A mixture of (S)-7-(1-aminoethyl)-6-(3-fluorophenyl)-3-methyl-5H-thiazolo[3,2-a]pyrimidin-5-one (108.2 g, 357 mmol), 6-chloropurine (71.73 g, 464.1 mmol), and N,N-diisopropylethylamine (74.6 mL, 428.4 mmol) in 1-butanol (952 mL) was degassed with nitrogen bubbling for 5 minutes. The reaction mixture was heated at 105° C. under nitrogen for 15 hours, at which point HPLC indicated amine was consumed. The reaction mixture was cooled down to room temperature before being treated with water (200 mL) at room temperature. The resulting mixture was concentrated under reduced pressure to give an oily residue and the residue was treated with CH2Cl2 (1000 mL) to give a brownish clear solution. The resulting solution was washed with 2.5% aqueous sodium carbonate solution (Na2CO3, 250 mL×2) and the organic layer was concentrated under reduced pressure to afford the crude desired product as a brownish solid. The solution of the crude desired product in CH2Cl2 was absorbed onto silica gel (300 g) and the dried silica gel was loaded onto a flash column. The flash column was eluted with pure CH2Cl2 and a mixture of CH2Cl2, MeOH and aqueous NH4OH (2000:10:5) to afford pure desired product. The fractions containing pure desired product were combined and concentrated under reduced pressure. The resulting yellowish solid (90.3 g) was dissolved in a mixture of CH2Cl2 and methanol (500:50 mL). The resulting solution was treated with ethyl acetate (900 mL) and the resulting mixture was distilled until the internal solution temperature reached 68° C. The mixture was then cooled to room temperature and subsequently to 0-5° C. for 1 hour. The solids were collected by filtration, washed with cold ethyl acetate (100 mL), and dried overnight on the filter under vacuum to afford (S)-7-(1-(9H-purin-6-ylamino)ethyl)-6-(3-fluorophenyl)-3-methyl-5H-thiazolo[3,2-a]pyrimidin-5-one (89.5 g, 59.4% yield, 99.4% ee) as a light yellowish solid. LCMS calculated for C20H17FN7OS (M+H)+ for the free base: m/z 422.1. Found: 422.0; 1H NMR (500 MHz, DMSO-d6) δ 8.40 (s, 1H), 7.99 (br s, 1H), 7.45 (m, 1H), 7.21-7.12 (m, 3H), 6.8 (m, 1H), 6.42 (s, 1H), 5.52 (br s, 1H), 2.79 (d, J=1.3 Hz, 3H), 1.43 (d, J=7.0 Hz, 3H).
