Organometallated Halonium Salts of the Type [{Cp(CO)2Fe}2X]SbY6 (X = Cl,Br, I; Y = F,Cl)

The reaction of CpFe(CO)₂X (X = Cl, Br, I) with SbY₅ (Y = F, Cl) in toluene leads to the cationic, halogen‐bridged compounds [{Cp(CO)₂Fe}₂X]SbY₆ ( 1 – 6 ). The halide of CpFe(CO)₂X is eliminated by the Lewis acid SbY₅, and the fragment “CpFe(CO)₂⁺” reacts with further CpFe(CO)₂X to form the halogen bridge between both the organometallic substituents. The exclusive formation of the counter anion SbY₆^– is caused by the oxidizing action of the antimony pentahalides, by which SbY₃ and the interhalogens XY are always obtained. The compounds have been characterized by their NMR‐, IR‐ and Mass spectra, the compounds 1 – 3 and 6 additionally by single crystal structure analyses. They show decreasing bond angles Fe–X–Fe following the range Cl → Br → I and the VSEPR concept; the two CpFe(CO)₂ groups are staggered with the dihedral angle Cp(centre)–Fe–Fe–Cp(centre) of about 160°.     

Koordination und Reaktivität von Sulfinylimiden RNSO in Cyclopentadienyldicarbonylmangan‐Komplexen

Coordination and Reactivity of Sulfinylimides RNSO in Cyclopentadienyldicarbonylmanganese Complexes The coordination behaviour of sulfinylimides RNSO (R = SiMe₃, Ph) towards CpMn(CO)₃ (Cp = C₅H₅, C₅H₄CH₃) has been studied. Under total exclusion of water the substitution products CpMn(CO)₂{S(O)NR} ( 2 a , b , 2 ′ a , b ) with μ¹‐S‐linkage are formed. In the presence of water, however, the SO₂ complexes 3 , 3 ′ as iminofree hydrolysis products are obtained. There are only spectroscopic indications for the directed hydrolytic conversion of the silyl derivatives 2 a , 2 ′ a into the unstable HNSO complexes 4 , 4 ′ by cleavage of the Si–N‐bond and retention of the cumulene moiety. The molecular structures of π‐C₅H₅Mn(CO)₂{S(O)NSiMe₃} ( 2 a ) and π‐C₅H₄CH₃Mn(CO)₂SO₂ ( 3 ′) were elucidated by single crystal X‐ray analyses.     

Dendritic polyols based on carbosilanes - lipophilic dendrimers with hydrophilic skin

Carbosilane dendrimers with up to 108 end groups were prepared by a divergent synthetic route via repeated alternating Grignard-reaction with allylmagnesium bromide and hydrosilylation. Quantitative hydroboration of the dendrimers was achieved with 9-BBN/H₂O₂,OH⁻. Employing this synthetic route, rather monodisperse dendritic polyols with lipophilic carbosilane core and hydrophilic shell were prepared. All dendrimers were characterized with MALDI-TOF, showing low polydispersities. The novel dendritic polyols constitute interesting molecular scaffolds for the preparation of unusual supermolecules. As an example, reaction with cholesteryl chloro-formiate was used to prepare a series of dendrimers with 12, 36 and 108 mesogenic cholesteryl end groups.     

Stereoselective Synthesis of 8‐Oxabicyclo[3.2.1]octane‐2,3,4,6,7‐pentols and Total Asymmetric Synthesis of 2,6‐Anhydrohepturonic Acid Derivatives and of β‐C‐manno‐Pyranosides Suitable for the Construction of (1→3)‐C,C‐Linked Trisaccharides

Enantiomerically pure (+)‐(1S,4S,5S,6S)‐6‐endo‐(benzyloxy)‐5‐exo‐{[(tert‐butyl)dimethylsilyl]oxy}‐7‐oxabicyclo[2.2.1]heptan‐2‐one ((+)‐ 5 ) and its enantiomer (−)‐ 5 , obtained readily from the Diels‐Alder addition of furan to 1‐cyanovinyl acetate, can be converted with high stereoselectivity into 8‐oxabicyclo[3.2.1]octane‐2,3,4,6,7‐pentol derivatives (see 23 – 28 in Scheme 2). A precursor of them, (1R,2S,4R,5S,6S,7R,8R)‐7‐endo‐(benzyloxy)‐8‐exo‐hydroxy‐3,9‐dioxatricyclo[4.2.1.0^2,4]non‐5‐endo‐yl benzoate ((−)‐ 19 ), is transformed into (1R,2R,5S, 6S,7R,8S)‐6‐exo,8‐endo‐bis(acetyloxy)‐2‐endo‐(benzyloxy)‐4‐oxo‐3,9‐dioxabicyclo[3.3.1]non‐7‐endo‐yl benzoate ((−)‐ 43 ) (see Scheme 5). The latter is the precursor of several protected 2,6‐anhydrohepturonic acid derivatives such as the diethyl dithioacetal (−)‐ 57 of methyl 3,5‐di‐O‐acetyl‐2,6‐anhydro‐4‐O‐benzoyl‐D ‐glycero‐D ‐galacto‐hepturonate (see Schemes 7 and 8). Hydrolysis of (−)‐ 57 provides methyl 3,5‐di‐O‐acetyl‐2,6‐anhydro‐4‐O‐benzoyl‐D ‐glycero‐D ‐galacto‐hepturonate 48 that undergoes highly diastereoselective Nozaki‐Oshima condensation with the aluminium enolate resulting from the conjugate addition of Me₂AlSPh to (1S,5S,6S,7S)‐7‐endo‐(benzyloxy)‐6‐exo‐{[(tert‐butyl)dimethylsilyl]oxy}‐8‐oxabicyclo[3.2.1]oct‐3‐en‐2‐one ((−)‐ 13 ) derived from (+)‐ 5 (Scheme 12). This generates a β‐C‐mannopyranoside, i.e., methyl (7S)‐3,5‐di‐O‐acetyl‐2,6‐anhydro‐4‐O‐benzoyl‐7‐C‐[(1R,2S,3R,4S,5R,6S,7R)‐6‐endo‐(benzyloxy)‐7‐exo‐{[(tert‐butyl)dimethylsilyl]oxy}‐4‐endo‐hydroxy‐2‐exo‐(phenylthio)‐8‐oxabicyclo[3.2.1]oct‐3‐endo‐yl]‐L ‐glycero‐D ‐manno‐heptonate ((−)‐ 70 ; see Scheme 12), that is converted into the diethyl dithioacetal (−)‐ 75 of methyl 3‐O‐acetyl‐2,6‐anhydro‐4,5‐dideoxy‐4‐C‐{[methyl (7S)‐3,5,7‐tri‐O‐acetyl‐2,6‐anhydro‐4‐O‐benzoyl‐L ‐glycero‐D ‐manno‐heptonate]‐7‐C‐yl}‐5‐C‐(phenylsulfonyl)‐L ‐glycero‐D ‐galacto‐hepturonate ( 76 ; see Scheme 13). Repeating the Nozaki‐Oshima condensation to enone (−)‐ 13 and the aldehyde resulting from hydrolysis of (−)‐ 75 , a (1→3)‐C,C‐linked trisaccharide precursor (−)‐ 77 is obtained.     

Fimbrolide Natural Products Disrupt Bioluminescence of Vibrio By Targeting Autoinducer Biosynthesis and Luciferase Activity

Vibrio is a model organism for the study of quorum sensing (QS) signaling and is used to identify QS‐interfering drugs. Naturally occurring fimbrolides are important tool compounds known to affect QS in various organisms; however, their cellular targets have so far remained elusive. Here we identify the irreversible fimbrolide targets in the proteome of living V. harveyi and V. campbellii via quantitative mass spectrometry utilizing customized probes. Among the major hits are two protein targets with essential roles in Vibrio QS and bioluminescence. LuxS, responsible for autoinducer 2 biosynthesis, and LuxE, a subunit of the luciferase complex, were both covalently modified at their active‐site cysteines leading to inhibition of activity. The identification of LuxE unifies previous reports suggesting inhibition of bioluminescence downstream of the signaling cascade and thus contributes to a better mechanistic understanding of these QS tool compounds.     

Total Asymmetric Synthesis of a New 9,12‐Anhydroerythronolide Aglycone

Two novel, potentially antimicrobial erythronolide aglycon analogs ((−)‐ 9 and (−)‐ 30 , respectively), which incorporate a large number of contiguous stereogenic centers, have been prepared by multistep synthesis from simple chirons. The chemistry presented demonstrates the power of the so‐called ‘naked sugars of the second generation' approach. As for the sporeamicins, our macrolides are 9,12‐anhydroerythronolides, yet presenting a higher degree of complexity due to additional functional groups.     

Single-electron tunneling in silicon nanostructures

We present a brief overview on different realizations of single-electron devices fabricated in silicon-on-insulator films. Lateral structuring of highly doped silicon films allows us to observe quasi-metallic Coulomb blockade oscillations in shrunken wires where no quantum dot structure is geometrically defined. Embedding quantum dot structures into the inversion channel of a silicon-on-insulator field-effect transistor Coulomb blockade up to 300 K is observed. In contrast to the quasi-metallic structures, in these devices the influence of the quantum mechanical level spacing inside the dot becomes visible. Suspending highly doped silicon nanostructures leads to a novel kind of Coulomb blockade devices allowing both high-power application as well as the study of electron–phonon interaction. Received: 14 April 2000 / Accepted: 17 April 2000 / Published online: 6 September 2000     

Resolvent Estimates and Quantification of Nonlinear Stability

Abstract The aim of this paper is to clarify the role played by resolvent estimates for nonlinear stability. We will give examples showing that a large resolvent may lead to a small domain of nonlinear stability. In other examples the resolvent is large, but the domain of nonlinear stability is completely unrestricted. Which case prevails depends on the details of the problem. We will also show that the size of the resolvent depends in an essential way on the norms that are used. * Supported by Office of Naval Research n00014 90 j 1382 ** Supported by NSF Grant DMS-9404124 and DOE Grant DE-FG03-95ER25235