Reaction SMILES: [H-].[Na+].[Cl:3][C:4]1[C:5]2[CH:12]=[CH:11][NH:10][C:6]=2[N:7]=[CH:8][N:9]=1.[C:13]([O:19][CH2:20]Cl)(=[O:18])[C:14]([CH3:17])([CH3:16])[CH3:15].O>O1CCCC1>[C:13]([O:19][CH2:20][N:10]1[C:6]2[N:7]=[CH:8][N:9]=[C:4]([Cl:3])[C:5]=2[CH:12]=[CH:11]1)(=[O:18])[C:14]([CH3:17])([CH3:16])[CH3:15]
Reactant: CC(C)(C)C(=O)OCCl
Reactant: Clc1ncnc2[nH]ccc12
Agent: [H-]
Agent: [Na+]
Solvent: O
Solvent: C1CCOC1
Product: CC(C)(C)C(=O)OCn1ccc2c(Cl)ncnc21
Temperature: 1.5 °C
Reaction time: 1 h

Procedure:
To a oven dried 2 L 4-neck round bottom flask equipped with overhead stirring, septa, thermocouple, 500 mL addition funnel and nitrogen inlet was charged sodium hydride (NaH, 60 wt %, 29.7 g, 0.742 mol, 1.34 equiv) and anhydrous tetrahydrofuran (THF, 400 mL, 5.0 mol) and the resulting mixture was cooled to 0-3° C. To a oven dried 1 L round bottom flask was charged 4-chloro-7H-pyrrolo[2,3-d]pyrimidine (1, 85.0 g, 0.553 mol) and tetrahydrofuran (600 mL, 7.0 mol) resulting in a slurry. This resulting slurry was then portion wise added to the suspension of sodium hydride in THF via large bore canula over 27 minutes at 0-5° C. The resulting solution was heterogeneous and green in color. Following the addition, the cold bath was removed and the mixture was gradually warmed to room temperature and allowed to stir at room temperature for 1 hour before being cooled to 0-5° C. Chloromethyl pivalate (pivaloyloxymethyl chloride, POM-Cl, 103 ml, 0.692 mol, 1.25 equiv) was added portion wise into the reaction mixture over 25 minutes via syringe with stirring at 0-5° C. The addition of chloromethyl pivalate (POM-Cl) was mildly exothermic and the reaction temperature went to as high as 14° C. After addition of chloromethyl pivalate (POM-Cl), the cooling bath was removed and the reaction mixture was allowed to return to room temperature and stirred at room temperature for overnight. When the reaction was deemed complete after about 16 hours, the reaction was quenched with 20% aqueous brine (250 mL) and ethyl acetate (250 mL) producing a slurry. Additional amount of water (250 mL) was added until the mixture becomes a homogeneous solution. The two layers were separated and the aqueous layer was extracted with ethyl acetate (250 mL). The combined organic fractions were dried over magnesium sulfate (MgSO4), filtered, and concentrated under reduced pressure. The residue was purified by flash column chromatography (SiO2, 10% to 15% ethyl acetate/hexane gradient elution) to afford the desired product as yellow, crystalline solids (155 g). The combined solids were treated with hexanes (750 mL) and the resulting slurry was warmed to 55° C. to produce a homogeneous solution. The resulting solution was then gradually cooled to room temperature and stirred at room temperature for overnight before being cooled to 0-5° C. for 2 h. The solids were collected by filtration, washed with pre-cooled hexanes (2×30 mL), dried in vacuum to afford 4-chloro-7H-pyrrolo[2,3-d]pyrimidin-7-yl)methyl pivalate (3f, 134.9 g, 148.0 g theoretical, 91% yield) as white solids. For 3f: 1H NMR (DMSO-d6, 400 MHz) δ ppm 8.71 (s, 1H), 7.83 (d, 1H, J=3.7 Hz), 6.73 (d, 1H, J=3.8 Hz), 6.23 (s 2H), 1.06 (s, 9H); 13C NMR (DMSO-d6, 100 MHz) δ ppm 176.9, 151.2, 151.1, 151.0, 131.6, 117.1, 99.9, 66.9, 38.3, 26.5; C12H14ClN3O2 (MW, 267.71), LCMS (EI) m/e 268/270 (M++H).