EPR/spin trapping study of the spontaneous addition of dioxygen on a dienol

Theoretical Studies of Autoxidation of 2-Alkylidene-1,3-cyclohexadione Leading to Bicyclic-Hemiketal Endoperoxides

Mechanism of the addition of molecular oxygen on the dienolic form of the 2-alkylidene-1,3-cyclohexadione was investigated by quantum chemical calculations using the approximate projection method developed by Yamaguchi. The complete reaction pathway of the formation of the endoperoxide is described. The crossing between triplet and singlet potential energy surfaces has been located. A multireference complete active space self-consistent field calculation has been performed to strengthen the results.

The mechanism of Rubisco-catalysed oxygenation

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the cornerstone of photosynthetic carbon assimilation because it catalyses the fixation of CO2 onto ribulose-1,5-bisphosphate (RuBP). The enzyme also catalyses RuBP oxygenation, thereby evolving phosphoglycolate which is recycled along the photorespiratory pathway. Oxygenation is quantitatively important, because under ordinary gaseous conditions, more than one third of RuBP molecules are oxygenated rather than carboxylated. However, contrary to carboxylation, the chemical mechanism of oxygenation is not well known, and little progress has been made since the early 80s. Here, I review recent experimental data that provide some new insights into the reaction mechanism, and carry out simple calculations of kinetic parameters. Isotope effects suggest that oxygenation is less likely initiated by a redox phenomenon (such as superoxide production) and more likely involves concerted chemical events that imply interactions with protons. A possible energy profile of the reaction is drawn which suggests that the generation of the oxygenated reaction intermediate (peroxide) is irreversible. Possible changes in oxygenation-associated rate constants between Rubisco forms are discussed.

Spontaneous Oxygenation of Siloxy-N-silylketenimines to α-Ketoamides

Siloxy-N-silylketenimines generated in situ from O-silyl cyanohydrins were converted to α-ketoamides by brief exposure to air or oxygen. Oxidation under extremely mild conditions can be explained by assuming the intermediacy of a 3-imino-1,2-dioxetane derivative generated via triplet-singlet intersystem crossing after the reaction of siloxy-N-silylketenimines with triplet oxygen.

Divergent Total Syntheses of Rhodomyrtosones A and B

Herein, we report total syntheses of the tetramethyldihydroxanthene natural product rhodomyrtosone B and the related bis-furan β-triketone natural product rhodomyrtosone A. Nickel-(II)-catalyzed 1,4-conjugate addition of an α-alkylidene-β-dicarbonyl substrate was developed to access the congener rhodomyrtosone B, and oxygenation of the same monoalkylidene derivative followed by cyclization was employed to obtain the bis-furan natural product rhodomyrtosone A.

Synthetic studies toward (+)-cortistatin A

We describe herein the synthesis of a late-stage intermediate en route to cortistatin A. Key transformations included a Snieckus-like cascade sequence culminating in a 6π-electrocyclization, an alkylative dearomatization, and the stereoselective functionalization of the cortistatin A-ring. While the total synthesis we sought was not accomplished, the work sets the stage for several approaches to the preparation of novel analogs via diverted total synthesis.

Weak chemiluminescence emission during base induced rearrangement of G-factors

A rearrangement in basic medium of the natural endoperoxide G3-factor extracted from Eucalyptus grandis is described. Evidence to support a 1,2-dioxetane intermediate that decomposes with weak luminescence emission (quantum yield) is presented.

Intramolecular azide trapping of the Nazarov intermediate: formation of peroxy-bridged indolizidinones via a deep-seated rearrangement and aerobic oxidation

Cross-conjugated dienones with pendent azide side chains undergo interrupted Nazarov trapping, leading to peroxy-bridged indolizidinones in good yields. This process is proposed to involve skeletal rearrangement of the initial trapping product, with loss of dinitrogen, to give an intermediate 1,4-betaine, which then undergoes reaction with atmospheric oxygen. The endoperoxide products can be reduced under catalytic hydrogenation conditions to furnish alpha-hydroxylactams. [reaction: see text].

Theoretical Investigation of Linalool Oxidation

This study concerns the autoxidation of one of the most used fragrances in daily life, linalool (3,7-dimethyl-1,6-octadien-3-ol). It reacts with O2 to form hydroperoxides, which are known to be important contact allergens. Pathways for hydroperoxide formation are investigated by means of quantum mechanical electronic structure calculations. Optimized molecular geometries and harmonic vibrational frequencies are determined using density functional theory (DFT). Insight into how the addition of O2 to linalool occurs is obtained by establishing a theoretical framework and systematically investigating three smaller systems: propene, 2-methyl-2-butene, and 2-methyl-2-pentene. 2-Methyl-2-pentene was chosen as a model system and used to compare with linalool. This theoretical study characterizes the linalool-O2 biradical intermediate state, which constitutes a branching point for the further oxidation reactions pathways. Thus, the observed linalool oxidation product spectrum is discussed in terms of a direct reaction path, the ene-type mechanism, and the radical mechanism. The major hydroperoxide found in experiments is 7-hydroperoxy-3,7-dimethyl-octa-1,5-diene-3-ol, and the calculated results support this finding.