Enantioselective partial reduction of 2,5-disubstituted pyrroles via a chiral protonation approach

[reaction: see text] The partial reduction of 2,5-pyrrole diester 1 followed by enantioselective protonation in situ to furnish synthetically useful building blocks is described. An enantiomeric excess of up to 74% was achieved using (-)-ephedrine and related analogues as chiral proton sources. The pyrroline product obtained could be recrystallized to give enantiomerically pure material.     

A Nuclear Singlet Lifetime of More than One Hour in Room‐Temperature Solution

Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are supremely important techniques with numerous applications in almost all branches of science. However, until recently, NMR methodology was limited by the time constant T₁ for the decay of nuclear spin magnetization through contact with the thermal molecular environment. Long‐lived states, which are correlated quantum states of multiple nuclei, have decay time constants that may exceed T₁ by large factors. Here we demonstrate a nuclear long‐lived state comprising two ¹³C nuclei with a lifetime exceeding one hour in room‐temperature solution, which is around 50 times longer than T₁. This behavior is well‐predicted by a combination of quantum theory, molecular dynamics, and quantum chemistry. Such ultra‐long‐lived states are expected to be useful for the transport and application of nuclear hyperpolarization, which leads to NMR and MRI signals enhanced by up to five orders of magnitude.     

Dynamic covalent chemistry

Dynamic covalent chemistry relates to chemical reactions carried out reversibly under conditions of equilibrium control. The reversible nature of the reactions introduces the prospects of "error checking" and "proof-reading" into synthetic processes where dynamic covalent chemistry operates. Since the formation of products occurs under thermodynamic control, product distributions depend only on the relative stabilities of the final products. In kinetically controlled reactions, however, it is the free energy differences between the transition states leading to the products that determines their relative proportions. Supramolecular chemistry has had a huge impact on synthesis at two levels: one is noncovalent synthesis, or strict self-assembly, and the other is supramolecular assistance to molecular synthesis, also referred to as self-assembly followed by covalent modification. Noncovalent synthesis has given us access to finite supermolecules and infinite supramolecular arrays. Supramolecular assistance to covalent synthesis has been exploited in the construction of more-complex systems, such as interlocked molecular compounds (for example, catenanes and rotaxanes) as well as container molecules (molecular capsules). The appealing prospect of also synthesizing these types of compounds with complex molecular architectures using reversible covalent bond forming chemistry has led to the development of dynamic covalent chemistry. Historically, dynamic covalent chemistry has played a central role in the development of conformational analysis by opening up the possibility to be able to equilibrate configurational isomers, sometimes with base (for example, esters) and sometimes with acid (for example, acetals). These stereochemical "balancing acts" revealed another major advantage that dynamic covalent chemistry offers the chemist, which is not so easily accessible in the kinetically controlled regime: the ability to re-adjust the product distribution of a reaction, even once the initial products have been formed, by changing the reaction's environment (for example, concentration, temperature, presence or absence of a template). This highly transparent, yet tremendously subtle, characteristic of dynamic covalent chemistry has led to key discoveries in polymer chemistry. In this review, some recent examples where dynamic covalent chemistry has been demonstrated are shown to emphasise the basic concepts of this area of science.     

Critical velocity of continuous vortex nucleation in a slab of superfluid 3He-A

A flow-induced Fréedericksz transition is observed in a 0.26 mm thick disk-shaped slab of superfluid 3He-A using a rotating cryostat and a torsional oscillator, and it is used to detect vortices in zero magnetic field. The phenomena are studied as a function of magnetic field normal to the slab. In defect-free l texture the critical velocity for vortex nucleation is 0.5 mm/s, but in the presence of a domain wall it is reduced to approximately Planck's over 2pi /2ma(c), where a(c)(H) is the field-dependent radius of the vortex soft core. The vortices nucleate at a distance at least 0.3 mm from the outer edge of the disk.     

Semisuperfluidity of 3He in aerogel?

According to hydrodynamic, acoustic, and NMR studies the superfluid transition temperature of 3He in aerogel ( T(a)(c)) is significantly suppressed with respect to that of bulk 3He. We have found in the range of temperatures between T(c) and T(a)(c) a large and unexpected NMR satellite line attributable to the liquid inside the aerogel. We propose that this anomalous behavior of liquid 3He corresponds to a new type of superfluid ordering related to magnetic and possibly orbital coherence.     

Frequency effects on the conduction electron spin resonance linewidth in aluminium

The CESR linewidth of pure aluminium is shown to have a temperature dependent phonon contribution which is largely frequency independent. A second term in the linewidth is found to be temperature independent but linearly dependent upon frequency giving 0.11 mT per GHz. Finally it is shown that if partial breakdown of motional narrowing occurs it contributes a term proportional to the frequency and not, as expected, to the frequency squared.     

Beta-alpha correlation in the decay of 20Na

We have measured the β-α angular correlation in the allowed positron decay of ²⁰Na to the 7.42 MeV state of ²⁰Ne. The correlation is of the form $\text{1 +}\text{a}\text{cos}\text{θ +}\text{p}\text{cos}{}^2\text{θ}$. We find $\text{p}\text{= ?0.003 ± 0.005}$ in reasonable agreement with expectations based on the conserved vector current theory.     

Simultaneous selection,amplification and isolation of a pseudo-peptide receptor by an immobilised N-methyl ammonium ion template

Immobilised N-methyl ammonium ions have been used to simultaneously select, amplify and isolate a pseudo-peptide receptor from a dynamic library of hydrazones.