SEAr-SNAr couplings of indolizines and related pyrrole derivatives with superelectrophilic nitrobenzoxadiazoles

The nucleophilic aromatic substitutions of 7-chloro-4,6-dinitrobenzofurazan (DNBZ-Cl) and 7-chloro-4,6-dinitrobenzofuroxan (DNBF-Cl) with a series of differently substituted indolizines (5a-f) and a series of dihyropyrroloisoquinolines (11a-f) have been investigated. In accord with previous reports emphasizing the superelectrophilic character of these compounds in σ-complexation processes, DNBZ-Cl and DNBF-Cl react very readily and quantitatively with the weak carbon nucleophiles 5a-f and 11a-f at room temperature in acetonitrile. In the case of DNBZ-Cl, the resulting products (7Z,a-f and 12Z,a-f) are those expected from the displacement of the chlorine atom through a S_EAr-S_NAr mechanism. A significant result is that these compounds, despite the lack of coplanarity of the two rings, are characterized by an intense intramolecular charge transfer between the donor pyrrole-type moiety and the electron-deficient acceptor DNBZ moiety. Contrasting with this behaviour, the DNBF-Cl reactions show a totally different pattern, proceeding with loss of the N-oxide functionality and expansion of the pyrrole moiety to afford stable zwitterionic spiro adducts (8F,a-f and 13F,a-f) of a so far unknown type. Rapid NMR recordings have revealed that the formation of these adducts occurs after initial formation of the expected substitution products 7F,a-f and 12F,a-f. A mechanism accounting for the overall rearrangement leading to the spirobenzofurazan adducts is suggested. It is based on an initial nucleophilic attack of the oxygen atom of the N-oxide functionality at the electron-deficient and strongly olefinic C-C coupling bond generated by the aforementioned intramolecular charge transfer. This results in the formation of an unstable five-membered isoxazole ring whose decomposition goes along with loss of the N-oxide functionality and enlargement of the pyrrole moiety into a pyridine one. Also discussed are the factors accounting for the high thermodynamic stability of the spiro adducts, and their relevance to the stability of previously reported spiro adducts.     

AKNS-KN孤子方程族的可积耦合与Hamilton结构

首先通过引入高维圈代数,在零曲率方程框架下得到了AKNS-KN孤子族(记为AKNS-KN-SH)的一个新的可积耦合系统;再由二次型恒等式得到了该系统的双-Hamilton结构形式.最后引进了一个新的Lie代数A_4,可通过建立其不同的圈代数与等价的列向量Lie代数,研究AKNS-KN-SH的多分量可积耦合系统及其Hamilton结构.     

Simplified multiplet pattern HSQC-TOCSY experiment for accurate determination of long-range heteronuclear coupling constants

A new two-dimensional pulse sequence for accurately determining heteronuclear coupling constants is presented. It is derived from HSQC and HECADE techniques with B0 gradient coherence selection. The main feature of the proposed method is spectra with only one component of the IS doublet; i.e., the final result is equivalent to a selective broadband excitation of either Salpha or Sbeta spin states and a preservation of these states during the entire experiment. The effect is obtained by an appropriate combination of in- and antiphase coherences. It is demonstrated that heteronuclear single-bond as well as long-range coupling constants and their relative signs are readily evaluated. The proposed sequence is equally or less sensitive to a variation of heteronuclear one-bond couplings than previously published, closely related sequences. The new method is applied to a peptide sample for determination of 3JN, Hbeta.     

Weitreichende H,H-Kopplungskonstanten in N-methylierten Azolen

Long-range H,H-couplings in N-methylated azoles were studied.⁴ J couplings between N-methyl and adjacent ring protons could always be observed, whereas similar⁵ J couplings were not present. This result may be a useful tool for structure elucidation of isomeric N-methylated azoles.

     

Total Synthesis of Sanglifehrin A

The immunosuppressive agent sanglifehrin A has been prepared for the first time by total synthesis. The construction of the macrocyclic unit of the target molecule was achieved through a selective intramolecular Stille coupling, and the spirolactam unit by Paterson–aldol reactions. The final steps involve an intermolecular Stille coupling and the opening of the internal acetal unit. This convergent synthesis opens the way for the synthesis of libraries of novel sanglifehrin analogues for biological screening.     

Protein Backbone N Relaxation Rates as a Tool for the Diagnosis of Structure Quality

In the work reported herein we define a structure validation factor that depends on protein backbone ¹⁵N relaxation rates. This is an alternative method to the previously defined quality factors derived from anisotropic chemical shifts or residual dipolar couplings. We have used the structure dependence of ¹⁵N relaxation rates of anisotropically tumbling proteins to calculate this structure diagnosis factor and have used it to demonstrate the improvement of protein structures refined with residual dipolar couplings.     

Couplings of uniform spanning forests

We prove the existence of an automorphism-invariant coupling for the wired and the free uniform spanning forests on connected graphs with residually amenable automorphism groups.     

Magnon energy gap in a four-layer ferromagnetic superlattice

The magnon energy band in a four-layer ferromagnetic superlattice is studied by using the linear spin-wave approach and Green＇s function technique. It is found that three modulated energy gaps exist in the magnon energy band along Kx direction perpendicular to the superlattice plane. The spin quantum numbers and the interlayer exchange couplings all affect the three energy gaps. The magnon energy gaps of the four-layer ferromagnetic superlattice are different from those of the three-layer one. For the four-layer ferromagnetic superlattice, the disappearance of the magnon energy gaps △ω12, △ω23 and △ω34 all correlates with the symmetry of this system. The zero energy gap △ω23 correlates with the symmetry of interlayer exchange couplings, while the vanishing of the magnon energy gaps △ω12 and △ω34 corresponds to a translational symmetry of x-direction in the lattice. When the parameters of the system deviate from these symmetries, the three energy gaps will increase.