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901.
Jens Reinbold Martin Bertau Torsten Voss Dieter Hunkler Lothar Knothe Horst Prinzbach Dmytro Neschchadin Georg Gescheidt Bernhard Mayer Hans‐Dieter Martin Jürgen Heinze G.K.Surya Prakash GeorgeA. Olah 《Helvetica chimica acta》2001,84(6):1518-1560
Secodecahedradiene 2a , featuring very proximate, perfectly syn‐periplanar and significantly pyramidalized C=C bonds, was synthesized as testing object for in‐plane(σ)‐homoconjugational electron delocalization, starting from the available pagodane 15b . The response of 2a (and in part its diester 2b ) – in π,π‐distance (average 3.08 Å), olefinic pyramidalization (average 26.9°), and π,π‐split (PE, 1.15 eV) intermediate between disecododecahedradiene 1a and 1,16‐dodecahedradiene 3a – to selected 4π‐reagents, electrophiles, and radicals was explored experimentally and by calculations. Intriguing multistep reaction sequences attest to the ease of competing stabilization pathways for the 3C/2e in‐plane homoconjugated cationic intermediates. PE, CV, and ESR measurements and calculations (DFT) characterize the radical cation generated from 2a as in‐plane homoconjugated 4C/3e‐species 2a .+, persistent in a Freon matrix, but only very shortly existent in solution (CIDNP). Consequently, NMR control of the two‐electron oxidation in SbF5/SO2ClF did not disclose the σ‐bis‐homoaromatic dication 4C/2e (see 2a 2 + ), but a bis‐allylic dication 75 as persistent species. In support of 2a 2 + as intermediate, evidence is presented for very limited kinetic protection offered by the secododecahedral framework to through H‐cage σ‐homoconjugated cations. 相似文献
902.
903.
904.
Lars Einfeldt Katrin Petzold Wolfgang Günther Armin Stein Manfred Kussler Dieter Klemm 《Macromolecular bioscience》2001,1(8):341-347
Reaction of starch 1 dissolved in dimethyl sulfoxide (DMSO) with bulky thexyldimethylchlorosilane (TDSCl) in the presence of pyridine leads to regioselectively functionalized silyl ethers with a degree of substitution (DS) up to 1.8. The control of the DSSi, of the regioselectivity, and of the reaction pathway is described in detail. The reaction proceeds homogeneously up to DSSi of 0.6. With ongoing silylation the polymers form a separate phase incorporating the silylating agent to form TDS‐starches with DSSi values higher than 1.0. After peracetylation of the silyl starches, the substitution pattern has been characterized not only in the anhydroglucose repeating units (AGU) but also in the non‐reducing terminal end groups (TEG) by means of two‐dimensional 1H NMR techniques. Up to DSSi 1.0, a very high regioselective functionalization of the primary 6‐OH groups in the AGU as well as in the TEG is detectable. With increasing silylation (DSSi > 1.0), the subsequent silylation takes place at the 2‐OH groups of the AGU and at the 3‐OH groups of the TEG. These results are compared with our own investigations on the silylation of starch in the reaction system N‐methylpyrrolidone (NMP)/ammonia and on the silylation of cellulose in N,N‐dimethylacetamide (DMA)/LiCl/pyridine solution. 相似文献
905.
Dieter Seebach JürgV. Schreiber Stefan Abele Xavier Daura WilfredF. vanGunsteren 《Helvetica chimica acta》2000,83(1):34-57
A careful CD analysis (Figs. 1 – 3 and 5; MeOH or H2O solutions) of β‐oligopeptides ( 1 – 6 , B , C ) containing four to seven β‐amino acids reveals that seemingly small structural changes cause a switch from the CD pattern (maxima of opposite sign near 215 and 200 nm) associated with a 314‐helical structure to the CD pattern (single Cotton effect at ca. 205 nm) considered characteristic of a so‐called 12/10‐helical structure, but also exhibited by a β‐peptide adopting a hair‐pin conformation with a ten‐membered H‐bonded ring as the turn motif. Comparison of these CD spectra with those of the trans‐2‐aminocyclohexanecarboxamide oligomers, which give rise to the long‐wavelength Cotton effect only, suggests that the H‐bonded 14‐, 12‐, and 10‐membered ring conformations of the β‐peptides, and not just the entire helix structures, might actually generate the Cotton effects. This interpretation would be compatible with our previous NMR structure determinations of β‐peptides and with previously reported temperature dependences of CD and NMR spectra of β‐peptides. To further substantiate this suggestion, we have performed a statistical analysis of the β‐peptidic conformations generated by molecular‐dynamics calculations (GROMOS96) for a β‐hexapeptide ( C ; the 12/10 helix) and a β‐heptapeptide ( 6 ; the 314 helix) in MeOH (Figs. 6 – 9). Up to 400,000 conformations at 0.5‐ps intervals were analyzed from up to 200‐ns simulations (at 298 to 360 K). The analysis reveals the co‐existence of the various H‐bonded rings. Remarkably, the central section of the β‐peptide 6 (containing a β2,3‐amino‐acid residue of like‐configuration!) adopts a ten‐membered‐ring conformation for ca. 5% of the simulation time, while the central section of the β‐peptide C adopts a 14‐membered‐ring conformation for ca. 3% of the time, according to this computational analysis. Further experimental and theoretical work will be necessary to find out to which extent the components (H‐bonded rings) and the entire helical secondary structures of β‐peptides contribute to the observed Cotton effects. 相似文献
906.
The known solid‐state structure (Fig. 1, top) of cyclo(β‐HAla)4 was used to model the structure of the title compound 1 as a prospective somatostatin mimic (Fig. 1, bottom). The synthesis started with the N‐protected natural amino acids Boc‐Phe‐OH, Boc‐Trp‐OH, Boc‐Lys(2‐Cl‐Z)‐OH, and Boc‐Thr(OBn)‐OH, which were homologated to the corresponding β‐amino‐acid derivatives (Scheme 1) and coupled to the β‐tetrapeptide Boc‐β‐HTrp‐β‐HPhe‐β‐HThr(OBn)‐β‐HLys(2‐Cl‐Z)‐OMe ( 16 ); the (N‐Me)‐β‐HThr‐(N‐Me)‐β‐HPhe analog 17 was also prepared. C‐ and N‐terminal deprotection and cyclization through the pentafluorophenyl ester gave the insoluble β‐tetrapeptide with protected Thr and Lys side chains ( 18 ). Solubilization and debenzylation could only be effected in LiCl‐containing THF (ca. 10% yield; with ca. 55% recovery). HPLC Purification provided a sample of the title compound 1 , the structure of which, as determined by NMR‐spectroscopy (Fig. 2, left) was drastically different from the `theoretical' model (Fig. 1). There is a transannular H‐bond dividing the macrocyclic 16‐membered ring, thus forming a ten‐ and a twelve‐membered H‐bonded ring, the former mimicking, or actually being superimposable on, an α‐peptidic so‐called β‐turn. Still, the four side chains occupy equatorial positions on the ring, as planned, albeit with somewhat different geometry as compared to the `original'. The cyclo‐β‐tetrapeptide has micromolar affinities to the human somatostatin receptors (hsst 1 – 5). Thus, we have demonstrated for the first time that it is possible to mimic a natural peptide hormone with a small β‐peptide. Furthermore, we have discovered a simple way to construct the ubiquitous β‐turn motif with β‐peptides (which are known to be stable to mammalian peptidases). 相似文献
907.
908.
909.
910.
The crystal structure of lead cyanamide, PbNCN, has been refined on the basis of a single crystal grown from solution. PbNCN crystallizes in space group Pnma (Z = 4) with a = 555.66(4) pm, b = 386.77(2) pm, and c = 1173.50(8) pm. The cyanamide anion exhibits C–N bond lengths of 116 pm and 130 pm, and the N–C–N angle is 176°. Quantum‐chemical DFT calculations indicate that the cyanamide unit is comparatively easy to distort. 相似文献