13C CPMAS NMR, XRD and DFT study of selected 2,6-diketopiperazines

A series of enantiopure compounds 2,6-diketopiperazines (3–6) and 2-carbamoylpyrrolidines (1, 2) was studied by a combined approach of solid-state NMR, X-ray diffraction (XRD) and theoretical methods. These derivatives of 2,6-diketopiperazines (3–6) are useful building blocks for the development of new biologically active compounds. Crystal structures of 1 and 4 are known from previous crystallographic measurements; the structure of 6 was determined in the present study. 13C CPMAS NMR spectra were recorded and assigned. The splitting of signals into doublets was observed in the CPMAS spectra of 1, 3 and 5. The calculations of shielding constants were performed at the GIAO/DFT (B3LYP/6-311++G∗∗) level. Satisfactory agreement between calculated and experimental chemical shifts enabled the assignment of resonances to particular conformers of 5.