Synthesis of N-glycosyl amides: conformational analysis and evaluation as inhibitors of β-galactosidase from E. coli

Abstract

The synthesis of N-glycosyl amides typically involves the use of glycosyl amines as direct precursors, resulting in low yields due to hydrolysis and the loss of stereocontrol through anomerization processes. In this study, a sequential synthesis of N-glycosyl amides is proposed, employing glycosyl amines as intermediates obtained from glycosyl azides. Derivatives with gluco, galacto, or xylo configurations were synthesized. Hexose derivatives were obtained under stereocontrol to give only the β anomer, while the xylo derivatives provided a mixture of α and β anomers. Conformational analysis revealed that all β anomers adopted the ⁴C₁ conformation, while α anomers were found in the ¹C₄ chair as the major conformer. After de-O-acetylation, the derivatives containing a galactose unit were evaluated as inhibitors of β-galactosidase from E. coli and were found to be moderate inhibitors.

Scheme 1. Some methodologies described for the synthesis of N-glycosil amides.

Scheme 2. Synthesis of glycosyl azides of gluco (5a), galacto (5b), or xylo (5c) configuration.

Chart 1. Carboxylic acids selected for amidation reactions with glycosyl amines 2a–c.

Scheme 3. Synthesis of N-glycosylamides 14a–c, 15a–c, and 16a–c.

Chart 2. Structure and isolated yields of per-O-acetylated N-glycosyl amides 14a–c, 15a–c, and 16a–c.

Scheme 4. Synthesis of glycosyl amides with the xylo configuration.

Scheme 5. Hydrogenation of azide 5c to afford N-glycosyl amine 2cβ and anomerization to 2cα.

Fig. 1. ¹H NMR spectrum of the mixture of N-glycosyl amines 2cβ and 2cα.

Fig. 2. Stable conformations of xylopyranosyl amides, determined by NMR and MM calculations.

Scheme 6. de-O-Acetylation of N-glycosyl amides 14a-cβ, 15a-cβ, and 16a-cβ.

Fig. 3. Effect of the concentration of N-glycosyl amides 17b, 18b, 19a, 19b, and 19cβ on the enzymatic activity of the β-galactosidase from E. coli.