Potassium tert-butoxide (1.0 M THF solution, 0.3 mL, 0.30 mmol, 1.10 eq.) was slowly added dropwise to a solution of tert-butyl (S)-2-(5-(4-chlorophenyl)-6,7-dimethyl-2-oxo-2,3-dihydro-1H-thieno[2,3-e][1,4]diazepin-3-yl)acetate (114 mg. 0.27 mmol, 1 equiv) in a solution of THF (1.8 mL, 0.15 M). The reaction mixture was slowly warmed to -10 °C and stirred at 23 °C for 30 min. Subsequently, the reaction mixture was re-cooled to -78°C and diethyl chlorophosphate (0.047 mL, 0.32 mmol, 1.20 equiv) was added. The resulting mixture was slowly warmed to -10°C over 45 min. Next, acetylhydrazine (30 mg, 0.40 mmol, 1.50 eq.) was added and the reaction mixture continued to be stirred at 23°C. After 1 hr, 1-butanol (2.25 mL) was added and the reaction mixture was heated to 90°C. After an additional 1 hr, all solvents were removed under reduced pressure. The residue was purified by fast column chromatography (Combiflash system, 4 g silica gel, gradient 0 to 100% ethyl acetate-hexane) to afford the target product (S)-2-(4-(4-chlorophenyl)-2,3,9-trimethyl-6H-thieno[3,2-F][1,2,4]triazolo[4,3-A][1,4]diazepin-6-yl)acetic acid tertiary Butyl ester (114 mg, 92% yield) as a white solid with 90% enantiomeric purity (determined by Berger Supercritical Fluid Chromatography (SFC) using an AS-H column, 85% hexanes-methanol, 210 nm detection wavelength, tR (R-enantiomer) = 1.59 min, tR (S-enantiomer) = 3.67 min). Further purification by chiral preparative HPLC (Agilent high-pressure liquid chromatography using an OD-H column) yielded the S-enantiomer with an enantiomeric purity of >99%.1H NMR (600 MHz, CDCl3, 25°C) δ 7.39 (d, J = 8.4 Hz, 2H), 7.31 (d, J = 8.4 Hz, 2H), 4.54 (t, J = 6.6 Hz, 2H), 4.54 (t, J = 6.6 Hz, 2H). J = 6.6 Hz, 1H), 3.54-3.52 (m, 2H), 2.66 (s, 3H), 2.39 (s, 3H), 1.67 (s, 3H), 1.48 (s, 9H).13C NMR (150 MHz, CDCl3, 25°C) δ 171.0, 163.8, 155.7, 150.0, 136.9 131.1, 130.9, 130.6, 130.3, 128.9, 81.2, 54.1, 38.1, 28.4, 14.6, 13.5, 12.1. HRMS (ESI) calculated value of C21H24ClN2O3S [M + H]+: 457.1460, measured value 457.1451 m/z. TLC (EtOAc ), Rf: 0.32 (UV). [α]22D = +75 (c 0.5, CHCl3).