| CPC C07H 21/02 (2013.01) [A61K 31/7084 (2013.01); A61K 45/06 (2013.01)] | 20 Claims | 
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               1. A compound of Formula (I): 
              ![]() or a pharmaceutically acceptable salt or stereoisomer thereof, wherein: 
                Z is either S or O; 
                B1 is a natural purinyl nucleobase and B2 is a natural pyrimidinyl nucleobase or B2 is a natural purinyl nucleobase and B1 is a natural pyrimidinyl nucleobase; 
                each of X1 and X2 is independently O or S; 
                each of Y1 and Y2 is independently O, S, or NR5; 
                L1 is absent, C1-C6 alkyl or C1-C6 heteroalkyl and L2 is C1-C6 alkyl or C1-C6 heteroalkyl, wherein each alkyl and heteroalkyl is optionally substituted with R6; or L1 is C1-C6 alkyl or C1-C6 heteroalkyl, and L2 is absent, C1-C6 alkyl, or C1-C6 heteroalkyl, wherein each alkyl and heteroalkyl is optionally substituted with R6; 
                each of R1 and R2 is independently hydrogen, halo, —CN, C1-C20 alkyl, or OR7; 
                each of R3 and R4 is independently hydrogen, C1-C20 alkyl, C1-C20 heteroalkyl, OC(O)OC1-C20 alkyl, cycloalkyl, heterocyclyl, aryl, or heteroaryl, wherein each alkyl, heteroalkyl, cycloalkyl, heterocyclyl, aryl, and heteroaryl is optionally substituted with one or more R8; 
                R5 is hydrogen or C1-C20 alkyl; 
                R6 is halo, —CN, C1-C20 alkyl, OR7, oxo, cycloalkyl, heterocyclyl, aryl, or heteroaryl, wherein each alkyl, cycloalkyl, heterocyclyl, aryl, and heteroaryl is optionally substituted with one or more R9; 
                R7 is hydrogen, C1-C20 alkyl, cycloalkyl, heterocyclyl, aryl, or heteroaryl, wherein each alkyl, cycloalkyl, heterocyclyl, aryl, and heteroaryl is optionally substituted with one or more R9; 
                each R8 is independently C1-C20 alkyl, C1-C20 heteroalkyl, C(O)—C1-C20 alkyl, OC(O)—C1-C20 alkyl, C(O)O—C1-C20 alkyl, OC(O)O—C1-C20 alkyl, C(O)N(R5)—C1-C20 alkyl, N(R5)C(O)—C1-C20 alkyl, OC(O)N(R5)—C1-C20 alkyl, O-aryl, O-heteroaryl, C(O)-aryl, C(O)-heteroaryl, OC(O)-aryl, C(O)O-aryl, OC(O)-heteroaryl, C(O)O-heteroaryl, C(O)N(R5)-aryl, C(O)N(R5)-heteroaryl, N(R5)C(O)-aryl, N(R5)2C(O)-aryl, N(R5)C(O)-heteroaryl, or S(O)2N(R5)-aryl, wherein each alkyl, heteroalkyl, aryl, and heteroaryl is optionally substituted by one or more R9; and 
                each R9 is independently C1-C20 alkyl, O—C1-C20 alkyl, C1-C20 heteroalkyl, halo, —CN, OH, oxo, aryl, heteroaryl, O-aryl, or O-heteroaryl. 
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