Thermoanalytical studies of rubber oxidation catalyzed by metallic ions

The effects of several metallic ions on the oxidation of natural rubber were studied by differential scanning calorimetry (DSC). The activation energy of oxidation was evaluated from DSC curves. Cobalt, manganese, iron and copper ions decreased the activation energy of oxidation. Nickel, zinc and tin ions did not affect the activation energy. The effective ions are those which are able to undergo one-electron transfer redox reaction with hydroperoxide. Based on the activation energy values, the catalytic activities of metallic ions are: Co > Mn > Cu ? Fe > Ni ? Zn ? Sn.     

Regio- and enantioselective intermolecular rhodium-catalyzed [2+2+2] carbocyclization reactions of 1,6-enynes with methyl arylpropiolates

Transition metal-catalyzed [m+n+o] carbocyclization reactions provide powerful methods for the construction of complex polycyclic systems that are generally not accessible through classical pericyclic reactions. We have developed the first regio- and enantioselective crossed intermolecular rhodium-catalyzed [2+2+2] carbocyclization of carbon- and heteroatom-tethered 1,6-enynes with unsymmetrical 1,2-disubstituted alkynes. This study clearly delineates the ligand requirements for obtaining excellent regio- and enantioselectivity. Furthermore, the ability to utilize various electron-withdrawing groups, and to introduce quaternary carbon stereogenic centers, provides the level of versatility necessary for its application to target-directed synthesis. Additional studies on the development and application of this novel methodology to the total synthesis of natural products are currently underway.     

Ab initio calculation of molar volumes: Comparison with experiment and use in solvation models

A simple and efficient procedure of calculating molecular volume (V_M) based on the Monte Carlo method is presented. The volume of a molecule is defined by the volume occupied by the 0.001-au electron density envelope. We have employed this method to compute the molecular volumes (V_M) of a large selection of organic molecules and compare them with the corresponding molar volumes (V_m) measured in the liquid state. A strong correlation is found to exist between the V_M and V_m values (V_M/V_m ≈ 0.75). Using this linear relationship, the calculated molecular volume may provide an estimate of the cavity-volume parameter in solvent-effect calculations. As a chemical application of molecular volume, we have investigated the conformational equilibrium of 1,2-difluoroethane in the liquid state using the self-consistent reaction field theory. © 1995 by John Wiley & Sons, Inc.     

Photochemical Degradation of the Plant Growth Regulator 2‐(1‐Naphthyl) acetamide in Aqueous Solution Upon UV Irradiation

The photochemical degradation of 2‐(1‐naphthyl) acetamide (NAD) in aqueous solution using simulated sunlight excitation as well as UV light within the 254–300 nm range was investigated to obtain an insight into the transformation mechanism that could occur under environmental conditions. Several photoproducts were identified using HPLC/MS/MS techniques. The degradation quantum yield was found to be independent of the excitation wavelength, but showed a dependence of oxygen concentration. This increased by a factor of approximately 3 from aerated to oxygen‐free solutions. There is a clear involvement of both triplet and singlet excited states in NAD photoreactivity. The participation of singlet oxygen as a significant route in NAD degradation was ruled out by comparison with the behavior using Rose Bengal as a photosensitizer. A mechanistic pathway implying hydroxylation process through NAD radical cation species as well as an oxidation reaction by molecular oxygen is proposed. The photochemical behavior of NAD appears to mainly involve the aromatic moieties without any participation of the amide side chain. Toxicity tests clearly show that the generated primary photoproducts are responsible for a significant increase in the toxicity. However, upon prolonged irradiation this toxicity tends to decrease.     

Synthesis of structurally diverse biflavonoids

Synthetic biflavonoids are associated with interesting biological activities, yet they remain poorly explored within drug discovery. Recent years have witnessed a growing interest in synthetic approaches that can provide access to structurally novel biflavonoids so that the biological usefulness of this compound class can be more fully investigated. Herein, we report upon the exploration of strategies based around Suzuki-Miyaura cross-coupling and alcohol methylenation for the synthesis of two classes of biflavonoids: (i) rare ‘hybrid’ derivatives containing flavonoid monomers belonging to different subclasses, and (ii) homodimeric compounds in which the two flavonoid monomers are linked by a methylenedioxy group. Application of these strategies enabled the preparation of a structurally diverse collection of novel biflavonoids from readily-available starting materials, thereby facilitating the probing of uncharted regions of biologically interesting chemical space.