Phosphitylation process

A process for the preparation of a compound of formula:

• which comprises reacting a compound of formula:
• with a compound of formula:
• NCCH2CH2O—P(N(R16)2)2
• in the presence of an activator, where the activator comprises a compound of formula:
• and an organic base, wherein R4 is an alcohol protecting group, R5 is —H, —F —OR6, —NR7R8, —SR9, or a substituted or unsubstituted aliphatic group, such as methyl or allyl, R6 for each occurrence is —H, a substituted or unsubstituted aliphatic group, a substituted or unsubstituted aryl group, a substituted or unsubstituted aralkyl, an alcohol protecting group, or —(CH2)q—NR11R12, R7 and R8 are each, independently, —H, a substituted or unsubstituted aliphatic group, or an amine protecting group or R7 and R8 taken together with the nitrogen to which they are attached are a heterocyclyl group, R9 is —H, a substituted or unsubstituted aliphatic group, or a thiol protecting group, R11 and R12 are each, independently, —H, a substituted or unsubstituted aryl group, a substituted or unsubstituted heteroaryl group, a substituted or unsubstituted aliphatic group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted heteroaralkyl group or an amine protecting group or R11 and R12 taken together with the nitrogen to which they are attached form a heterocyclyl group, q is an integer from 1 to about 6, B is —H, a natural or unnatural nucleobase, protected nucleobase, protected natural or unnatural nucleobase, heterocycle or a protected heterocycle and R16 represents a C1-6 alkyl group, preferably an isopropyl group.