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901.
Modified nucleosides in tRNAs play an important role in the translational process. They fine tune the codon-anticodon interactions and they influence the folding and stabilisation of the tRNA structure. Herein, we present a novel synthetic route to the highly modified nucleosides PreQ(0) and archaeosine. The synthesis involves coupling of a protected 7-cyano-7-deazaguanosine nucleobase with a TBDMS and isopropylidene protected chloro-ribose unit yielding the PreQ(0) nucleoside after deprotection. This PreQ(0) nucleoside is then used as the starting material for the synthesis of archaeosine providing the first total synthetic access to this hypermodified RNA nucleoside. 相似文献
902.
903.
Florian Luca 《Discrete Mathematics》2007,307(13):1672-1678
In this note, we supply the details of the proof of the fact that if a1,…,an+Ω(n) are integers, then there exists a subset M⊂{1,…,n+Ω(n)} of cardinality n such that the equation
904.
Synthesis,Structural Characterization,and Physical Properties of Cs2Ga2S5, and Redetermination of the Crystal Structure of Cs2S6
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Daniel Friedrich Florian Pielnhofer Marc Schlosser Richard Weihrich Prof. Dr. Arno Pfitzner 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(4):1811-1817
The reaction of CsN3 with GaS and S at elevated temperatures results in Cs2Ga2S5. Its crystal structure was determined from single‐crystal X‐ray diffraction data. The colorless solid crystallizes in space group C2/c (no. 15) with V=1073.3(4) Å3 and Z=4. Cs2Ga2S5 is the first compound that features one‐dimensional chains ${{{\hfill 1\atop \hfill \infty }}}$ [Ga2S3(S2)2?] of edge‐ and corner‐sharing GaS4 tetrahedra. The vibrational band of the S22? units at 493 cm?1 was revealed by Raman spectroscopy. Cs2Ga2S5 has a wide bandgap of about 3.26 eV. The thermal decomposition of CsN3 yields elemental Cs, which reacts with sulfur to provide Cs2S6 as an intermediate product. The crystal structure of Cs2S6 was redetermined from selected single crystals. The red compound crystallizes in space group ${P\bar 1}$ with V=488.99(8) Å3 and Z=2. Cs2S6 consists of S62? polysulfide chains and two Cs positions with coordination numbers of 10 and 11, respectively. Results of DFT calculations on Cs2Ga2S5 are in good agreement with the experimental crystal structure and Raman data. The analysis of the chemical bonding behavior revealed completely ionic bonds for Cs, whereas Ga?S and S?S form polarized and fully covalent bonds, respectively. HOMO and LUMO are centered at the S2 units. 相似文献
905.
Ionic‐Radius‐Driven Selection of the Main‐Group‐Metal Cage for Intermetalloid Clusters [Ln@PbxBi14−x]q− and [Ln@PbyBi13−y]q− (x/q=7/4, 6/3; y/q=4/4, 3/3)
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Dr. Rodica Ababei Prof. Dr. Werner Massa Bastian Weinert Patrik Pollak Dr. Xiulan Xie Dr. Rodolphe Clérac Dr. Florian Weigend Prof. Dr. Stefanie Dehnen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(1):386-394
Reactions of the binary, pseudo‐homoatomic Zintl anion (Pb2Bi2)2? with Ln(C5Me4H)3 (Ln=La, Ce, Nd, Gd, Sm, Tb) in the presence of [2.2.2]crypt in ethane‐1,2‐diamine/toluene yielded ten [K([2.2.2]crypt)]+ salts of lanthanide‐doped semimetal clusters with 13 or 14 surface atoms. Single‐crystal X‐ray diffraction and energy‐dispersive X‐ray spectroscopy indicated the presence of the anions [Ln@Pb6Bi8]3?, [Ln@Pb3Bi10]3?, [Ln@Pb7Bi7]4?, or [Ln@Pb4Bi9]4? in single or double salts; the latter showed various ratios of the components in the solid state. The anions are the first ternary intermetalloid clusters comprising only elements of the sixth period of the periodic table, namely, Pb, Bi and lanthanides. This study, which was complemented by ESI mass spectrometry and 139La NMR spectroscopy in solution, rationalizes a continuous development of the ratio of 13:14‐atom cages with the ionic radius of the embedded Ln3+ ion, which seems to select the most suitable cage type. Quantum chemical investigations helped to analyze this situation in more detail and to explain the observed subtle influence of the atomic radii. Magnetic measurements confirmed that the embedded Ln3+ ions keep their expected paramagnetic or diamagnetic nature. 相似文献
906.
Functionalization of Quinoxalines by Using TMP Bases: Preparation of Tetracyclic Heterocycles with High Photoluminescene Quantum Yields
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Julia Nafe Dr. Simon Herbert Dr. Florian Auras Prof. Dr. Konstantin Karaghiosoff Prof. Dr. Thomas Bein Prof. Dr. Paul Knochel 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(3):1102-1107
Tetracyclic heterocycles that exhibit high photoluminescence quantum yields were synthesized by anellation reactions of mono‐, di‐, and trifunctionalized 2,3‐dichloroquinoxalines. Thus, treatment of 2,3‐dichloroquinoxaline with TMPLi (TMP=2,2,6,6‐tetramethylpiperidyl) allows a regioselective lithiation in position 5. Quenching with various electrophiles (iodine, (BrCl2C)2, allylic bromide, acid chloride, aryl iodide) leads to 5‐functionalized 2,3‐dichloroquinoxalines. Further functionalization in positions 6 and 8 can be achieved by using TMPLi or TMPMgCl ? LiCl furnishing a range of new di‐ and tri‐functionalized 2,3‐dichloroquinoxalines. The chlorine atoms are readily substituted by anellation with 1,2‐diphenols or 1,2‐dithiophenols leading to a series of new tetracyclic compounds. These materials exhibit strong, tunable optical absorption and emission in the blue and green spectral region. The substituted O‐heterocyclic compounds exhibit particularly high photoluminescence quantum yields of up to 90 %, which renders them interesting candidates for fluorescence imaging applications. 相似文献
907.
Evolution of a Unified Strategy for Complex Sesterterpenoids: Progress toward Astellatol and the Total Synthesis of (−)‐Nitidasin
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Dr. Daniel T. Hog Dr. Florian M. E. Huber Dr. Gonzalo Jiménez‐Osés Dr. Peter Mayer Prof. Dr. Kendall N. Houk Prof. Dr. Dirk Trauner 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(39):13646-13665
Astellatol and nitidasin belong to a subset of sesterterpenoids that share a sterically encumbered trans‐hydrindane motif with an isopropyl substituent. In addition, these natural products feature intriguing polycyclic ring systems, posing significant challenges for chemical synthesis. Herein, the evolution of our stereoselective strategy for isopropyl trans‐hydrindane sesterterpenoids is detailed. These endeavors included the synthesis of several building blocks, enabling studies toward all molecules of this terpenoid subclass, and of advanced intermediates of our initial route toward a biomimetic synthesis of astellatol. These findings provided the basis for a second‐generation and a third‐generation approach toward astellatol that eventually culminated in the enantioselective total synthesis of (?)‐nitidasin. In particular, a series of substrate‐controlled transformations to install the ten stereogenic centers of the target molecule was orchestrated and the carbocyclic backbone was forged in a convergent fashion. Furthermore, the progress toward the synthesis of astellatol is disclosed and insights into some observed yet unexpected diastereoselectivities by detailed quantum‐mechanical calculations are provided. 相似文献
908.
M. Sc. Florian Glaus Prof. Karl‐Heinz Altmann 《Angewandte Chemie (International ed. in English)》2015,54(6):1937-1940
Tiacumicin B (lipiarmycin A3, fidaxomicin) is an atypical macrolide antibiotic which is used for the treatment of Clostridium difficile infections. Tiacumicin B is also a potent inhibitor of Mycobacterium tuberculosis, but due to its limited oral bioavailability is unsuitable for systemic therapy. To provide a basis for structure–activity studies that might eventually lead to improved variants of tiacumicin B, we have developed an efficient approach to the synthesis of the tiacumicin B aglycone. The synthesis features a high‐yielding intramolecular Suzuki cross‐coupling reaction to effect macrocyclic ring closure. Key steps in the synthesis of the macrocyclization precursor were a highly selective, one‐pot Corey–Peterson olefination and an ene–diene cross‐metathesis reaction. Depending on the reaction conditions, the final deprotection delivered either the fully deprotected tiacumicin B aglycone or partially protected versions thereof. 相似文献
909.
The Precise Synthesis of Phenylene‐Extended Cyclic Hexa‐peri‐hexabenzocoronenes from Polyarylated [n]Cycloparaphenylenes by the Scholl Reaction
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Dr. Martin Quernheim Dr. Florian E. Golling Wen Zhang Dr. Manfred Wagner Dr. Hans‐Joachim Räder Dr. Tomohiko Nishiuchi Prof. Dr. Klaus Müllen 《Angewandte Chemie (International ed. in English)》2015,54(35):10341-10346
The longitudinal extension of cycloparaphenylenes (CPP) towards ultrashort carbon nanotubes (CNTs) is essential for the solution based bottom‐up synthesis of CNTs. Herein, the longitudinal extension of the CPP skeleton by the introduction of hexaphenylbenzene units towards polyarylated [n]CPPs is described. Further, the applicability of the Scholl reaction to selectively form graphenic sidewalls is demonstrated. The ring size and substitution patterns of the polyarylated [n]CPPs were varied to overcome strain‐induced side reactions during the oxidative cyclodehydrogenation and cyclic para‐hexa‐peri‐hexabenzocoronene trimers ([3]CHBCs) were selectively obtained. This concept is envisioned as an access to ultrashort carbon nanotubes subject to the condition that further benzene rings with the right connectivity will be inserted. 相似文献
910.
Molecular Solution Behaviour of an Intermediate Biofuel Feedstock: Acetone–Butanol–Ethanol (ABE)
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Mixtures of acetone, butanol, and ethanol (ABE) are common intermediate products in the production of biofuels via biomass fermentation. Their separation to yield, for example, bio‐butanol, is still difficult due to the lack of a fundamental understanding of these mixtures at the molecular level. In order to bridge this gap, a detailed analysis of characteristic features of the vibrational spectrum is carried out. A systematic study of the binary solutions of acetone with ethanol and butanol does not only reveal a universal behaviour at the molecular level when acetone is mixed with short‐chain alcohols, it also shows that the phenomena at a length scale between the molecules and in the macroscopic solution need to be taken into account to understand the structure–property relationships. The size of self‐associated molecule clusters seems to determine whether or not a system exhibits an azeotrope. When a second alcohol is added to an acetone/alcohol solution, no additional non‐idealities are induced, which is advantageous for modelling ternary ABE mixtures and for improving their processing in the production of biofuels. 相似文献