A new NMR approach for structure determination of thermally unstable biflavanones and application to phytochemicals from Garcinia buchananii

作者：Stark Timo D^*; Loesch Sofie; Salger Mathias; Balemba Onesmo B; Wakamatsu Junichiro; Frank Oliver; Hofmann Thomas

来源：Magnetic Resonance in Chemistry, 2015, 53(10): 813-820.

DOI：10.1002/mrc.4269

• NMR
• H-1
• C-13
• 1,1-ADEQUATE
• manniflavanone
• manniflavanone-7 ''-O-beta-D-glucopyranoside
• GB2 7 ''-O-beta-D-glucopyranoside
• variable temperature NMR
• biflavanone glucosides
• biflavanones

Previous activity-guided phytochemical studies on Garcinia buchananii stem bark, which is traditionally used in Africa to treat various gastrointestinal and metabolic illnesses, revealed xanthones, polyisoprenylated benzophenones, flavanone-C-glycosides, biflavonoids, and/or biflavanones as bioactive key molecules. Unequivocal structure elucidation of biflavonoids and biflavanones by means of NMR spectroscopy is often complicated by the hindered rotation of the monomers around the C-C axis (atropisomerism), resulting in a high spectral complexity. In order to facilitate an unrestricted rotation, NMR spectra are usually recorded at elevated temperatures, commonly over 80 degrees C, which effects in a single set of resonance signals. However, under these conditions, one of the target compounds of this investigation, (2R, 3S, 2 '' R, 3 '' R)-manniflavanone (1), undergoes degradation. Therefore, we demonstrated in the present study that the 1,1-ADEQUATE could be successfully used as a powerful alternative approach to confirm the C-C connectivities in 1, avoiding detrimental conditions. However, a moderate increase in temperature up to 50 degrees C was sufficient to deliver sharp signals in the proton NMR experiment of (2R, 3S, 2 '' R, 3 '' R)-isomanniflavanone (2) and (2 '' R, 3 '' R)-preussianone (3). In addition, two new compounds could be isolated, namely (2R, 3S, 2 '' R, 3 '' R)-GB-2 7 ''-O-beta-D-glucopyranoside (4) and (2R, 3S, 2 '' R, 3 '' R)-manniflavanone-7 ''-O-beta-D-glucopyranoside (5), and whose structures were elucidated by spectroscopic analysis including 1D and 2D NMR and mass spectrometry methods. The absolute configurations were determined by a combination of NMR and electronic circular dichroism(ECD) spectroscopy. The aforementioned compounds exhibited high anti-oxidative capacity in the H2O2 scavenging, hydrophilic Trolox equivalent antioxidant capacity (H-TEAC) and hydrophilic oxygen radical absorbance capacity (H-ORAC) assays.

• 出版日期2015-10