Experimental and DFT 1H NMR study of conformational equilibria in trans-4',7-dihydroxyisoflavan-4-ol and trans-isoflavan-4-ol

Synthesis, structure-activity relationship analysis and kinetics study of reductive derivatives of flavonoids as Helicobacter pylori urease inhibitors

In a continuing study for discovering urease inhibitors based on flavonoids, nineteen reductive derivatives of flavonoids were synthesized and evaluated against Helicobacter pylori urease. Analysis of structure-activity relationship disclosed that 4-deoxy analogues are more potent than other reductive products. Out of them, 4',7,8-trihydroxyl-2-isoflavene (13) was found to be the most active with IC50 of 0.85 μM, being over 20-fold more potent than the commercial available urease inhibitor, acetohydroxamic acid (AHA). Kinetics study revealed that 13 is a competitive inhibitor of H. pylori urease with a Ki value of 0.641 μM, which is well matched with the results of molecular docking. Biological evaluation and mechanism study of 13 suggest that it is a good candidate for discovering novel anti-gastritis and anti-gastric ulcer agent.

Formation of 5-Hydroxy-3-methoxy-1,4-naphthoquinone and 8-Hydroxy-4-methoxy-1,2-naphthoquinone from Juglone

From the treatment of 5-hydroxy-1,4-naphthoquinone (juglone) with acetic anhydride and H2SO4 followed subsequently by treatment with methanolic HCl, 5-hydroxy-3-methoxy-1,4-naphthoquinone (3-methoxy juglone) and 8-hydroxy-4-methoxy-1,2-naphthoquinone were obtained as products rather than the anticipated product 2,5-dihydroxy-1,4-naphthoquinone (2-hydroxy juglone). The reaction and the identification of the products are discussed in terms of NMR and DFT calculations.

Conversion of (3S,4R)-tetrahydrodaidzein to (3S)-equol by THD reductase: proposed mechanism involving a radical intermediate

To elucidate the mechanism of (3S)-equol biosynthesis, (2,3,4-d(3))-trans-THD was synthesized and converted to (3S)-equol by THD reductase in Eggerthella strain Julong 732. The position of the deuterium atoms in (3S)-equol was determined by (1)H NMR and (2)H NMR spectroscopy, and the product was identified as (2,3,4(alpha)-d(3))-(3S)-equol. All the deuterium atoms were retained, while the OH group at C-4 was replaced by a hydrogen atom with retention of configuration. To explain the deuterium retention in this stereospecific reduction, we propose a mechanism involving radical intermediates.

Absolute configurations of isoflavan-4-ol stereoisomers

Isoflavan-4-ol has been synthesized quantitatively from the reduction of isoflavone in the presence of Pd/C and ammonium formate under N(2) atmosphere. Isolation of cis- and trans-isomers was achieved by flash column chromatography and each enantiomer was separated by Sumi-Chiral column chromatography. Absolute configurations of four stereoisomers were determined by circular dichroism spectroscopy.

Multiple hydride reduction pathways in isoflavonoids

BACKGROUND

Isoflavonoids are of interest owing to their appearance in metabolic pathways of isoflavones, and their estrogenic and other physiological properties, making them promising lead compounds for drug design.

RESULTS

The reduction of isoflavones by various hydride reagents occurs by a 1,4-pathway in contrast to ordinary beta-alkoxy-alpha,beta-unsaturated ketones. Isoflavan-4-ones, cis- and trans-isoflavan-4-ols, alpha-methyldeoxybenzoins or 1,2-diphenylprop-2-en-1-ols are obtained depending on the hydride reagent, mostly in good yields. The stereoselective reduction of isoflavan-4-ones is also discussed.

CONCLUSION

The work described in this paper shows that most structural types of reduced isoflavonoids are now reliably available in satisfactory or good yields by hydride reductions to be used as authentic reference compounds in analytical and biological studies.

Oxidised derivatives of silybin and their antiradical and antioxidant activity

Carboxylic acids derived from silybin (1) and 2,3-dehydrosilybin (2) with improved water solubility were prepared by selective oxidation of parent compounds and a new inexpensive method for preparation of 2,3-dehydrosilybin from silybin was developed and optimised. The antioxidative properties of the above-mentioned compounds and of side product 3a from oxidation of compound 1 were determined by cyclic voltammetry, free radical scavenging (DPPH, superoxide) assays, and by inhibition of in vitro generated liver microsomal lipid peroxidation. Dehydrogenation at C((2))-C((3)) in flavonolignans (silybin vs 2,3-dehydrosilybin; silybinic acid vs 2,3-dehydrosilybinic acid) strongly improved antioxidative properties (analogously as in flavonoids taxifolin vs quercetin). Thus, in antioxidative properties, dehydrosilybin was superior to silybin by one order, but its water solubility is too low for application in aqueous milieu. On the other hand, 2,3-dehydrosilybinic acid is a fairly soluble derivative with antilipoperoxidation and antiradical activities better than that of silybin.

Conformation of the galactose ring adopted in solution and in crystalline form as determined by experimental and DFT 1H NMR and single-crystal X-ray analysis

The solution-state conformations of various galactose derivatives were determined by comparison of the experimental (1)H-(1)H vicinal coupling constants to those calculated using density functional theory (DFT) at the B3LYP/cc-pVTZ//B3LYP/6-31G(d,p) level of theory. The agreement between the experimental and calculated vicinal coupling constants for 1,2:3,4-di-O-isopropylidene-alpha-d-galactopyranose was good, thereby confirming an (O)S(2) skew conformation for it and its derivatives on the basis of their similar observed couplings. Single-crystal X-ray analysis of 1,2:3,4-di-O-isopropylidene-6-O-(3,4,6-tri-O-acetyl-2-deoxy-2-N-phthalimido-beta-d-glucopyranosyl)-alpha-d-galactopyranose and 1,2,3,4,6-penta-O-acetyl-alpha-d-galactopyranose provided (O)S(2) and (4)C(1) conformations, respectively, for the galactose ring in the solid state. The solid-state structures proved to be suitable starting structures for further DFT structure refinement or for immediate calculation of the coupling constants.