Asymmetric strecker synthesis of alpha-amino acids via a crystallization-induced asymmetric transformation using (R)-phenylglycine amide as chiral auxiliary

[reaction: see text]. Diastereoselective Strecker reactions based on (R)-phenylglycine amide as chiral auxiliary are reported. The Strecker reaction is accompanied by an in situ crystallization-induced asymmetric transformation, whereby one diastereomer selectively precipitates and can be isolated in 76-93% yield and dr > 99/1. The diastereomerically pure alpha-amino nitrile obtained from pivaldehyde (R1 = t-Bu, R2 = H) was converted in three steps to (S)-tert-leucine in 73% yield and >98% ee.     

Complex dynamic behavior of fluctuating smectic- A films as studied by scattering with coherent X-rays

Coherent dynamic x-ray scattering has been used to study the thermally excited layer fluctuations in freely suspended smectic films of the compound 4O.8. Using 8-keV x rays and films with a thickness around 0.3 &mgr;m we resolve relaxation times down to a few &mgr;s. A combination of damped and oscillatory behavior is observed for the layer undulations, which can be attributed to inertial effects. These are due to the surface contribution to the free energy which cannot be disregarded for thin films.     

Dichlorocarbene Addition to

In contrast to the terminal phosphinidene complex PhPW(CO)(5) (2), which adds to [5]metacyclophane (1) in a 1,4-fashion, dichlorocarbene preferentially adds in a 1,2-fashion to the formal "anti-Bredt" type double bond of the aromatic ring of 1 to afford the norcaradiene 11b, which immediately rearranges to the bridged cycloheptatriene 12b and further by a [1,5] sigmatropic chlorine migration to the isomeric 13b as the first observable product. More slowly, the latter isomerizes via a dissociative mechanism to give 15b. A computational study supports the notion that the [1,5] chlorine migration in the rearrangement 12b --> 13b, for which an activation barrier of 70.2 kJ mol(-)(1) was calculated, is essentially concerted with minor charge separation. In contrast, the analogous [1,5] chlorine migration in the flat model compound 7,7-dichlorocycloheptatriene (12a) displays features of a dissociative pathway.     

Enantioselective high-performance liquid chromatographic separation of N-methyloxycarbonyl unsaturated amino acids on macrocyclic glycopeptide stationary phases

This paper describes the enantiomeric resolution of a series of unsaturated N-methyloxycarbonyl-alpha-H-alpha-amino acids (N-MOC-alpha-amino acids) on macrocyclic glycopeptide stationary phases by means of high-performance liquid chromatography (HPLC). Three types of glycopeptide phases, i.e. Chirobiotic T, V and R, were evaluated in both reversed-phase (RP) and polar ionic mode (PIM). The best results in terms of enantioselectivity and resolution were obtained on Chirobiotic R phase, with the PIM mobile phase giving the highest resolution per min. Investigation of the pH of the reversed-phase mobile phase in the pH range 4.1-5.9 showed little effect on enantioselectivity. The method was applied for monitoring the conversion and product enantiomeric excess of an enzymatic hydrolysis reaction using N-MOC-alpha-H-alpha-amino acid esters as substrate.     

Measurement of leptonic branching ratios of the Γ(9.46)

The electronic width Γ _ee and the muonic branching ratioB _μμ =Γ _μμ /Γ_tot of the Γ(9.46) have been measured as Γ _ee =(1.33±0.14) andB _μμ=(2.2±2.0)%. From these values a lower limit of Γ_tot<23keV for the total width of the Γ(9.46) is obtained.