Carbon-13 NMR Spectroscopy Data

The 13C NMR spectrum of *N*-acylated *L*-valine **5** presented a signal at 58.43 ppm attributed to the C-4, for the 1-methylethyl group revealed a signal at 29.48 ppm due to the methine carbon (C-18) and two signals at 18.61 and 19.25 ppm due to the C-19 and C-20 non-equivalent carbon atoms. The 13C NMR spectrum of **6** highlighted the C-4 signal at 71.04 ppm, which is deshielded by 12.61 ppm as a result of cyclization of the open-chain precursor **5**. Further, the C-2 atom of 4*H*-1,3-oxazol-5-one **6** resonated at 160.36 ppm, being more shielded with 5.42 ppm compared with the C-2 of **5**, and the C-5 at 176.98 ppm, being more deshielded with 4.19 ppm than the corresponding carbon atom of its precursor (**5**).

The carbon-13 NMR spectra of aromatic *O*,*N*-heterocycles **8a**,**b** showed a signal assigned to the C-4 atom at 143.63 (**8b**) or 144.19 ppm (**8a**), which is more deshielded by ≈ 84.77 ppm than the corresponding signal of the compounds from which they were obtained by cyclization, from 59.05 (**7b**) or 59.23 ppm (**7a**). The cyclization of **7a**,**b** to compounds **8a**,**b** induced a shielding effect for the C-2 of the 1,3-oxazole ring, resulting in an about 7.87 ppm lower chemical shift. The C-5 signal of 1,3-oxazoles **8a**,**b** was observed at 145.47 (**8a**) or 145.69 ppm (**8b**), whereas the corresponding signal of acyclic precursors **7a**,**b** appeared at 198.57 (**7b**) or 199.19 ppm (**7a**), revealing a shift of this carbon signal at a smaller *δ*<sup>C</sup> of approximately 53.30 ppm.
