Hemiortho Esters and Hydrotrioxides as the Primary Products in the Low-Temperature Ozonation of Cyclic Acetals: An Experimental and Theoretical Investigation

Discovery of neo-Clerodane Diterpenoids from Ajuga campylantha as Neuroprotective Agents against Ferroptosis and Neuroinflammation

Twelve new neo-clerodane diterpenoids, eight undescribed methoxy/ethoxy acetal analogues, and one new nor-iridane monoterpenoid were isolated from Ajuga campylantha. Their structures were elucidated using a combination of spectroscopic data, quantum chemical calculations, and X-ray crystallography. This research reveals the distinctive structural features of A. campylantha diterpenes, including distinct C rings and 4,18-double bonds, distinguishing them from diterpenes of other plants in the Ajuga genus. Compound 2 represents the first example of a 19(5→6)-abeo-clerodane formed through a Wagner-Meerwein rearrangement. The isolated compounds were assessed for their neuroprotective effects against RSL3-induced ferroptosis in HT22 cells and LPS-induced neuroinflammation in BV-2 cells. Notably, compound 7 inhibits ferroptosis (EC50 = 10 μM) with a potentially new mechanism of action. The preliminary structure-activity relationship studies revealed that the furan-clerodane diterpenoids possess potential ferroptosis inhibitory activity, while the lactone-clerodanes do not. This study represents the first report of furan-containing clerodanes within the Ajuga genus, providing fresh insights into the phytochemistry and pharmacological potential of A. campylantha.

Mechanistic and Synthetic Implications of the Diol-Ritter Reaction: Unexpected Yet Reversible Pathways in the Regioselective Synthesis of Vicinal-Aminoalcohols

The Ritter reaction of 1,2-diolmonoesters with nitriles to 1- vic-amido-2-esters proceeds through dioxonium and nitrilium cation intermediates. To provide the basis for the reaction mechanism, novel forms of these cations were isolated, characterized, and studied by spectroscopic methods and single crystal X-ray analysis. Ground and transition state energies were determined both experimentally and theoretically. Taken together, these data suggest that the reaction proceeds via rapid formation of the dioxonium cation 9, followed by rate determining yet reversible ring opening by acetonitrile to the corresponding nitrilium cation 10 (computed Δ G^⧧ = 24.7 kcal at 50 °C). Rapid, irreversible hydration of the latter affords the corresponding vic-acetamido ester. Controlled addition of H₂O to the dioxonium cation 9 in acetonitrile- d₃ results in near-quantitative production of deuterated acetamido ester 13a. Kinetics of this conversion (9 to 13a) are biphasic, and the slow phase is ascribed to either direct cation 9 attack by acetamide to form cation 16 via O-alkylation or by reversible ether formation. Deuterium labeling studies suggest O-alkylated cation 16 does not directly isomerize to N-alkylated cation 18; instead, it reverts to vic-amidoester 13a via the nitrilium pathway. Preliminary results indicate high regioselectivity for primary amide formation in the diol-Ritter sequence.

Mechanistic insights into ozone-initiated oxidative degradation of saturated hydrocarbons and polymers

Accurate quantum chemistry is used to identify the mechanism of ozonation of saturated C–H hydrocarbons and various polymer dimer units, and to show that the process can be catalysed by water and plays an important role in environmental polymer degradation.

Theoretical study on the atmospheric transformation mechanism of pirimiphos-methyl initiated by O3

Pirimiphos-methyl (PMM) is a widely used organophosphorus pesticide that can be released into the atmosphere in gas and condensed phases. It possesses a PS bond and an N,N-dialkyl group adjacent to pyrimidine, which are common functional groups for pesticides. Currently, the reaction mechanisms of O3 with these functional groups are poorly understood. In this study, the mechanisms and possible degradation products for O3-initiated atmospheric oxidation of PMM were investigated using the Density Functional Theory (DFT) method. The results show that H abstraction from the alpha carbon of the N,N-diethyl group and its subsequent reactions (hydroxylation, N-dealkylation, and carbonylation reactions), as well as the transformation of the PS bond to the PO oxone form, are the most favorable reaction pathways for PMM and O3. The Gibbs free energy (ΔG) indicates that the subsequent reactions tend to take place more spontaneously once the initial reaction occurs. In addition, theoretical calculations indicate that water can serve as an effective catalyst in the N-dealkylation reaction process. Water-assisted reactions lead to the activation energy decreasing by 20.2 kcal mol(-1) compared with direct reactions, and thus may represent a dominant reaction pathway for the N-dealkylation process in the atmosphere. These theoretical results provide new insights into O3-initiated degradation of PMM and its analogues.

Products and mechanisms of the heterogeneous reaction of three suspended herbicide particles with NO₃ radicals

Over 60% of herbicides are capable of disrupting the endocrine and/or reproductive systems of animals. These herbicides may be released into the atmosphere in both gas and particulate phases, but most of their degradation processes in the atmosphere are not well known. In this study, the heterogeneous reactions of suspended isopropalin, trifluralin, and alachlor particles with NO₃ radicals were investigated using an online vacuum ultraviolet photoionization aerosol time-of-flight mass spectrometer. The reaction products for the three herbicides were determined by the assistance of the gas chromatography-mass spectrometer analysis. Mono-dealkylated derivatives were detected as the main reaction products of isopropalin and trifluralin. In addition, an α-amino alcohol product was detected for isopropalin. The carbonylation derivative and the nitro-substituted derivative were the main reaction products observed for alachlor. The reaction mechanism of NO₃ radical-induced N-dealkylation for isopropalin was clarified by density functional theory calculations. It began with the H-abstraction from the N-propyl group, followed by the formation of the α-peroxypropyl radical, α-propyloxy radical, and α-amino alcohol, as well as by the collapse of the α-amino alcohol. The oxidation mechanism for trifluralin is similar to that of isopropalin, whereas the mechanism for alachlor involves carbonylation and nitro-substitution. These results provided insights on the chemical transformation of these herbicides in the atmosphere. The data generated from this study can be used as fundamental information for future studies on their toxic effects to the environment.

The Ozonation of Silanes and Germanes: An Experimental and Theoretical Investigation

Ozonation of various silanes and germanes produced the corresponding hydrotrioxides, R3SiOOOH and R3GeOOOH, which were characterized by 1H, 13C, 17O, and 29Si NMR, and by infrared spectroscopy in a two-pronged approach based on measured and calculated data. Ozone reacts with the E-H (E = Si, Ge) bond via a concerted 1,3-dipolar insertion mechanism, where, depending on the substituents and the environment (e.g., acetone-d6 solution), the H atom transfer precedes more and more E-O bond formation. The hydrotrioxides decompose in various solvents into the corresponding silanols/germanols, disiloxanes/digermoxanes, singlet oxygen (O2(1delta(g))), and dihydrogen trioxide (HOOOH), where catalytic amounts of water play an important role as is indicated by quantum chemical calculations. The formation of HOOOH as a decomposition product of organometallic hydrotrioxides in acetone-d6 represents a new and convenient method for the preparation of this simple, biochemically important polyoxide. By solvent variation, singlet oxygen (O2(1delta(g))) can be generated in high yield.

The Rotational Spectrum and Structure of HOOOH

Dihydrogen trioxide, HOOOH, which is a species with fundamental importance for understanding the chain formation ability of the oxygen atom, was detected in a supersonic jet by a Fourier transform microwave spectrometer with a pulsed discharge nozzle, together with double resonance and triple resonance techniques. Its precise molecular structure was determined from the experimentally determined rotational constants of HOOOH and its isotopomer, DOOOD. Many of the microwave and millimeter wave transitions can now be accurately predicted, which could be facilitated for remote sensing of the molecule to elucidate its roles in various chemical processes.