## 2 Materials and Methods
### Synthesis of the Nucleoside Derivatives
#### Synthesis of 4-amino-6-oxo-1-(3',5'-O-t-butyldimethylsilyl-2',4'-dideoxy-p-ribityl) triazine (4)
for C\({}_{30}\)H\({}_{63}\)N\({}_{4}\)O\({}_{3}\)Si\({}_{2}\)\({}^{+}\) [M + H]\({}^{+}\) _m/z_ 615.4154, found _m/z_ 615.4158. 2.3 Synthesis of 2-(2-octylthioethyl)-6-oxo-4-acetylamino-1-(3',5'-O-t-butyldimethylsilyl-2', 4'-dideoxy-p-ribityI) triazine (7)  
AcCl (5.3 \(\upmu\)L, 74.7 \(\upmu\)mol) was added to a solution of **6** (15 mg, 24.9 \(\upmu\)mol) in pyridine (250 \(\upmu\)L). The mixture was stirred at rt for 19 h. The resulting solution was diluted with Et\({}_{2}\)O and quenched with NH\({}_{4}\)Cl. The product was extracted with Et\({}_{2}\)O and the combined organic layer was washed with brine, dried over Na\({}_{2}\)SO\({}_{4}\), filtered and evaporated. The crude product was purified by silica gel column chromatography (CHCl\({}_{3}\), then CHCl\({}_{3}\) with 1% Et\({}_{3}\)N) to afford **7** (13.4\(\upmu\)mol, 54%) as a colorless oil: \({}^{1}\)H-NMR (600 MHz, CDCl\({}_{3}\)) \(\delta\) 7.71 (br, 1H), 4.08-3.98 (m, 3H), 3.66 (t, 2H, \(J\) = 6.0 Hz), 3.65 (t, 2H, \(J\) = 6.0 Hz), 3.00 (t, 2H, \(J\) = 6.6 Hz), 2.91 (t, 2H, \(J\) = 6.6 Hz), 2.61 (s, 3H), 2.54 (t, 2H, \(J\) = 7.2 Hz), 1.95 (m, 1H), 1.81-1.64 (m, 3H), 1.58 (qt, 2H, \(J\) = 7.2, 7.2 Hz), 1.39-1.26 (m, 10 H), 0.92 (s, 9H), 0.91 (t, 3H, \(J\) = 7.2 Hz), 0.88 (s, 9H), 0.095 (s, 3H), 0.093 (s, 3H), 0.041 (s, 3H), 0.03 (s, 3H); \({}^{13}\)C-NMR (150 MHz, CDCl\({}_{3}\)) \(\delta\) 172.0, 170.2, 161.5, 154.8, 41.9, 40.0, 34.9, 34.1, 32.8, 29.7, 29.5, 29.2, 28.8, 27.7, 25.87, 25.86, 25.85, 22.6, 18.2, 18.0, 14.1, -4.52, -4.55, -5.38, -5.41; HRMS (ESI) calcd. for C\({}_{32}\)H\({}_{64}\)N\({}_{4}\)O\({}_{4}\)SSi\({}_{2}\)\({}^{+}\) [M + H]\({}^{+}\) _m/z_ 657.4246, found _m/z_ 657.4246. 2.2.4 Synthesis of 2-(2-octylthioethyl)-6-oxo-4-acetylamino-1-(2',4'-dideoxy-p-ribityI) triazine (8)  
Boron trifluoride-diethylether complex (51 \(\upmu\)L, 398 \(\upmu\)mol) was slowly added to a solution of **7** (131 mg, 199 \(\upmu\)mol) in CH\({}_{3}\)CN (800 \(\upmu\)L) at 0 \({}^{\circ}\)C. After the addition, the mixture was stirred at 0 \({}^{\circ}\)C for 2 h. The resulting solution was diluted with EtOAc, then quenched with phosphate buffer (pH = 6.0, 1 M). The product was extracted with EtOAc and the combined organic layer was washed with brine, dried over Na\({}_{2}\)SO\({}_{4}\), filtered and evaporated. The crude product was purified by silica gel chromatography, eluting with CH\({}_{3}\)OH/CHCl\({}_{3}\), 1:50, then 1:10 to give **8** (103 \(\upmu\)mol, 52%) as a white solid: \({}^{1}\)H-NMR (600 MHz, CDCl\({}_{3}\)) 6 7.80 (br, 1H), 4.38-4.33 (m, 1H), 4.00-3.96 (m, 1H), 3.91-3.88 (m, 1H), 3.85-3.81 (m, 2H), 3.15 (dt, \(J\) = 16.8, 1H, 7.2 Hz), 3.05 (dt, 1H, \(J\) = 16.8, 7.2 Hz), 2.93 (t, 2H, \(J\) = 7.2 Hz), 2.61 (s, 3H), 2.56 (t, 2H, \(J\) = 7.2 Hz), 1.95-1.90 (m, 1H), 1.82-1.71 (m, 3H), 1.59 (qt, 2H, \(J\) = 7.2, 7.2 Hz0.88 (t, 3H, \(J\) = 7.2 Hz); \({}^{13}\)C-NMR (150 MHz, CDCl\({}_{3}\)) \(\delta\) 171.9, 170.9, 161.7, 156.0, 68.1, 61.63, 61.59, 41.7, 38.0, 36.2, 34.1, 32.8, 31.8, 29.6, 29.2, 28.8, 27.9, 25.9, 22.6,14.1; HRMS (ESI) calcd. for C\({}_{20}\)H\({}_{36}\)N\({}_{4}\)O\({}_{4}\)S\({}^{+}\) [M + H]\({}^{+}\)\(m\)/z 429.2528, found _m/z_ 429.2530. 2.5 Synthesis of 2-(2-octylthioethyl)-6-oxo-4-acetylamino-1-(5'-O-(4,4'-dimethoxyytrityl)-2', 4'-dideoxy-p-ribityl) triazine (9)  
DMTrCl (58 mg, 170 \(\mu\)mol) was added to a solution of **8** (497 mg, 114 \(\mu\)mol) in pyridine (487 \(\mu\)L) at 0 \({}^{\circ}\)C. After the addition, the mixture was stirred at 0 \({}^{\circ}\)C for 1 h. The resulting mixture was diluted with CH\({}_{2}\)Cl\({}_{2}\), then quenched with NaHCO\({}_{3}\). The product was extracted with CH\({}_{2}\)Cl\({}_{2}\) and the combined organic layer was washed with brine, dried over Na\({}_{2}\)SO\({}_{4}\), filtered and evaporated.