Flash Chemistry Extensively Optimized: High‐Temperature Swern–Moffatt Oxidation in an Automated Microreactor Platform

The generally accepted benefits of small lateral dimensions of microreactors (1 μm to 1 mm) enable a different way of performing synthetic chemistry: Extremely short contact times in the millisecond range can circumvent the need for performing highly exothermic and fast reactions at very low temperatures. In order to fully exploit this technology, such fast processes need to be redesigned and investigated for optimal reaction conditions, which can differ drastically from the ones traditionally applied. In a comprehensive study, we optimized the selective Swern–Moffatt oxidation of benzyl alcohol to benzaldehyde by varying five experimental parameters, including reaction time and temperature. Employing an ultrashort mixing and reaction time of only 32 ms, the optimal temperature was determined to be 70 °C, approximately 150 °C higher than in the conventional batch conditions. This remarkable difference shows both the potency of continuous‐flow chemistry as well as the urgency of a paradigm shift in reaction design for continuous‐flow conditions.     

Enantiomerically Pure Thrombin Inhibitors for Exploring the Molecular‐Recognition Features of the Oxyanion Hole

A new route via intermediate pseudoenantiomers was developed to synthesize racemic and enantiomerically pure new non‐peptidic inhibitors of thrombin, a key serine protease in the blood‐coagulation cascade. These ligands feature a conformationally rigid tricyclic core and are decorated with substituents to fill the major binding pockets (distal (D), proximal (P), selectivity (S1), and oxyanion hole) at the thrombin active site (Fig. 1). The key step in the preparation of the new inhibitors is the 1,3‐dipolar cycloaddition between an optically active azomethine ylide, prepared in situ from L ‐(4R)‐hydroxyproline and 4‐bromobenzaldehyde, and N‐piperonylmaleimide (Scheme 1). According to this protocol, tricyclic imide (compounds (±)‐ 15 ‐(±)‐ 18 and (+)‐ 21 ) and lactam (compound (+)‐ 2 ) inhibitors with OH or ether substituents at C(7) in the proline‐derived pyrrolidine ring were synthesized to specifically explore the binding features of the oxyanion hole (Schemes 2–4). Biological assays (Table) showed that the polar oxyanion hole in thrombin is not suitable for the accommodation of bulky substituents of low polarity, thereby confirming previous findings. In contrast, tricyclic lactam (+)‐ 2 (K_i=9 nM , K_i(trypsin)/K_i(thrombin)=1055) and tricyclic imide (+)‐ 21 (K_i=36 nM , K_i(trypsin)/K_i(thrombin)=50) with OH‐substituents at the (R)‐configured C(7)‐atom are among the most‐potent and most‐selective thrombin inhibitors in their respective classes, prepared today. While initial modeling predicted H‐bonding between the OH group at C(7) in (+)‐ 2 and (+)‐ 21 with the H₂O molecule bound in the oxyanion hole (Fig. 2), the X‐ray crystal structure of the complex of (+)‐ 21 (Fig. 7, b) revealed a different interaction for this group. The propionate side chain of Glu192 undergoes a conformational change, thereby re‐orienting towards the OH group at C(7) under formation of a very short ionic H‐bond (O? H???^?OOC; d(O???O)=2.4 Å). The energetic contribution of this H‐bond, however, is negligible, due to its location on the surface of the protein and the unfavorable conformation of the H‐bonded propionate side chain.     

High spin states in 210Pb

High spin states in nuclei in the ²⁰⁸Pb region were populated in deep inelastic collisions of ¹³⁶Xe and ²⁰⁸Pb projectiles with thick ²⁰⁸Pb targets at about 50 MeV above the Coulomb barrier. Prompt and prompt-delayed γ-ray coincidences were measured. Four new states above the 8⁺ isomer in ²¹⁰Pb were found and spins and parities assigned from theoretical expectations. The results are discussed in the frame of shell model theory.     

2 μm semiconductor disk laser with a heterodyne linewidth below 10 kHz

We demonstrate a 2 μm semiconductor disk laser emitting in a single longitudinal mode with a linewidth in the <10 kHz range. A heterodyne detection scheme was used for precise linewidth measurements. In these experiments, the output beams of two identical laser cavities were superposed in order to generate a beat note signal on a photodiode. In the absence of active frequency stabilization, a linewidth of 45 kHz was measured at an output power of 100 mW. When using a frequency stabilization consisting of a feedback loop with a Fabry-Perot interferometer as wavelength reference, the linewidth could be further reduced to 9 kHz.     

An extension of Tikhonov's theorem in singular perturbations for the planar case

A classical error estimate for singularly perturbed ordinary differential equations due to A. N. Tikhonov is generalized in the neighbourhood of a so-called leaving point where a certain stability assumption ceases to be valid. This is done for the planar case, and an application to the Van der Pol relaxation oscillator is given.     

TiCl4-catalyzed intermolecular hydroamination reactions of norbornene

[reaction: see text] An intermolecular hydroamination reaction of norbornene is presented that uses catalytic amounts of user-friendly TiCl(4) and tolerates a variety of functional groups. In addition, a secondary amine is converted using this methodology.     

Synthesis and characterization of vanadium(IV) complexes with cis-inositol in aqueous solution and in the solid-state

The complex formation of vanadium(IV) with cis-inositol (ino) and the corresponding trimethyl ether 1,3,5-trideoxy-1,3,5-trimethoxy-cis-inositol (tmci) was studied in aqueous solution and in the solid-state. With increasing pH, the formation of [VO(H-2L)], [(VO)2L2H-5]-, [VO(H-3L)]- (L = ino) or [(VO)2L2H-6]2- (L = tmci), [V(H-3L)2]2-, and [VO(H-3L)(OH)2]3- was observed. For the vanadium(IV)/ino system, [(VO)2L2H-7]3- was observed as an additional dinuclear species. The formation constants of these complexes were determined by potentiometric titrations (25 degrees C, 0.1 M KCl). In addition, the vanadium(IV)/ino system was investigated by means of UV-vis spectrophotometric methods. EPR spectroscopy and cyclic voltammetry confirmed this complexation scheme. EPR measurements indicated the formation of three distinct isomers of the non-oxo complex [V(H-3ino)2]2- in weakly basic solution. This type of isomerism, which is not observed for the vanadium(IV)/tmci system, was assigned to the ability of ino to bind the vanadium(IV) center with three alkoxo groups having either a 1,3,5-triaxial or an 1,2,3-axial-equatorial-axial arrangement. The structures of [V(H-3ino)2][K2(ino)2].4H2O (1) and [Na6V(H-3ino)2](SO4)2.6H2O (2) were determined by single-crystal X-ray analysis. In both compounds, the coordination of each ino molecule to the vanadium(IV) center via three axial deprotonated oxygen donors was confirmed. The centrosymmetric structure of the coordination spheres corresponds to an almost regular octahedral geometry with a twist angle of 60 degrees. The crystal structure of the potassium complex 1 represents an unusual 1:1 packing of [V(H-3ino)2]2- dianions and [K2(ino)2]2+ dications, in which both K+ ions have a coordination number of nine and are bonded simultaneously to a 1,3,5-triaxial and an 1,2,3-axial-equatorial-axial site of ino. In 2, the [V(H-3ino)2]2- complexes are surrounded by six Na+ counterions that are bonded to the axial alkoxo oxygens and to the equatorial hydroxy oxygens of the cis-inositolato moieties. The six Na+ centers are further interlinked by bridging sulfate ions. According to EPR spectroscopy, the D3d symmetric structure of the [V(H-3ino)2]2- anion is retained in H2O, in dimethylformamide, and in a mixture of CHCl3/toluene 60:40 v/v.