Compositions and methods for phosphoramidite and oligonucleotide synthesis

A composition comprising a compound having the structure of formula I:

• or a salt thereof and a silylating reagent;
• BA is
• SU is
• wherein SU is connected to the phosphorus atom in formula I through the oxygen atom;
• R5s is R′ or —OR′;
• R2s is —H, —F, —CN, —N3, —NO, —NO2, —O—(C1-10 alkyl), or —O-(optionally substituted C1-6 alkylene)-OR′, or R2s is L connecting C2 with C1, C2, C3, C4 or C5, wherein L is —O—CH2— wherein the —CH2— is optionally substituted;
• each R′ is independently —R, —C(O)R, —CO2R, or —SO2R, or:
• two or more R′ are taken together with their intervening atoms to form an optionally substituted monocyclic, bicyclic or polycyclic, saturated, partially unsaturated, or aryl 3-30 membered ring having, in addition to the intervening atoms, 0-10 heteroatoms independently selected from oxygen, nitrogen, sulfur, phosphorus and silicon;
• and
• each R is independently hydrogen, or an optionally substituted group selected from C1-30 aliphatic, C1-30 heteroaliphatic having 1-10 heteroatoms independently selected from oxygen, nitrogen, sulfur, phosphorus and silicon, C6-30 aryl, a 5-30 membered heteroaryl ring having 1-10 heteroatoms independently selected from oxygen, nitrogen, sulfur, phosphorus and silicon, and a 3-30 membered heterocyclic ring having 1-10 heteroatoms independently selected from oxygen, nitrogen, sulfur, phosphorus and silicon.