Effects of resolution agents on the configuration of (R)-1^a.

Entry	Resolution agent	Solvent	Configuration (ee%)
1	(−)-l-Tartaric acid	MeOH	4%
2	(−)-Di-p-toluoyl-l-tartaric acid	MeOH	96%

^aAll reactions were carried out using racemates 13 (0.5 g, 1.23 mmol) and resolution agents such as (−)-di-p-toluoyl-l-tartaric acid (0.24 g, 0.62 mmol) in solvent (15 mL) at 65 °C for 1 h. White sitagliptin tartrate was obtained after filtration. And tartrate was hydrolyzed by using ammonia water (10 mL), and extracted twice by ethyl acetate (30 mL × 2). The organic layer was concentrated and isopropanol (15 mL) was added. Then 85 wt% H₃PO₄ was added dropwise and the reaction mixture was aged at 78 °C for 1 h to afford sitagliptin 1.