5′ phosphate mimics

1. A nucleoside represented by formula (IV):

or isomers thereof, wherein:

D 1 is H, a hydroxyl protecting group, a solid support, or a reactive phosphorus group;

K is O, S, NR′, or optionally substituted alkyl;

B is selected from the group consisting of hydrogen, unsubstituted or substituted aliphatic, a natural nucleobase, a modified nucleobase, and a universal nucleobase;

R is H, OH, SH, halogen, optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted alkoxy, optionally substituted alkylthio, optionally substituted alkylamino, or optionally substituted dialkylamino;

L and J are independently absent or selected from the group consisting of S, O, NR′, alkylene, alkenylene, and alkynylene;

R′ is hydrogen, acyl, aliphatic or substituted aliphatic; and

W 1 and V 1 are each independently selected from the group consisting of a boronic group B(OH) 2 , and

or alternatively, W 1 and V 1 , taken together with the atoms to which they are bonded, form a phosphorous-atom-containing heterocyclic ring;

Z is O, S or NR′;

Z 1 is O or S,

X and Y are each independently selected from the group consisting of H, O − , OH, SH, SM, OM, borane (BH 3 ), optionally substituted alkyl, optionally substituted alkoxy, optionally substituted alkylthio, optionally substituted alkylamino, and optionally substituted dialkylamino; or X and Y, taken together with the atoms to which they are bonded, form

wherein

is an optionally substituted C 2 -C 8 alkylene;

M is a metal counter ion with an overall charge of +1; and

m and n are independently 1-10.

2. The nucleoside of claim 1 , wherein V 1 and W 1 are taken together with the atom to which they are bonded to form a nucleoside represented by formula (V):

or isomers thereof, wherein:

D 1 is H, a hydroxyl protecting group, a solid support, or a reactive phosphorus group;

K is O, S, NR′, or an optionally substituted alkyl;

B is selected from the group consisting of hydrogen, unsubstituted or substituted aliphatic, a natural nucleobase, a modified nucleobase, and a universal nucleobase;

X is H, OH, OM, SH, SM, optionally substituted alkyl, optionally substituted alkoxy, optionally substituted alkylthio, optionally substituted alkylamino, or optionally substituted dialkylamino, where M is a metal counter ion;

Z is O, S, or NR′;

R is H, OH, SH, halogen, optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted alkoxy, optionally substituted alkylthio, optionally substituted alkylamino, or optionally substituted dialkylamino;

R′ is hydrogen, acyl, or unsubstituted or substituted aliphatic;

L and J are each independently absent or selected from the group consisting of S, O, NR′, alkylene, alkenylene, and alkynylene; and

m and n are each independently 1-10.

3. An oligonucleotide comprising a nucleoside represented by formula (IVa):

or isomers thereof, wherein:

R is H, OH, SH, halogen, optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted alkoxy, optionally substituted alkylthio, optionally substituted alkylamino, optionally substituted dialkylamino, or an internucleoside linking group attaching the nucleoside to the oligonucleotide;

T 1 is hydroxyl, a protected hydroxyl, a linked conjugate group or an internucleoside linking group attaching the nucleoside to the oligonucleotide, wherein at least one of T 1 and R is an internucleoside linking group attaching the nucleoside to the oligonucleotide;

K is independently for each occurrence O, S, NR′, or optionally substituted alkyl;

each B is selected from the group consisting of hydrogen, unsubstituted or substituted aliphatic, a natural nucleobase, a modified nucleobase, and a universal nucleobase;

L and J are each independently absent or selected from the group consisting of S, O, NR′, alkylene, alkenylene, and alkynylene;

W 1 and V 1 are independently selected from the group consisting of a boronic group (B(OH) 2 ), and

or alternatively, W 1 and V 1 , taken together with the atoms to which they are bonded, form a phosphorous-atom-containing heterocyclic ring;

Z is O, S, or NR′;

Z 1 is O or S;

R′ is hydrogen, acyl, or unsubstituted or substituted aliphatic;

X and Y are each independently selected from the group consisting of H, O − , OH, SH, SM, OM, borane (BH 3 ), optionally substituted alkyl, optionally substituted alkoxy, optionally substituted alkylthip, optionally substituted alkylamino, and optionally substituted dialkylamino; or X and Y, taken together with the atoms to which they are bonded, form

where

is an optionally substituted C 2 -C 8 alkylene;

M is a metal counter ion; and

m and n are each independently 1-10.

4. The oligonucleotide of claim 3 , wherein the nucleoside is represented by formula (Va):

or isomers thereof, wherein:

R is H, OH, SH, halogen, optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted alkoxy, optionally substituted alkylthio, optionally substituted alkylamino, optionally substituted dialkylamino, or an internucleoside linking group attaching the nucleoside to the oligonucleotide;

T 1 is hydroxyl, a protected hydroxyl, a linked conjugate group or an internucleoside linking group attaching the nucleoside to the oligonucleotide, wherein at least one of Ti and R is an internucleoside linking group attaching the nucleoside to the oligonucleotide;

K is independently for each occurrence O, S, NR′, or optionally substituted alkyl;

each B is selected from the group consisting of hydrogen, unsubstituted or substituted aliphatic, a natural nucleobase, a modified nucleobase, and a universal nucleobase;

X is H, OH, OM, SH, SM, optionally substituted alkyl, optionally substituted alkoxy, optionally substituted alkylthio, optionally substituted alkylamino, or optionally substituted dialkylamino, where M is a metal counter ion;

Z is O, S, or NR′;

R′ is hydrogen, acyl, or unsubstituted or substituted aliphatic;

L and J are each independently absent or selected from the group consisting of S, O, NR′, alkylene, alkenylene, and alkynylene; and

m and n are each independently 1-10.

5. The oligonucleotide of claim 3 , wherein said oligonucleotide comprises at least one non-phosphodiester backbone linkage.

6. The oligonucleotide of claim 5 , wherein the non-phosphodiester of at least one of said non-phosphodiester backbone linkages is selected from a group consisting of phosphorothioate, phosphorodithioate, alkyl-phosphonate, and phosphoramidate.

7. The oligonucleotide of claim 5 , wherein at least one of the non-phosphodiester backbone linkages is placed between the first two nucleosides at the 5′-end and/or between the first two nucleosides at the 3′-end.

8. The oligonucleotide of claim 3 , wherein said oligonucleotide comprises at least one ligand conjugate.

9. The oligonucleotide of claim 3 , wherein said oligonucleotide is a double-stranded oligonucleotide comprising a first strand and a second strand.

10. The oligonucleotide of claim 9 , wherein the nucleoside is present either in the first strand or the second strand of the oligonucleotide.

11. The oligonucleotide of claim 10 , wherein said first strand is the sense strand.

12. The oligonucleotide of claim 10 , wherein said second strand is the antisense strand.

13. The oligonucleotide of claim 9 , wherein the nucleoside is present in both strands of the oligonucleotide.

14. The oligonucleotide of claim 3 , wherein said oligonucleotide has a hairpin structure.

15. The oligonucleotide of claim 3 , wherein said oligonucleotide is an antisense or a siRNA.

16. The oligonucleotide of claim 15 , wherein said oligonucleotide is a single-stranded siRNA.

17. A pharmaceutical composition, comprising the oligonucleotide of claim 3 and a pharmaceutically acceptable excipient.