Table 1. Phosphitylation Reaction Conditionsa.
| isolated
phosphoramidite ratio |
||||||
|---|---|---|---|---|---|---|
| entry | ribonucleoside mixture (1 equiv) | mixture ratio (2′-OAc:3′-OAc) | reaction conditions (equiv) | 2′-OAc (a) | 3′-OAc (b) | |
| 1 | 14a+14b | 1:3 | 17 (1.3) | DIPEA (4) | 1 | 5 |
| 2 | 14a+14b | 1:3 | 18 (2.0) | DCI (2) | 1 | 4 |
| 3 | 14a+14b | 1:3 | 18 (1.2) | 1H-Tetrazole (1) | 1 | 4 |
| 4 | 14a+14b | 1:3 | 18 (1.2) | BTT (1) | 1 | 3 |
| 5 | 16a+16b | 1:5 | 17 (1.3) | DIPEA (4) | 1 | 7 |
| 6 | 16a+16b | 1:5 | 18 (1.2) | BTT (1) | 1 | 3.7 |
a
Reactions were conducted in anhydrous THF (0.1 M) at room temperature (rt) and reactions times within a range of 1–3 h. 1H-Tetrazole (0.45 M) and 5-benzylthio-1H-tetrazole (BTT, 0.3 M) were added to the reaction as a solution in anhydrous MeCN. DIPEA = N,N-diisopropylethylamine (Hünig’s base), DCI = 4,5-dicyanoimidazole.