Ultraviolet and Visible Absorption Spectroscopy Data

The UV/VIS absorption spectra of novel compounds **5**–**8** dissolved in methanol presented bands at 202.6 nm (**5**–**8**), at 227.3 nm (**6**), in the 249.3–255.5 nm region (**5**–**8**), and at 333.9 (**8a**) or 337.4 nm (**8b**). When acetonitrile was used as solvent, the first peak was shifted in the interval of 195.3–198.0 nm (**5**–**8**), while the rest of the bands were recorded at approximately the same wavelength values: at 228.2 nm (for **5**, this peak appearing as a "shoulder" when methanol was used as solvent) or 229.1 nm (**6**), in the range of 248.6–254.8 nm (**5**–**8**), and at 335.6 (**8a**) or 340.1 nm (**8b**). The last absorption maximum was present only in the electronic spectra of compounds **8a** and **8b** due to the extension of the π-electron conjugation by the formation of the 1,3-oxazole chromophore.

#### Fourier-Transform Infrared Absorption Spectroscopy Data

The FTIR absorption spectra of acyclic precursors **5** and **7** showed a characteristic peak in the range of 3347–3281 cm−<sup>1</sup> due to the valence vibration of the N–H bond. In addition, the absorption maximum at 1746 cm−<sup>1</sup> due to carbonyl valence vibration, and another at 1635 cm−<sup>1</sup> due to stretching vibration of amidic carbonyl were remarked in the FTIR spectrum of **5**. For α-acylamino ketones **7a** and **7b**, these two carbonyl absorption bands are overlapped, as suggested by the very strong single peak recorded in their FTIR spectra at 1655 cm<sup>−</sup>1. A broad absorption band in the spectral region from 3300 to 2500 cm<sup>−</sup>1, centered at ≈ 3000 cm<sup>−</sup>1, due to stretching vibration of the O–H bond and two noticeably weak satellite peaks at 2676 and 2599 cm−<sup>1</sup> are also characteristic of the hydrogen-bonded *N*-acylated *L*-valine **5**.

As evidence that the intramolecular cyclocondensations occurred, significant changes were observed in the FTIR absorption spectra of five-membered *O*,*N*-heterocyclic compounds **6**, and **8a**,**b** compared with the corresponding spectra of open-chain intermediates **5**, and **7a**,**b**, respectively. The FTIR spectrum of 4*H*-1,3-oxazol-5-one **6** presented a peak at 1825 cm−<sup>1</sup> due to carbonyl stretching vibration, which is shifted at a higher wavenumber compared with the C=O absorption maximum from the spectrum of **5**. The FTIR absorption spectra of 5-membered heterocycles **6** and **8** showed an absorption band at 1650 (**6**), 1602 (**8a**), or 1601 cm−<sup>1</sup> (**8b**) due to the valence vibration of the C=N bond. The absorption maxima at 1099 (**8a**), 1097 (**8b**), or 1040 cm−<sup>1</sup> (**6**) due to the C–O–C symmetric stretching vibration and at 1280 (**8a** and **8b**) or 1243 cm−<sup>1</sup> (**6**) due to the asymmetric stretching vibration of the C–O–C group were also observed.

#### Nuclear Magnetic Resonance Spectroscopy Data

The nuclear magnetic resonance spectroscopic data also confirmed the structures of the new *L*-valine analogs. Complete assignments of the signals from the 1H and 13C NMR spectra of the new compounds **5**–**8** were performed using combinations of standard NMR spectroscopic techniques, namely 2D COSY and HETCOR experiments.

The molecular structures of the new bromine-containing derivatives **5**–**8** with the numbering of the atoms, used for the assignment of NMR signals, are presented in Figure 3.

**Figure 3.** Molecular structures of the new 4-[(4-bromophenyl)sulfonyl]benzoic acid derivatives **5**–**8** with the atom numbering (for assigning NMR signals).
