Sitagliptin n has a chemical name of (2R)-4-oxo-4-(3-(trifluoromethyl)-5,6-dihydro(1,2,4)triazolo[4,3-a]pyrazin-7(8H)-yl)-1-(2,4,5-trifluorophenyl)butan-2-amine, hydrochloride form of which has a structural formula shown as follows:

Sitagliptin is the first agent in the class of Dipeptidyl Peptidase-IV (DPP-IV) inhibitors developed by Merck America. In October 2006, Sitagliptin phosphate became the first Dipeptidyl Peptidase-VI (DPP-VI) inhibitor to gain FDA approval for clinical use in the treatment of type 2 diabetes. This medication has the advantages of fewer adverse effects, a lower risk of hypoglycemia, and no weight gain (bodyweight neutrality).
The current preparation methods of Sitagliptin intermediate compounds are disclosed in the following references:
Reference 1: Journal of Medicinal Chemistry (J. Med. Chem.), 2005, Vol. 48, No. 11, page 141-151, by Kim D et al. The scheme disclosed is depicted below:

Reference 2: Org. Proc. Res. Dev, 2005, 9(5): 634-639, by Hansen K B, Balsells J, et al. The scheme disclosed is depicted below:

From the above references, it is clear that they have common disadvantages of expensive starting materials, complexity of the reactions, long synthetic routes, strict operating conditions, low yields, difficulty in optimize the production, and being unable to carry out large scale production, which limit the wide application of Sitagliptin.