Modified guanines

1 . A compound according to Formula (1a) or (1b):

wherein,

R 1 is a phosphate or polyphosphate group or salt thereof, optionally containing one or more sulfur atoms;

R 3 is selected from H, OH, F, OCH 3 , or OCH 2 CH 2 OMe; and

either X is N and Y is N, or X is CR 2 and Y is CH or N, where

R 2 is an electron withdrawing group (EWG) selected from the group consisting of: F, Cl, nitrile; halomethyl, dihalomethyl, trihalomethyl; SOR 4 ; SO 2 R 4 ; SO 3 R 4 ; COR 4 ; CO 2 R 4 ; CONR 4 R 5 ; wherein R 4 and R 5 are independently selected from H and C 1-6 alkyl optionally substituted with OH or halo atoms.

2 . The compound according to claim 1 which is a compound of Formula (2a) or (2b):

wherein,

R 1 is a phosphate or polyphosphate group or salt thereof, optionally containing one or more sulfur atoms;

R 3 is selected from H, OH, F, OCH 3 , or OCH 2 CH 2 OMe; and

X is CR 2 , where R 2 is an electron withdrawing group (EWG) selected from the group consisting of: nitrile; halomethyl, dihalomethyl, trihalomethyl; SOR 4 ; SO 2 R 4 , SO 3 R 4 ; COR 4 ; CO 2 R 4 ; CONR 4 R 5 ;

wherein R 4 and R 5 are independently selected from H and C 1-6 alkyl optionally substituted with OH or halo atoms.

3 . The compound according to claim 1 , wherein R 2 is nitrile, halomethyl, dihalomethyl or trihalomethyl.

4 . The compound according to claim 3 , wherein R 2 is F or Cl.

5 . The compound according to claim 1 , wherein R 1 is —(PO 3 ) − x (PO 2 S) − y (PO 3 ) − z where x, y and z are independently 0-5 and x+y+z is 1-5.

6 . The compound according to claim 1 , wherein R 1 is a monophosphate, diphosphate, triphosphate, tetraphosphate, pentaphosphate, or (alpha-thio)triphosphate group.

7 . The compound according to claim 1 , wherein R 1 is a triphosphate group.

8 . The compound according to claim 1 , wherein R 3 is H.

9 . The compound according to claim 1 which is selected from the group consisting of

or a salt thereof.

10 . A method of nucleic acid synthesis comprising reacting a compound according to claim 1 with an oligonucleotide in the presence of a polymerase or terminal deoxynucleotidyl transferase (TdT) enzyme and treating the extended oligonucleotide with a nitrite salt.

11 . The method according to claim 10 , wherein the oligonucleotide sequence is a solid-supported oligonucleotide sequence.

12 . The method according to claim 9 , wherein the nitrite salt is sodium nitrite.

13 . A method of synthesizing a compound according to formula (1a):

wherein

R 1 is a phosphate or polyphosphate group or salt thereof, optionally containing one or more sulfur atoms;

R 3 is selected from H, OH, F, OCH 3 , or OCH 2 CH 2 OMe; and

either X is N and Y is N, or X is CR 2 and Y is CH or N, where

R 2 is an electron withdrawing group (EWG) selected from the group consisting of: F, Cl, nitrile; halomethyl, dihalomethyl, trihalomethyl; SOR 4 ; SO 2 R 4 ; SO 3 R 4 ; COR 4 ; CO 2 R 4 ; CONR 4 R 5 ; wherein R 4 and R 5 are independently selected from H and C 1-6 alkyl optionally substituted with OH or halo atoms;

comprising taking a compound according to Formula (1b):

wherein,

R 1 is a phosphate or polyphosphate group or salt thereof, optionally containing one or more sulfur atoms;

R 3 is selected from H, OH, F, OCH 3 , or OCH 2 CH 2 OMe; and

either X is N and Y is N, or X is CR 2 and Y is CH or N, where

R 2 is an electron withdrawing group (EWG) selected from the group consisting of: F, Cl, nitrile; halomethyl, dihalomethyl, trihalomethyl; SOR 4 ; SO 2 R 4 ; SO 3 R 4 ; COR 4 ; CO 2 R 4 ; CONR 4 R 5 ; wherein R 4 and R 5 are independently selected from H and C 1-6 alkyl optionally substituted with OH or halo atoms;

and treating the compounds of Formula (1b) with an aminooxy compound.

14 . The method according to claim 13 wherein the aminooxy compound is hydroxylamine, methoxylamine or ethoxylamine.

15 . A kit comprising:

a. a compound according to claim 1 ;

b. a terminal deoxynucleotidyl transferase (TdT) enzyme; and

c. a nitrite salt.

16 . An oligonucleotide according to Formula (1a) or (1b):

wherein,

R 1 is an oligonucleotide;

R 3 is selected from H, OH, F, OCH 3 , or OCH 2 CH 2 OMe; and

either X is N and Y is N, or X is CR 2 and Y is CH or N, where R 2 is an electron withdrawing group (EWG) selected from the group consisting of: F, Cl, nitrile; halomethyl, dihalomethyl, trihalomethyl; SOR 4 ; SO 2 R 4 ; SO 3 R 4 ; COR 4 ; CO 2 R 4 ; CONR 4 R 5 ; wherein R 4 and R 5 are independently selected from H and C 1-6 alkyl optionally substituted with OH or halo atoms.

17 . The oligonucleotide according to claim 16 wherein either Y is N and X is N; or Y is CH and X is CR 2 where R 2 is SO 2 CH 3 , CN or CF 3 .

18 . The oligonucleotide according to claim 16 , wherein R 3 is H.