ISSN:0947-6539    

DOI:10.1002/chem.19970031003

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractUltimate success in the design of solid oxide catalysts as well as other covalently bonded or heterogenised organometallic catalysts predicates knowledge of precisely what structure it is that has to be targetted. This, in turn, demands the greatest possible precision in determining, under operating conditions, the structure of the catalyst in general and of the active site in particular. Combined X‐ray absorption spectroscopy and X‐ray diffraction are ideal tools for such in situ investigations. Examples of such studies and of engineered catalysts, the structure of which have been determined in atomic detail, are gi

ISSN:0947-6539    

DOI:10.1002/chem.19970031004

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractAt first glance the topic of chiral dendrimers seems to be a contradiction in terms. However, recent studies reveal that both the building blocks of the dendrimer and the overall dendritic architecture can be chiral and that chirality can be introduced at various levels. The expression of optical activity in these enantiomerically pure dendrimers as a result of conformational (dis)order has proven to be of special interest. In this Concepts article we present the different approaches to introducing chirality in dendritic architectures, organized through their possible impact in fields such as biocompatibility, catalysis, molecular recognition, and surface chemistry. Also, the relation between molecular chirality of core or building block and the macroscopic chirality of dendritic objects is discussed.

ISSN:0947-6539    

DOI:10.1002/chem.19970031005

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractA molecular modeling tool for the rational design of E‐selectin antagonists based on the lead structure sialyl Lewisxhas been developed. The binding affinity to the receptor is considerably influenced by the entropy and consequently by the antagonist's ability to place its pharmacophores in an optimal spatial arrangement, i.e., by its preorganization for binding. The computational model assesses the preorganization of a potential selectin antagonist with the aid of Monte Carlo (jumping between wells)/stochastic dynamics [MC(JBW)/SD] simulations. The model has been validated by correlating preorganization and bioactivity of several selectin antagonists. The results suggest that only preorganized compounds are likely to bind to E‐selec

ISSN:0947-6539    

DOI:10.1002/chem.19970031006

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

Abstract[Cp*Mo(NO)(CH2SiMe3)2] exhibits three principal types of reaction with the various lithium reagents investigated during this study, namely: regioselective deprotonation, reduction, and addition. Deprotonation of the reactant, achieved by treatment with lithium amide reagents, leads ultimately to the formation of the alkylidene „ate”︁ complex [Cp*Mo(NO)(CH2SiMe3)2(CHSiMe3)]2‐[Li2(thf)3] (1). While LiN(SiMe3)2effects this conversion directly with no detectable intermediates, reaction with 1 equiv of LDA in THF for 15 min deprotonates the Cp*ligand to form the lithium salt of the „tucked‐in”︁ ate complex [(η5,η1‐C5Me4CH2)Mo(NO)(CH2SiMe3)2]‐[Li(thf)3] (2) in 40% isolated yield. Complex2slowly converts to the thermodynamically more stable1when left as a THF or C6D6solution at ambient temperature for 48 h. Reaction of the dialkyl starting material with eithertBuLi or PhLi leads to the production of the alkylidene complex1in irreproducible yields (10–50% NMR; not isolable). A kinetic analysis of the reaction of [Cp*Mo(NO)(CH2SiMe3)2] with LiN(SiMe3)2indicated that the reaction was first‐order in both the lithium and molybdenum reagents, and the activation parameters of ΔH*= 7.3∓1.0 kcal mol−1and ΔS*= − 34∓3 e.u. suggest an associative process. Treatment of the neutral dialkyl with 1 equiv of LiPPh2in THF results in a one‐electron reduction and production of {[Cp*Mo(NO)(CH2SiMe3)2[Li2(thf)3] (3). If left in solution, the 17e−dialkyl anion3is converted to the 18e−alkylidene anion1by the Ph2P–PPh2coproduct, which effects the requisite hydrogen‐atom abstraction. Finally, addition of a sterically undemanding alkyllithium reagent such as MeLi to the 16e−dialkyl reactant leads to the formation of the 18e−trialkyl anionic complex [[Cp*Mo(NO)(CH2SiMe3)2(CHSiMe3)]2‐[Li2(thf)3](Me)][Li(thf)3] (4). Warming of4in a C6D6solution results in loss of methane and production of1. In most cases, the chemistry exhibited by the Mo system is duplicated by the analogous W congener, [Cp*Mo(NO)(CH2SiMe3)2(CHSiMe3)]2‐[Li2(thf)3]. The solid‐state molecular structures of complexes1and4have been

ISSN:0947-6539    

DOI:10.1002/chem.19970031007

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe two isomeric [4+2] cyclo‐adducts from two different 1,3‐dienes may result from direct cycloadditions as well as from Cope rearrangements (Scheme 1). This general question is tackled by employing two energetically different types of dienes, protonated pyrazolines (1H+,2H+) or dihydropyridazines (3H+), prepared in situ from their trimers and alicyclic (4–6) or aliphatic (7–9) 1,3‐dienes. Depending on structural features and conditions (amount of acid, reaction time), various ratios of the two isomeric [4+2] cycloadductsAandBare obtained;AandBare azo compounds10, 14, 16, 20, 22, 24, 27, 32, 34, 36–39, 41, 42, pyrazolinesendo‐11,endo‐13,endo‐15,endo‐endo‐17,endo‐18,endo‐19, 21, 23, 25, 26, 28, and hydropyridazines31,endo‐33,endo‐35, 40and43(Schemes 3, 4). These results were backed by others from acid‐catalyzed isomerizations, trapping experiments, and calculations of the equilibria (ΔΔH) between the isomers (by analogy with the corresponding olefins). A critical discussion reveals: a) Azo compounds20, 22, 24, 27, 34, 38, and42must result from a [4++2] cycloaddition with inverse electron demand, whereas hydropyridazinesendo‐33,endo‐35, 40, and43originate from a [4+2+] cycloaddition with normal electron demand. b) All isomerizations occur by a [3,3]sigmatropic rearrangement; [4+2] cycloreversion is energetically disfavored. c) A clear‐cut distinction between the [4++2] or [4+2+] cycloaddition reaction routes to the energetically well‐balanced systems10→endo‐11and12→endo‐13is not possible. d) The two cycloadditions may well favor a nonconcerted reaction through an allylic cationic intermediate which

ISSN:0947-6539    

DOI:10.1002/chem.19970031008

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractA new upper limit of binding site size is defined for the 2:1 overlapped polyamide: DNA motif. Eight‐ring polyamides composed of four‐ring subunits containing pyrrole (Py) and imidazole (Im) amino acids linked by a central β‐alanine (β) spacer („4‐β‐4 ligands”︁) were designed for recognition of eleven base pair sequences as antiparallel dimer (4‐β‐4)2.DNA complexes in the minor groove. The DNA binding properties of three polyamides, ImPyPyPy‐β‐PyPyPyPy‐β‐Dp, ImImPyPy‐β‐PyPyPyPy‐β‐Dp, and ImImImPy‐β‐PyPyPyPy‐β‐Dp, were analyzed by footprinting experiments on DNA fragments containing the respective match sites 5'‐AGTAATTTACT‐3', 5'‐AGGTATTACCT‐3', and 5'‐AGGGATTCCCT‐3' (Dp = dimethylaminopropylamide). Quantitative footprint titrations reveal that each polyamide binds its respective target site with subnanomolar affinity and 7‐fold to over 30‐fold specificity over double‐base‐pair mismatch sites. A 20‐fold decrease in binding affinity is observed for placement of a side‐by‐side β‐β pairing opposite G.C/C.G relative to placement opposite a A.T/T.A base pair. The use of side‐by‐side antiparallel β‐alanine residues as an A.T/T.A‐specific DNA binding element provides a new pairing rule for polyamide design. Expanding the DNA binding site size targeted by pyrrole‐imidazole polyamides represents an important step in t

ISSN:0947-6539    

DOI:10.1002/chem.19970031009

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe compounds [M(SiR3)H3(PPh3)3] (1: M = Ru, R = 1 ‐ NC4H4= pyr;2a‐c: M = Os, R = pyr, Et, Ph) are prepared through reaction of either [RuH2(PPh3)4] or [OsH4(PPh3)3] with the appropriate silane HSiR3(3a‐c: R = pyr, Et, Ph). The X‐ray structure analysis of compound2 aand ab initio calculations on the model compounds [Os(SiR3)H3(PH3)3] (4a‐c: R = H, NH2, pyr) reveal a trigonal distortion along the Os‐Si axis from an idealised tetrahedral geometry for the central OsSiP3heavy‐atom skeleton. The structure can be described as two face‐shared octahedra, one based on osmium (OsH3P3) and the other based on silicon (SiH3N3). Studies of the bonding situation in2 areveal that theN‐pyrrolyl substituents have a marked shortening effect on the osmium‐silicon distance (229.3(3) pm) and that each of the three hydride ligands participates in partial three‐centre bonding involving osmium, silicon and hydrogen.1H,13C,29Si and31P NMR spectra were used to determine the solution structure

ISSN:0947-6539    

DOI:10.1002/chem.19970031010

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractN‐Linked glycopeptides were synthesized by condensation of a highmannose anomeric amine bearing a pentasaccharide with aspartic‐acid‐containing tri‐ and pentapeptides through the agency of IIDQ. The pentasaccharide portion, corresponding to the „core”︁ region of all asparagine‐linked glycoproteins, was assembled by means of glycal‐derived thioethyl donors and glycal acceptors. The central mannose residue was established by inversion of the C2 hydroxyl of a glucosyl precursor in the pentasaccharide. The protecting‐group scheme employed allows the extension of the pentasaccharide through the terminal mannose units. While a fully convergent coupling of the high‐mannose carbohydrate to the peptide domain has thus been accomplished for the first time with a fully synthetic sugar, the stereochemical integrity of the anomeric center of the carbohydrate domain was not maintained and a mixture of glyco

ISSN:0947-6539    

DOI:10.1002/chem.19970031011

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

Abstract2‐Methyl‐1,3‐dimorpholino‐1,3‐butadiene1reacted with α,β‐unsaturated Fischer carbene complexes to give a wide range of different products depending on the substitution pattern. Thus, sevenmembered rings (4, 5and6) could be obtained from chromium complexes2with aromatic or vinylic groups at the β position. Similar results were observed when α‐methyl‐substituted carbene complex7 awas used. Six‐membered carbocycles (derivatives of cycloadducts12and13) were isolated after reaction with both chromium and tungsten complexes bearing one or two alkyl groups at the β position (10and11). Moreover, cyclopentenones20were the main products when the starting carbene complexes were alkyl‐substituted at both α and β positions (19a, b) or when aromatic (19c, d) instead of vinylic complexes were used. A bicyclo[4.1.0]heptene system18has also been obtained in the special case of reaction with β,β‐dimethylvinylchromium complex13b; its formation could be explained as a formal carbene insertion into a C–H bond. The behaviour of diene1towards alkoxymethylcarbene complexes22was unusual. The different reaction products (cyclopentadienes23, bicyclo[3.1.0]hexenes24, aromatic amine25and metallatrienes26) imply a mechanism in which the deprotonation of the carbene complex by the diene is followed by Michael add

ISSN:0947-6539    

DOI:10.1002/chem.19970031012

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY