Improved Aldehyde Synthesis: Preparation of 3α, 7α, 12α-Triacetoxy-5β-Cholan-23-al with Ruthenium Tetroxide in Neutral Medium

In relation to our studies on the metabolism of neutral sterols² we have prepared several substituted 24-norcholanic acids. Ruthenium tetroxide was selected as the oxidizing agent^3,4 in the formation of 24-norcholic acid^5,6 from the olefin, 3α, 7α, 12α-triacetoxy-24, 24-diphenyl-5β-chol-23-ene (II), (Fig. 1) since CrO₃ provided multiple products⁵.     

The photo-dissociation of the pyrrole-ammonia complex--the role of hydrogen bonding in Rydberg states photochemistry

The photochemistry of the pyrrole-ammonia cluster is analyzed theoretically. Whereas in neat pyrrole the dominant photochemical reaction is H-atom cleavage, recent experiments show that in pyrrole-ammonia clusters the major reaction is H-transfer to form the NH(4) radical (solvated by ammonia molecules in the case of large clusters) and the pyrrolyl radical. A mechanism involving the hydrogen-bonded Rydberg state is offered to account for these results and verified computationally. Two minima are located on the lowest excited singlet PES. Both of them are Rydberg states, one leads to the formation of NH(4) and pyrrolyl radicals, the other is connected to the πσ* state through a relatively high barrier, leading to a 3-body dissociation reaction to form a pyrrolyl radical, ammonia and an H-atom. The former is the energetically and statistically preferred one.     

Intermediate-energy proton-4He elastic scattering with a microscopic optical potential

A microscopic, momentum space, optical potential calculation of elastic p-⁴He scattering is compared with 100–200 MeV data over the full angular range. The least sophisticated potential explains the occurrence and energy dependence of the back angle peak.     

Optimal insurance coverage in situations of pure and speculative risk and the risk-free asset

The insured's portfolio consists of an insurable (pure) risk, an uninsurable (speculative) risk, a (proportional) insurance policy and a risk-free asset. The optimal insurance policy (i.e., the proportion to be insured) is examined from the insured's point of view, using the reward to variability concept. The importance of the risk-free asset in reaching an exact and explicit solution is analyzed, while emphasizing the possibility of substitution of the risk-free investment and insurance mechanisms. The paper demonstrates possibilities of improving the insured's welfare by the use of the risk-free rate - which is sometimes less expensive than other risk reduction instruments. The analysis leads to a two-step solution, similar to the well- known Hirschleifer investment model and to the famous Capital Assets Pricing Model.     

Polarization of electronic transitions in some alicyclic saturated ketones

The direction of polarization of the symmetry-allowed short wavelength transition in alicyclic saturated ketones (λ_max ≈ 180 nm) has been estimated. It was found, from measurements of linear dichroism of steroidal polycyclic ketones incorporated in stretched polyethylene films that this band is polarized along the CO bond axis. This direction is in accord with both π-π^* and n-σ_CC^* assignments.     

Chemical and photochemical electron transfer of new helianthrone derivatives: aspects of their photodynamic activity

Helianthrones 2-4 are a new class of synthetic photosensitizers, which have a molecular skeleton related to that of hypericin. We established that irradiation of heliantrones with visible light leads to the formation of semiquinone radicals and reactive oxygen species. The structures of the paramagnetic anion species produced by electron transfer were calculated on the density functional level and investigated by cyclovoltammetry, UV/vis, and EPR/ENDOR spectroscopy. As with hypericin, the pi system of the helianthrones was found to be considerably deviated from planarity, and, upon electron transfer, deprotonation in the bay region occurs. The structure of the semiquinone radicals was found to be identical in THF, DMF, and aqueous buffered solutions regardless of the means by which reduction was achieved. Semiquinone radicals can be formed via self-electron transfer between the excited state and the ground state or via electron transfer from an electron donor to the excited state of helianthrone. Therefore, the presence of an electron donor significantly enhanced the photogeneration of semiquinone and superoxide radical. The kinetic studies showed that no significant photochemical destruction of helianthrones occurred upon irradiation. Generation of superoxide and singlet oxygen upon irradiation of helianthrones was established by spin trapping techniques. This shows that both type I and type II mechanisms are of importance for the photodynamic action of these compounds.     

Electronic degeneracies in symmetric (Jahn-Teller) and nonsymmetric aliphatic radical cations: global topology of sigma-bonded molecules

The ground-state potential surfaces of five aliphatic radical cations are investigated using a spin-pairing model. It is shown that the ground-state surface of an n-atomic system supports several stationary points (minima and transition states, including second-order ones). In addition, there are numerous nuclear configurations at which the ground state is electronically degenerate. The electronic degeneracies due to interactions between atoms bound to the same atom are either 2-fold (conical intersections) or 3-fold degenerate but not of a higher dimension. Each 3-fold degeneracy is accompanied by an even number of conical intersections (four or two). A systematic procedure for locating all of these nuclear configurations (that are in fact 3n - 8 or 3n - 11 dimensional hypersurfaces) is described. The model allows for the qualitative determination of the structure and charge distribution of the system at all of the stationary points and electronic degeneracies. Quantum chemical calculations confirm the predictions of the model, which is used to direct and facilitate the calculations.