; or iii) R 1 is hydrogen, alkyl or cycloalkyl, and R 2 is m C Q 1, wherein Q 1 is OR 4, SR 4 or NR 5 R 6; each G is independently CR 7 R 8; and each m is 1 or 2; Q 1 is OR 4, SR 4 or NR 5 R 6; each G is independently CR 7 R 8; and each m is 1 or 2; R 3 and R 4 are each independently hydrogen, alkyl, alkenyl, alkynyl, aryl, aralkyl, cycloalkyl or cycloalkenyl; R 5 and R 6 are selected as follows: i) R 5 and R 6 are each independently hydrogen, alkyl, alkenyl, alkynyl, aryl, aralkyl, cycloalkyl or cycloalkenyl; or ii) R 5 and R 6 together with the nitrogen atom on which they are substituted form a 3-7 membered heterocyclic or heteroaryl ring; i) R 5 and R 6 are each independently hydrogen, alkyl, alkenyl, alkynyl, aryl, aralkyl, cycloalkyl or cycloalkenyl; or ii) R 5 and R 6 together with the nitrogen atom on which they are substituted form a 3-7 membered heterocyclic or heteroaryl ring; R 7 and R 8 are selected as follows: i) R 7 and R 8 are each independently hydrogen, alkyl, alkenyl, alkynyl, aryl, aralkyl, cycloalkyl, cycloalkenyl, alkylheterocyclyl or alkylheteroaryl, wherein alkyl is optionally substituted by alkoxy; or ii) R 7 and R 8 together with the carbon atom to which they are attached form a 3-7 membered cycloalkyl ring; i) R 7 and R 8 are each independently hydrogen, alkyl, alkenyl, alkynyl, aryl, aralkyl, cycloalkyl, cycloalkenyl, alkylheterocyclyl or alkylheteroaryl, wherein alkyl is optionally substituted by alkoxy; or ii) R 7 and R 8 together with the carbon atom to which they are attached form a 3-7 membered cycloalkyl ring; R 9 and R 10 are selected as follows: i) R 9 and R 10 are each independently hydrogen, alkyl, alkenyl, alkynyl, aryl, aralkyl, cycloalkyl or cycloalkenyl; or ii) R 9 and R 10 together with C and the oxygen or sufur atoms on which they are substituted form a 5 membered ring. i) R 9 and R 10 are each independently hydrogen, alkyl, alkenyl, alkynyl, aryl, aralkyl, cycloalkyl or cycloalkenyl; or ii) R 9 and R 10 together with C and the oxygen or sufur atoms on which they are substituted form a 5 membered ring.