A novel alkaloid isolated from Crotalaria paulina and identified by NMR and DFT calculations

Deciphering molecular structures: NMR spectroscopy and quantum mechanical insights of halogenated 4H-Chromenediones

Sesquiterpene lactones (SL) represent a class of secondary metabolites found in the Asteraceae family, notable for their unique structures. The SL α-santonin (1) and its derivatives are worthy of mention due to their diverse biological properties. Additionally, 4H-chromenes and 4H-chromones are appealing frameworks holding the capability to be used as structural motifs for new drugs. Furthermore, unambiguous structural elucidation is crucial for developing novel compounds for diverse applications. In this context, it is common to find in the literature molecules erroneously assigned. Therefore, the use of quantum mechanical calculations to simulate NMR chemical shifts has emerged as a valuable strategy. In this work, we conceived the synthesis of two halogenated 4H-chromenediones derived from photosantonic acid (2), a photoproduct arising from irradiation of α-santonin (1) in the ultraviolet region. The structure of the chlorinated and brominated products was determined by NMR analysis, with the aid of quantum mechanical calculations at the B3LYP/6-311 + G(2d,p)//M062x/6-31 + G(d,p) level of theory. All analyses were in agreement and led to the assignment of the brominated 4H-chromene-2,7-dione as (3S,3aS,5aR,9bS)-5a-(2-bromopropan-2-yl)-3-methyl-3,3a,5,5a,8,9b-hexahydro-4H-furo[2,3-f]chromene-2,7-dione (11b) and of the chlorinated 4H-chromene-2,7-dione as (3S,3aS,5aR,9bS)-5a-(2-chloropropan-2-yl)-3-methyl-3,3a,5,5a,8,9b-hexahydro-4H-furo[2,3-f]chromene-2,7-dione (12b). The diastereoselectivities of the reactions were explained based on products and intermediates formation energy calculated using B3LYP/6-31 + G(d,p) as the level of theory. Structures 11b and 12b were identified as the thermodynamic and kinetic products of the reaction among all candidates. Consequently, the strategy utilized in this study is robust and successfully illustrates the use of quantum mechanical calculations in the structural elucidation of new compounds with potential applications as novel drugs or products.

Computational 1 H and 13 C NMR in structural and stereochemical studies

Present review outlines the advances and perspectives of computational ¹ H and ¹³ C NMR applied to the stereochemical studies of inorganic, organic, and bioorganic compounds, involving in particular natural products, carbohydrates, and carbonium ions. The first part of the review briefly outlines theoretical background of the modern computational methods applied to the calculation of chemical shifts and spin-spin coupling constants at the DFT and the non-empirical levels. The second part of the review deals with the achievements of the computational ¹ H and ¹³ C NMR in the stereochemical investigation of a variety of inorganic, organic, and bioorganic compounds, providing in an abridged form the material partly discussed by the author in a series of parent reviews. Major attention is focused herewith on the publications of the recent years, which were not reviewed elsewhere.

Characterization of stereoisomeric 5-(2-nitro-1-phenylethyl)furan-2(5H)-ones by computation of 1 H and 13 C NMR chemical shifts and electronic circular dichroism spectra

In the present work, we describe the preparation of two diastereomers from the enantioselective Michael addition of furan-2(5H)-one to (E)-(2-nitrovinyl)benzene catalyzed by a dinuclear Zn-complex. The relative configurations of the diastereomeric products were assigned by comparing nuclear magnetic resonance (NMR) experimental chemical shift data with those computed by density functional theory (DFT) methods. Corrected mean absolute error (CMAE) and CP3 analyses were used to compare the data sets. The absolute configuration of each diastereomer was initially assigned by analysis of electronic circular dichroism (ECD) data, which was consistent with that of the known X-ray crystallographic structure of the product of a related reaction, namely, (R)-5-((R)-1-(4-chlorophenyl)-2-nitroethyl)furan-2(5H)-one.

Rapid identification and determination of pyrrolizidine alkaloids in herbal and food samples via direct analysis in real-time mass spectrometry

Pyrrolizidine alkaloids (PAs) are naturally occurring plant toxins associated with severe liver damage if excessive ingestion. Herein, a novel analytical strategy on utilizing direct analysis in real-time mass spectrometry (DART-MS) was developed, and applied in analysis of six representative PAs. The calibration curves in the range of 10-1000 ng·mL^-1 were established, and relative standard deviations (RSDs) were less than 10%. The limits of detection (LODs) and limits of quantitation (LOQs) were 0.55-0.85 ng·mL^-1 and 1.83-2.82 ng·mL^-1, respectively. The feasibility of method was indicated by analysing real samples including Gynura japonica, drug tablets, granules, and fresh cow's milk. Moreover, the results of DART-MS were in good agreement with those observed by high performance liquid chromatography mass spectrometry (HPLC-MS), but consumed less time without chromatographic separation. This research provides a facile fashion for safety assessment of herbal and food products containing PAs and presents promising applications in food, pharmaceutical and clinical analysis.

Synthesis and structural elucidation of a phthalide analog using NMR analysis and DFT calculations

Phtalides are secondary metabolites found in several fungi with a wide range of biological activities. A novel phthalide analog was synthesized by Diels-Alder reaction between cyclopentadiene and 3,4-dichlorofuran-2(5H)-one. Quantum mechanical calculations were used in conjunction with the spectrometric methods to determine the structure of the title compound. The calculated NMR chemical shifts for eight candidate pairs of enantiomers were compared with the experimental NMR chemical shifts applying the DP4 probability and mean absolute errors methodology. DP4 analysis using ¹ H and ¹³ C NMR chemical shifts without assignment of the signals presented 100% probability for the correct candidate structure 3d, proving the consistency of the method even without spectra interpretation. Results from theoretical calculation and NMR spectra interpretation were in agreement to the structure of rac-(3aR,4S,4aS,5R,8S,8aR,9R,9aS)-3a,9a-dichloro-3a,4,4a,5,8,8a,9,9a-octahydro-4,9:5,8-dimethanonaphtho[2,3-c]furan-1(3H)-one.

Computational 1 H NMR: Part 3. Biochemical studies

This is the third and the last part of three closely interrelated reviews dealing with computation of ¹ H nuclear magnetic resonance chemical shifts and ¹ H-¹ H spin-spin coupling constants. Present review deals with the computation of these parameters in biologically active natural products, carbohydrates, and other molecules of biological origin focusing on stereochemical applications of computational ¹ H nuclear magnetic resonance to these objects.

Computational protocols for calculating 13C NMR chemical shifts

The most recent results dealing with the computation of ¹³C NMR chemical shifts in chemistry (small molecules, saturated, unsaturated and aromatic compounds, heterocycles, functional derivatives, coordination complexes, carbocations, and natural products) are reviewed, paying special attention to theoretical background and accuracy, the latter involving solvent effects, vibrational corrections, and relativistic effects.

Computation and structural elucidation of compounds formed via epoxide alcoholysis

Isobenzofuranones are known for their wide range of biological activities such as fungicide, insecticide, and anticancer. The search for novel bioactive compounds was performed by reaction of epoxide 2 with methanol, ethanol, propan-1-ol, propan-2-ol, and butan-1-ol. The mechanism for the stereoselective and stereospecific epoxide opening with methanol was reasoned by calculating the transition states for the two putative structures (rac)-3a and (rac)-3b. The compound (rac)-3a is the kinetic product as inferred from the lower energies of its transition state (TS1). The ¹ H and ¹³ C nuclear magnetic resonance (NMR) chemical shifts for these two candidate structures were calculated and compared with the experimental data using mean absolute error (MAE) and DP4 analyses. Therefore, the relative stereochemistry of (rac)-3a was established by the mechanism, MAE, and DP4 approaches. The hydroxyl group was acetylated to surpass the problem of signal overlapping of H5 and H6 in the ¹ H NMR. The relative stereochemistry of the corresponding ester determined by NMR interpretation was in agreement with the structure of (rac)-3a.