Monomers and oligonucleotides comprising cycloaddition adduct(s)

1. A compound of formula XIIa or pharmaceutically acceptable salts thereof:

wherein:

R a and R b are independently hydrogen, an activated phosphate group, an activated phosphite group, a phosphoramidite, a solid support, —P(Z 1 )(Z 2 )-OH, —P(Z 1 )(Z 2 )—O-nucleoside, or —P(Z′)(Z 2 )—O-oligonucleotide;

Y 1 -Y 6 are each independently O, S, NR N , or CR P 2 ;

each linker can be the same or different;

J is independently for each occurrence absent, O, S, NR N , OC(O)NH, NHC(O)O, C(O)NH, NHC(O), NHSO, NHSO 2 , NHSO 2 NH, OC(O), C(O)O, OC(O)O, NHC(O)NH, NHC(S)NH, OC(S)NH, O-N═CH, OP(N(R N ) 2 )O, or OP(N(R N ) 2 );

X is O, S, NR N or CR P 2 ;

E is absent or C(O), C(O)O, C(O)NH, C(S), C(S)NH, SO, SO 2 , or SO 2 NH;

R L is hydrogen or a ligand;

R N is independently for each occurrence H, optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted cycloalkyl, optionally substituted aralkyl, optionally substituted heteroaryl or an amino protecting group;

R P is independently for each occurrence H, optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted cycloalkyl or optionally substituted heteroaryl; and

Z 1 and Z 2 are each independently for each occurrence O, S or optionally substituted alkyl.

2. The compound of claim 1 , where R L is selected from the group consisting of:

3. The compound of claim 1 , wherein each linker is represented by structure

[P-Q 1 -R] q -T-,

wherein:

P, R and T are each independently for each occurrence absent, CO, NH, O, S, S-S, OC(O), NHC(O), CH 2 , CH 2 NH, CH 2 O, NHCH(R a )C(O), —C(O)—CH(R a )—NH-, —C(O)-(optionally substituted alkyl)-NH-, CH═N-O,

acetal, ketal,

Q 1 is independently for each occurrence absent, -(CH 2 ) n -, -C(R 100 )(R 200 )(CH 2 ) n —, —(CH 2 ) n C(R 100 )(R 200 )—, —(CH 2 CH 2 O) m CH 2 CH 2 —, or -(CH 2 CH 2 O) m CH 2 CH 2 NH-; provided that at least one of Q 1 , P, R, and T is not absent;

R a is H or an amino acid side chain;

R 100 and R 200 are each independently for each occurrence H, CH 3 , OH, SH or N(R X ) 2 ;

R X is independently for each occurrence H, methyl, ethyl, propyl, isopropyl, butyl or benzyl;

q is independently for each occurrence 0-20;

n is independently for each occurrence 1-20; and

m is independently for each occurrence 0-50.

4. The compound of claim 1 , wherein R a or R b contains an oligonucleotide.

5. The compound of claim 4 , wherein the oligonucleotide is a single-stranded oligonucleotide.

6. The compound of claim 4 , wherein the oligonucleotide is a double-stranded oligonucleotide.