Phosphorous protecting groups and methods of preparation and use thereof

A compound having the structural formula (VII)

• wherein
• each of R1 and R2 is an isopropyl or R1 and R2 together form a pyrrolidine heterocyclic ring with N to which they are attached;
• R3 is an acid-labile protecting group;
• R′ is a group selected from a cyanoethyl group and a methyl group;
• and
• B is a protected nucleobase selected from:

An array of nucleic acids synthesized by a method comprising:

• (a) providing a nucleoside residue having an unprotected hydroxyl group;
• and
• (b) contacting the nucleoside residue with a nucleoside monomer to covalently bond the nucleoside monomer to the nucleoside residue, wherein the nucleoside monomer has the structural formula (I):
• wherein
• B is a nucleobase or an analogue thereof;
• wherein the B comprises an amidine nucleobase protecting group;
• and wherein the protecting group is described by the following structure:
• wherein R21 and R22 are each independently an alkyl, a substituted alkyl, or R21 and R22 are cyclically linked to form a 5- or 6-membered substituted or unsubstituted heterocycle;
• each of R1 and R2 is independently a linear, branched or cyclic, substituted or un-substituted alkyl, or R1 and R2 together form a 5-, 6-, 7- or 8-membered non-aromatic ring;
• R3 is an acid-labile protecting group;
• and
• R is a group selected from the group consisting of a benzyl, alpha-methyl aryl, naphthalene, bi-cyclic aliphatic, S-ethylthioate and an amino acid, with the proviso that R is not o-methyl benzyl.

A library comprising a plurality of nucleic acids synthesized by a method comprising:

• (a) providing a nucleoside residue having an unprotected hydroxyl group;
• and
• (b) contacting the nucleoside residue with a nucleoside monomer to covalently bond the nucleoside monomer to the nucleoside residue, wherein the nucleoside monomer has the structural formula (I):
• wherein
• B is a nucleobase or an analogue thereof;
• wherein the B comprises an amidine nucleobase protecting group;
• and wherein the protecting group is described by the following structure:
• wherein R21 and R22 are each independently an alkyl, a substituted alkyl, or R21 and R22 are cyclically linked to form a 5- or 6-membered substituted or unsubstituted heterocycle;
• each of R1 and R2 is independently a linear, branched or cyclic, substituted or un-substituted alkyl, or R1 and R2 together form a 5-, 6-, 7- or 8-membered non-aromatic ring;
• R3 is an acid-labile protecting group;
• and
• R is a group selected from the group consisting of a benzyl, alpha-methyl aryl, naphthalene, bi-cyclic aliphatic, S-ethylthioate and an amino acid, with the proviso that R is not o-methyl benzyl.