Conformationally Locked Cyclo[2]Dipyrrins Linked with Anthracene Subunits: Synthesis and Chiroptical Properties

Herein, we report the synthesis of anthracene-containing twisted cyclo[2]dipyrrin 1 by utilizing a non-planar building block, 1,5-dipyrrylanthracene (1,5-DPA). The non-planar nature of the macrocycle enhanced the solubility and helped in structural characterization. Macrocycle 1 adopts a twisted ‘figure of eight’ conformation stabilized by strong intramolecular H-bonding interactions and exists as a pair of helical enantiomers, as revealed by X-ray crystallographic analysis. More importantly, the sterically locked structure enabled facile optical resolution using chiral HPLC. The (P,P) and (M,M) enantiomers show moderate chiroptical properties, such as absorption dissymmetry factors |g_abs| in the order of 10⁻³, and luminescence dissymmetry factors |g_lum| of 3.8×10⁻³ and 2.9×10⁻³ at 702 nm, respectively.     

Biomimetic Design of a Robustly Stabilized Folded State Enabling Seed-Initiated Supramolecular Polymerization under Microfluidic Mixing**

We have investigated the folding and assembly behavior of a cystine-based dimeric diamide bearing pyrene units and solubilizing alkyl chains. In low-polarity solvents, it forms a 14-membered ring through double intramolecular hydrogen bonds between two diamide units. The spectroscopic studies revealed that the folded state is thermodynamically unstable and eventually transforms into more energetically stable helical supramolecular polymers that show an enhanced chiral excitonic coupling between the transition dipoles of the pyrene units. Importantly, compared to an alanine-based monomeric diamide, the dimeric diamide exhibits a superior kinetic stability in the metastable folded state, as well as an increased thermodynamic stability in the aggregated state. Accordingly, the initiation of supramolecular polymerization can be regulated using a seeding method even under microfluidic mixing conditions. Furthermore, taking advantage of a self-sorting behavior observed in a mixture of l -cysteine- and d -cysteine-based dimeric diamides, a two-step supramolecular polymerization was achieved by stepwise addition of the corresponding seeds.     

Merging the Isonitrile-Tetrazine (4+1) Cycloaddition and the Ugi Four-Component Reaction into a Single Multicomponent Process

Multicomponent reactions are of utmost importance at generating a unique, wide, and complex chemical space. Herein we describe a novel multicomponent approach based on the combination of the isonitrile-tetrazine (4+1) cycloaddition and the Ugi four-component reaction to generate pyrazole amide derivatives. The scope of the reaction as well as mechanistic insights governing the 4H-pyrazol-4-imine tautomerization are provided. This multicomponent process provides access to a new chemical space of pyrazole amide derivatives and offers a tool for peptide modification and stapling.     

Electrophilic S‐Trifluoromethylation of Cysteine Side Chains in α‐ and β‐Peptides: Isolation of Trifluoro‐methylated Sandostatin® (Octreotide) Derivatives

The new electrophilic trifluoromethylating 1‐(trifluoromethyl)‐benziodoxole reagents A and B (Scheme 1) have been used to selectively attach CF₃ groups to the S‐atom of cysteine side chains of α‐ and β‐peptides (up to 13‐residues‐long; products 7 – 14 ). Other functional groups in the substrates (amino, amido, carbamate, carboxylate, hydroxy, phenyl) are not attacked by these soft reagents. Depending on the conditions, the indole ring of a Trp residue may also be trifluoromethylated (in the 2‐position). The products are purified by chromatography, and identified by ¹H‐, ¹³C‐, and ¹⁹F‐NMR spectroscopy, by CD spectroscopy, and by high‐resolution mass spectrometry. The CF₃ groups, thus introduced, may be replaced by H (Na/NH₃), an overall Cys/Ala conversion. The importance of trifluoromethylations in medicinal chemistry and possible applications of the method (spin‐labelling, imaging, PET) are discussed.     

铑催化剂的制备及其催化取代苯乙炔不对称聚合研究

以1,5-环辛二烯(cod)和2,5-降冰片二烯(nbd)为配体,合成了[Rh(cod)Cl]_2和[Rh(nbd)Cl]_2两种铑催化剂,利用红外吸收光谱、核磁共振对其进行测试表征,准确确定其结构,且[Rh(nbd)Cl]_2的收率高达79.4%。再用四种手性氨作为助催化剂和[Rh(cod)Cl]_2或[Rh(nbd)Cl]_2形成催化体系催化含十二烷基的取代苯乙炔单体(1)聚合,聚合物重均分子量最高达49.39万,按照螺旋选择聚合机理诱起了聚合物主链上的螺旋结构;此外,首次在无助催化剂的作用下,用[Rh(nbd)Cl]_2催化含有L-丙氨醚、L-缬氨醚、L-苯丙氨醚取代基的苯乙炔单体(2、3、4)进行不对称聚合,三种聚合物产率高且分子量大,通过自配位螺旋选择聚合诱起三大分子主链上的螺旋结构,所得聚合物Poly(2)、Poly(3)、Poly(4)的主链顺式构型所占百分比分别为70%、77%、77%,因此取得了比较理想的催化效果。     

Control of EOF in CE by different ways of application of radial electric field

Various ways of application of radial electric field for the control of electrokinetic potential and EOF in a home-made device for CE are presented. The device comprises three high-voltage power supplies, which are used to form a radial electric field across the fused-silica capillary wall. One power supply provides the internal electric field - a driving force for electrophoretic migration of charged analytes and for the EOF. Two power supplies are connected to the ends of the outer low-conductivity polymeric coating, which is formed by the dispersion of insoluble conductive copolymer of aniline and p-phenylendiamine in polystyrene matrix (dissolved in N-methylpyrrolidone) attached to the original outer polyimide coating of the capillary. They are able to constitute the external longitudinal electric field with variable values of electric potential at both ends of the outer coating. The potential gradient between the external and internal electric field is perpendicular to the capillary wall and forms a radial electric field across the capillary wall, which affects the electrokinetic potential at the solid-liquid interface and EOF inside the capillary. The developed device and methodology has been applied for the analysis of both chiral and achiral molecules such as terbutaline enantiomers and oligopeptides (diglycine and triglycine). The effect of magnitude, orientation, and different ways of application of the radial electric field on the flow rate of the EOF and on the speed, efficiency, and resolution of CZE separations of the above analytes in the internally noncoated fused-silica capillaries have been evaluated.     

gem-Difluorocyclopropane as core molecule candidate for liquid crystal compounds

The synthesis of a novel chiral gem-difluorocyclopropane building block has been accomplished using chemo-enzymatic reaction protocol; the prochiral diol of 1,4-bis(2,2-difluoro-3-(hydroxymethyl)cyclopropyl)benzene (5) was converted to the corresponding chiral diacetate by Pseudomonas lipase (lipase SL-25, Meito)-catalyzed transesterification with vinyl acetate as acyl donor with >99% enantiomeric excess. Various types of diesters or dialkyl ether were prepared from the diol and their helical twisting power (HTP) was evaluated by addition of 1.0 wt% to a non-chiral nematic liquid crystal host; the HTP was significantly dependent on the structure of ester or ether moieties and diester of diol 5 with isopropylfumalic acid showed the largest HTP.     

Synthesis and structural characterization of metallated bioconjugates: C-terminal labeling of amino acids with aminoferrocene

Aminoferrocene, H₂N-Fc, has been substituted to the C-terminus of six amino acids using the HBTU/HOBt coupling protocol. The synthesized bioconjugates Boc-Aaa-NH-Fc, Aaa = Gly (1), Leu (2), Phe (3), Val (4), Cys(Acm) (5), Tyr(^tBu) (6) (Acm = acetamidomethyl, ^tBu = tert-butyl), have been characterized by ¹H NMR, ¹³C NMR, EI-MS, EI-HRMS, UV and CD spectroscopies. In addition, a VT NMR study on 4 and the X-ray structure of 1 are presented.     

用氨基酸加和法计算多肽的脂水分配系数

Based on amino acid addition model, a set of hydrophobicity contributions of amino acids was obtained from the multivariate linear regression analysis of peptides' octanol/water partition coefficients. Multivariate regression was performed on a training set of 219 peptides including dipeptides to pentapeptides which we compose 21 natural amino acids. The correlation coefficients for the whole set fitting are 0. 978 and 0- 974, for log P and log D respectively. In addition, a new test method -evolution test-for regression analysis was discussed. The result of evolution test for amino acid addition model shows the advantage of this new test method.     

Adaptive umbrella sampling of the potential energy: modified updating procedure of the umbrella potential and application to peptide folding

Adaptive umbrella sampling of the potential energy is used as a search method to determine the structures and thermodynamics of peptides in solution. It leads to uniform sampling of the potential energy, so as to combine sampling of low-energy conformations that dominate the properties of the system at room temperature with sampling of high-energy conformations that are important for transitions between different minima. A modification of the procedure for updating the umbrella potential is introduced to increase the number of transitions between folded and unfolded conformations. The method does not depend on assumptions about the geometry of the native state. Two peptides with 12 and 13 residues, respectively, are studied using the CHARMM polar-hydrogen energy function and the analytical continuum solvent potential for treatment of solvation. In the original adaptive umbrella sampling simulations of the two peptides, two and six transitions occur between folded and unfolded conformations, respectively, over a simulation time of 10 ns. The modification increases the number of transitions to 6 and 12, respectively, in the same simulation time. The precision of estimates of the average effective energy of the system as a function of temperature and of the contributions to the average effective energy of folded conformations obtained with the adaptive methods is discussed. Received: 11 July 1998 / Accepted: 22 September 1998 / Published online: 17 December 1998