Heteroaromatic N-Oxides Modified with a Chiral Oxazoline Moiety, Synthesis and Catalytic Applications

Abstract

Interesting properties of N-oxides and pyridine oxazoline compounds have become the starting point to synthesize compounds connecting both groups. A multi-step synthesis of a series of chiral oxazoline substituted pyridine N-oxides, alkyl derived of pyridine N-oxides, bipyridine N-oxides, and isoquinoline N-oxides, based on amino alcohols derived from natural amino acids or other previously prepared, is presented herein. Various synthetic pathways have been designed and tested according to the properties and limitations imposed by the target products. The encountered problems related to the stability of the products were discussed. The resulting compounds (eighteen structures) were tested as catalysts in th e allylation of benzaldehyde (obtaining up to 79% ee) as well as in nitroaldol reaction (obtaining up to 48% ee).

Oxazoline scaffold in synthesis of benzosiloxaboroles and related ring-expanded heterocycles: diverse reactivity, structural peculiarities and antimicrobial activity

“…11 B NMR spectrum of 3a shows a resonance at 30.0 ppm typical of boronic acid derivatives with trigonal planar boron atom which means that there is no tendency to selfaggregation resulting from plausible formation of N-B dative bonds. 23 29 Si NMR chemical shi of 19.61 ppm is in agreement with the values reported previously for other benzosiloxaboroles. 24,25 In addition, single crystal X-ray diffraction revealed that geometric parameters of benzosiloxaborole core in 3a are very similar to those found previously in other derivatives whilst the oxazoline ring is twisted with respect to aromatic ring by 47.3(2) (Fig.…”

“…1b). The comparison of 11 B and 29 Si NMR data of 3a vs. 3b shows that they are essentially unaffected by introduction of charge to the pendant oxazoline framework. The geometric parameters of the benzosiloxaborole cation of 3b and its precursor 3a are also very similar, although the oxazoline ring in crystal structure 3b is inversely twisted around C-C bond with respect to 3a molecule.…”

“…HCl resulted in precipitation of 3a which was isolated as a white solid; the process should be performed under precise pH control in order to avoid oxazoline protonation. The structure of compound 3a was conrmed by multinuclear ( 1 H, 13 C, 11 B, 19 F and 29 Si) NMR spectroscopy. 11 B NMR spectrum of 3a shows a resonance at 30.0 ppm typical of boronic acid derivatives with trigonal planar boron atom which means that there is no tendency to selfaggregation resulting from plausible formation of N-B dative bonds.…”

“…F NMR (376 MHz, DMSO-d 6 ) d À104.47 (t, J ¼ 6.1 Hz) ppm 10. B NMR (54 MHz, DMSO-d 6 ) d 29.0 ppm 29. Si NMR (99 MHz, DMSO-d 6 ) d 18.33 ppm.…”

“…F NMR (376 MHz, CDCl 3 ) d À98.86 (dd, J ¼ 6.3, 5.4 Hz) ppm 10. B NMR (54 MHz, acetone-d 6 ) d 30.1 ppm 29. Si NMR (99.3 MHz, acetone-d 6 ) d 18.35 ppm.…”