Compound of Formula IIn (Formula IIa where R15, R16 and R17 are H, R41 is P, R43 is S(CH2)0-3R1) is prepared by reacting compound of Formula IId (Formula IIa where R15, R16 and R17 are H, R41 is P, and R43 is a halogen, R59, e.g. chloro) with a compound of Formula R1???(CH2)0-3???SH in the presence of a base (e.g. sodium hydride, potassium carbonate) in an appropriate solvent (e.g. dimethylformami