Absolute Konfiguration von Picrocrocin

By correlation with (?)-3-methoxy-β-jonone picrocrocin has been shown to have R-configuration.     

Caged Thiophosphotyrosine Peptides

No Abstract     

Oxygen isotope corrections for online delta(34)S analysis

Elemental analyzers have been successfully coupled to stable-isotope-ratio mass spectrometers for online measurements of the delta(34)S isotopic composition of plants, animals and soils. We found that the online technology for automated delta(34)S isotopic determinations did not yield reproducible oxygen isotopic compositions in the SO(2) produced, and as a result calculated delta(34)S values were often 1-3 per thousand too high versus their correct values, particularly for plant and animal samples with high C/S ratio. Here we provide empirical and analytical methods for correcting the S isotope values for oxygen isotope variations, and further detail a new SO(2)-SiO(2) buffering method that minimizes detrimental oxygen isotope variations in SO(2).     

Selective nitrogen protection of hydroxyalkylbenzimidazoles using 2,2,2-trichloroethylchloroformate

A method for selective N-protection of hydroxyalkylbenzimidazoles using 2,2,2-trichloroethylchloroformate (Troc-Cl) applicable to various alkyl chain lengths has been developed. In the specific case of 5-(1-[2,2,2-trichloroethyl formyl]-benzimidazol-2-yl)-propan-1-ol, migration of Troc from the benzimidazole to the primary alcohol occurs in the presence of triethylamine, allowing the choice of selective N-Troc or O-Troc protection.     

Synthesis of aryl ketones by the PD-catalyzed C-H activation of arenes and intermolecular carbopalladation of nitriles

The palladium-catalyzed reaction of simple arenes and nitriles provides good to excellent yields of aryl ketones or the corresponding hindered imines. The addition of a small amount of DMSO increases the yields dramatically. Both intermolecular and intramolecular examples of this process are successful. This novel chemistry is believed to involve palladium-catalyzed C-H activation, followed by carbopalladation of the nitrile.     

Very long-lived doubly vibrationally excited states above dissociation threshold: non-RRKM behavior in model triatomic systems

For couplings and mass ratios appropriate to molecules such as H₂O, D₂O, and CH₂ large volumes of the classical vibrational phase space are found to be non-dissociating even well above threshold. Fully quantum calculations yield families of exceptionally long-lived quantum states corresponding to these trapped volumes.     

Über substituierte Bis-biphenylen-allyl-Radikale

Zusammenfassung Fluoren reagiert in Gegenwart starker Basen mit Biphenylenvinylbromiden vom Typ (Biph)C=C(R)-Br; dabei bilden sich unter Halogenwasserstoffabspaltung direkt die Anionen acider Propene, deren Entladung mit K₃Fe(CN)₆ sehr stabile Kohlenstoffradikale liefert. Die Dehydrierung des 1,4-Bis-[bis-biphenylen-propenyl-(2)]-benzols ergibt das Biradikal V. Die 2-Arylbis-biphenylen-allyl-Radikale liegen in Lösung und im Fest-zustand bis 77° K monomer vor; ihre Elektronen- und ihre ESR-Spektren wurden aufgenommen.     

Biomimetic chemistry of xylomollin

Kingiside aglucon ($\text{2}$$\text{b}$) undergoes stereoselective addition of CH₃OH to its C(3,4) enol ether double bond to give a C(1,3,9) diastereomer of xylomollin ($\text{1}$), a secoiridoid aglucon that has insect antifeedant properties.     

Cyclodextrin chemistry. Selective modification of all primary hydroxyl groups of α- and β-cyclodextrins

Two efficient methods are described for the selective modification of all six primary hydroxyl groups of α-cyclodextrin (α-CD, 1 1 ). One, using an indirect strategy, involves protection of all 18 hydroxyl functions as benzoate esters, followed by selective deprotection of the six primary alcohol groups. The other, using a direct strategy, involves selective activation of the primary hydroxyl groups via a bulky triphenylphosphonium salt, which is then substituted by azide anion as the reaction proceeds. A number of modified α-cyclodextrin derivatives have been prepared and fully characterized, among which are: the useful intermediate α-cyclodextrin-dodeca (2, 3) benzoate ( 3 ); hexakis (6-amino-6-deoxy)-α-cyclodextrin hexahydrochloride ( 7 ); hexakis (6-amino-6-deoxy)-dodeca (2, 3)-O-methyl-α-cyclodextrin hexahydrochloride ( 9 ), hexa (6)-O-methyl-α-cyclodextrin ( 13 ). The direct substitution is shown to be even more efficient for β-cyclodextrin ( 16 ), giving the heptakis (6-azido-6-deoxy)-β-CD-tetradeca (2, 3)acetate ( 17 ), while the indirect strategy fails. The compounds are characterized by extensive use of ¹³C- and ¹H-NMR. spectroscopy. The steric and statistical problems of selective polysubstitution reactions for the cyclodextrins are discussed, and possible reasons for the observed differences in reactivity between α- and β-cyclodextrins are examined. The dodecabenzoate 3 presents a very marked solvent effect on physical properties (IR. and NMR. spectra, optical rotation); the effects observed may be ascribed to an unusually strong intramolecular network of hydrogen bonds which severely distorts the α-cyclodextrin ring and lowers the symmetry from six-fold to three-fold.     

A new entry to the isogeissoschizoid skeleton

[reaction: see text] The tetracyclic isogeissoschizoid skeleton has been prepared by a novel route that involves the ozonolysis and double reductive amination of a cyclopentene, a nickel-catalyzed cyclization, and a late-stage Fischer indole synthesis.