Montecrinanes A–C: Triterpenes with an Unprecedented Rearranged Tetracyclic Skeleton from Celastrus vulcanicola. Insights into Triterpenoid Biosynthesis Based on DFT Calculations

Three new triterpenoids with an unprecedented 6/6/6/6‐fused tetracyclic carbon skeleton, montecrinanes A–C ( 1 – 3 ), were isolated from the root bark of Celastrus vulcanicola, along with known D:B‐friedobaccharanes ( 4 – 6 ), and lupane‐type triterpenes ( 7 – 12 ). The stereostructures of the new metabolites were elucidated based on spectroscopic (1D and 2D NMR) and spectrometric (HR‐EIMS and HR‐ESIMS) techniques. Their absolute configurations were determined by both NMR spectroscopy, with (R)‐(?)‐α‐methoxyphenylacetic acid as a chiral derivatizing agent, and biogenetic considerations. Biogenetic pathways for montecrinane and D:B‐friedobaccharane skeletons were proposed and studied by DFT methods. The theoretical results support the energetic feasibility of the putative biogenetic pathways, in which the 1,2‐methyl shift from the secondary baccharenyl cation represents a novel and key reaction step for a new montecrinane skeleton.     

Enantioselective Total Synthesis of (+)‐Steenkrotin A and Determination of Its Absolute Configuration

The first enantioselective total synthesis of (+)‐steenkrotin A has been achieved in 18 steps and 4.2 % overall yield. The key features of the strategy entail a Rh‐catalyzed O?H bond insertion followed by an intramolecular carbonyl‐ene reaction, two sequential SmI₂‐mediated Ueno–Stork and ketyl–olefin cyclizations, and a cascade intramolecular aldol condensation/vinylogous retro‐aldol/aldol process with inversion of the relative configuration at the C7 position. The absolute configuration of (+)‐steenkrotin A was determined based on the stepwise construction of the stereocenters during the total synthesis.     

Practical guide for selection of 1H qNMR acquisition and processing parameters confirmed by automated spectra evaluation

In our recent paper, a new technique for automated spectra integration and quality control of the acquired results in qNMR was developed and validated (Monakhova & Diehl, Magn. Res. Chem. 2017, doi: 10.1002/mrc.4591 ). The present study is focused on the influence of acquisition and postacquisition parameters on the developed automated routine in particular, and on the quantitative NMR (qNMR) results in general, which has not been undertaken previously in a systematic and automated manner. Results are presented for a number of model mixtures and authentic pharmaceutical products measured on 500‐ and 600‐MHz NMR spectrometers. The influence of the most important acquisition (spectral width, transmitter [frequency] offset, number of scans, and time domain) and processing (size of real spectrum, deconvolution, Gaussian window multiplication, and line broadening) parameters for qNMR was automatically investigated. Moderate modification of the majority of the investigated parameters from default instrument settings within evaluated ranges does not significantly affect the trueness and precision of the qNMR. Lite Gaussian window multiplication resulted in accuracy improvement of the qNMR output and is recommended for routine measurements. In general, given that the acquisition and processing parameters were selected based on the presented guidelines, automated qNMR analysis can be employed for reproducible high‐precision concentration measurements in practice.     

Development and Validation Method for Silica Determination by Spectrophotometry in Some Herbs and Pharmaceutical Formulations

ABSTRACT

A spectrophotometric method for determination of silica in herbs and herbal pharmaceutical products has been developed and validated. The method showed excellent accuracy and precision with S.D. 1.67 and RSD 1.72%, 2.75%, respectively. The established linearity range was 0.05–0.5 μg/mL(r² > 0.99974). The recovery of silica from spiked placebo was > 95% over the linear range. The method has been successfully used in the analysis of silica in raw material and herbal pharmaceutical formulations. This validated method proved to be rapid and less costly.