Diastereoselective synthesis of alpha-tocopherol: a new concept for the formation of chromanols

A diastereoselective synthesis of alpha-tocopherol 1 (93% de) was achieved via two key steps, (i) a highly diastereoselective Shi epoxidation of a trisubstituted alkene and (ii) an acid supported, "anti-Baldwin" epoxide ring opening under inversion of configuration leading to the 6-membered chromanol ring.     

Temporal processing deficits in the pre-senescent auditory system

This study tested the hypothesis that temporal processing deficits are evident in the pre-senescent (middle-aged) auditory system for listening tasks that involve brief stimuli, across-frequency-channel processing, and/or significant processing loads. A gap duration discrimination (GDD) task was employed that used either fixed-duration gap markers (experiment 1) or random-duration markers (experiment 2). Independent variables included standard gap duration (0, 35, and 250 ms), marker frequency (within- and across-frequency), and task complexity. A total of 18 young and 23 middle-aged listeners with normal hearing participated in the GDD experiments. Middle age was defined operationally as 40-55 years of age. The results indicated that middle-aged listeners performed more poorly than the young listeners in general, and that this deficit was sometimes, but not always, exacerbated by increases in task complexity. A third experiment employed a categorical perception task that measured the gap duration associated with a perceptual boundary. The results from 12 young and 12 middle-aged listeners with normal hearing indicated that the categorical boundary was associated with shorter gaps in the young listeners. The results of these experiments indicate that temporal processing deficits can be observed relatively early in the aging process, and are evident in middle age.     

Low-energy pions in nuclear matter and ππ photoproduction within a BUU transport model

In the present paper we investigate a method to describe low-energy scattering events of pions and nuclei within a Boltzmann-Uehling-Uhlenbeck (BUU) transport model. Implementing different scenarios of medium modifications, we studied the mean-free path of pions in nuclear matter at low momenta and compared pion absorption simulations to data. Pursuing these studies we have shown, that also in a regime of a long pionic wave length the semi-classical BUU model still generates reasonable results. We present results on π-induced events in the regime of 10 MeV≤T _kin ^π ≤130 MeV and photo-induced ππ production at incident beam energies of 400–460 MeV.     

Bis-ortho-substitution by methyl groups dramatically increases the racemization barrier of Tröger bases

We have shown through racemization kinetics studies that the enantiomerization barriers of the bis‐ortho‐methyl substituted Tröger bases 2 and 3 in acidic media are raised by 30 kJ mol^?1 relative to the parent compound 1 , that is 130.4(4) and 131.6(4) kJ mol^?1, respectively (105 °C, pH 1, ethylene glycol). The enantiomerization barrier of para‐methoxy‐para‐nitro substituted Tröger base 4 was determined by dynamic capillary electrophoresis to 96.3(2) kJ mol^?1 (25 °C, pH 2.2, H₂O), which is lower by 5 kJ mol^?1 relative to 1 . The influence of deutero‐substitution on the racemization rates was also studied. The influence of steric and electronic factors on the enantiomerization barrier was investigated by quantum‐mechanical (DFT) calculations. It is shown that enantiomerization takes place in two steps: ring‐opening and further interconversion of the monocyclic intermediate. For the interconversion to occur a transition state has to be passed which is sensitive to steric effects. Ortho‐substitution by methyl groups significantly increases the energy of this state. Thus, compounds 2 and 3 are the simplest Tröger bases which are configurationally stable in acidic media.