Table 1.
Optimization of the conditions for the 5’-phosphitylation.
 | ||||
| entrya | activatorb | solvent | concentration (mM) | yield (%)c,d |
| 1 | BMT | CH2Cl2 | 5 | 61 (24) |
| 2 | BMT | CH2Cl2 | 25 | 91 (0) |
| 3 | BMT | CH2Cl2 | 100 | 0 (81) |
| 4 | BMT | THF | 25 | 0 (86) |
| 5 | tetrazole | CH2Cl2 | 25 | 83 (11) |
| 6 | DCI | CH2Cl2 | 25 | 40 (46) |
aReactions were carried out using 3.0 equiv of 2-cyanoethyl-N,N,N',N'-tetraisopropylphosphorodiamidite in the presence of 3 Å MS at rt for 30 min. b3.0 equiv was used. cYields were determined by 31P NMR analysis. dRecovered starting material was reported in parenthesis.