A compound of Formula I wherein: R is O-R 1 or NHR 1; or R and R 5 together form a bond; R' is N(R 4)C(R 2a)(R 2b)C(=O)OR 3 or -OR 3; R 1 is H, lower haloalkyl, or aryl, wherein aryl is phenyl or naphthyl, optionally substituted with one or more lower alkyl, lower alkenyl, lower alkynyl, lower alkoxy, halo, lower haloalkyl, -N(R 1a) 2, acylamino, -SO 2 N(R 1a) 2, -COR 1b, -SO 2 (R 1c), -NHSO 2 (R 1c), nitro or cyano; each R 1a is independently H or lower alkyl; each R 1b is independently -OR 1a or -N(R 1a) 2; each R 1c is lower alkyl; R 1' is -C(R 2a)(R 2b)C(=O)OR 3; each R 2a and R 2b are independently H, lower alkyl, -(CH 2) r N(R 1a) 2, lower hydroxyalkyl, -CH 2 SH, -(CH 2)S p Me, -(CH 2) 3 NHC(=NH)NH 2, (1 H -indol-3-yl)methyl, (1 H- indol-4-yl)methyl, - m C(=O)R 1b, aryl and aryl lower alkyl, wherein aryl may optionally be substituted with one or more hydroxy, lower alkyl, lower alkoxy, halo, nitro or cyano; or R 2a is H and R 2b and R 4 together form (CH 2) n; each R 3 is H, lower alkyl, lower haloalkyl, phenyl or phenyl lower alkyl; each R 4 is H, lower alkyl, or R 2b and R 4 together form (CH 2) 3; R 5 is H, C(=O)R 1c, C(=O)R 1b, P(=O)(OR 1)(OR 1a), orP(=O)(OR 1)(NR 4 R 7); R 6 is OH or F; R 7 is C(R 2a R 2b)C(=O)OR 3 m is 0 to 3; n is 3, 4 or 5; p is 0 to 2; r is 1 to 6; X is O or S; and Base is cytosine, which may optionally be substituted with one or more hydroxy, lower alkyl, lower alkoxy, halo, nitro or cyano; or a pharmacologically acceptable salt thereof. R is O-R 1 or NHR 1; or R and R 5 together form a bond; R' is N(R 4)C(R 2a)(R 2b)C(=O)OR 3 or -OR 3; R 1 is H, lower haloalkyl, or aryl, wherein aryl is phenyl or naphthyl, optionally substituted with one or more lower alkyl, lower alkenyl, lower alkynyl, lower alkoxy, halo, lower haloalkyl, -N(R 1a) 2, acylamino, -SO 2 N(R 1a) 2, -COR 1b, -SO 2 (R 1c), -NHSO 2 (R 1c), nitro or cyano; each R 1a is independently H or lower alkyl; each R 1b is independently -OR 1a or -N(R 1a) 2; each R 1c is lower alkyl; R 1' is -C(R 2a)(R 2b)C(=O)OR 3; each R 2a and R 2b are independently H, lower alkyl, -(CH 2) r N(R 1a) 2, lower hydroxyalkyl, -CH 2 SH, -(CH 2)S p Me, -(CH 2) 3 NHC(=NH)NH 2, (1 H -indol-3-yl)methyl, (1 H- indol-4-yl)methyl, - m C(=O)R 1b, aryl and aryl lower alkyl, wherein aryl may optionally be substituted with one or more hydroxy, lower alkyl, lower alkoxy, halo, nitro or cyano; or R 2a is H and R 2b and R 4 together form (CH 2) n; each R 3 is H, lower alkyl, lower haloalkyl, phenyl or phenyl lower alkyl; each R 4 is H, lower alkyl, or R 2b and R 4 together form (CH 2) 3; R 1 is H, lower haloalkyl, or aryl, wherein aryl is phenyl or naphthyl, optionally substituted with one or more lower alkyl, lower alkenyl, lower alkynyl, lower alkoxy, halo, lower haloalkyl, -N(R 1a) 2, acylamino, -SO 2 N(R 1a) 2, -COR 1b, -SO 2 (R 1c), -NHSO 2 (R 1c), nitro or cyano; each R 1a is independently H or lower alkyl; each R 1b is independently -OR 1a or -N(R 1a) 2; each R 1c is lower alkyl; each R 1a is independently H or lower alkyl; each R 1b is independently -OR 1a or -N(R 1a) 2; each R 1c is lower alkyl; R 1' is -C(R 2a)(R 2b)C(=O)OR 3; each R 2a and R 2b are independently H, lower alkyl, -(CH 2) r N(R 1a) 2, lower hydroxyalkyl, -CH 2 SH, -(CH 2)S p Me, -(CH 2) 3 NHC(=NH)NH 2, (1 H -indol-3-yl)methyl, (1 H- indol-4-yl)methyl, - m C(=O)R 1b, aryl and aryl lower alkyl, wherein aryl may optionally be substituted with one or more hydroxy, lower alkyl, lower alkoxy, halo, nitro or cyano; or R 2a is H and R 2b and R 4 together form (CH 2) n; each R 3 is H, lower alkyl, lower haloalkyl, phenyl or phenyl lower alkyl; each R 4 is H, lower alkyl, or R 2b and R 4 together form (CH 2) 3; R 5 is H, C(=O)R 1c, C(=O)R 1b, P(=O)(OR 1)(OR 1a), orP(=O)(OR 1)(NR 4 R 7); R 6 is OH or F; R 7 is C(R 2a R 2b)C(=O)OR 3 m is 0 to 3; n is 3, 4 or 5; p is 0 to 2; r is 1 to 6; m is 0 to 3; n is 3, 4 or 5; p is 0 to 2; r is 1 to 6; X is O or S; and Base is cytosine, which may optionally be substituted with one or more hydroxy, lower alkyl, lower alkoxy, halo, nitro or cyano;