The correlation of NMR chemical shifts with EHT calculated electron densities for 3-pyridinols

The NMR spectra of a series of five 3-pyridinols in dioxane and dimethyl sulfoxide have been correlated with electron densities calculated by the Extended Hückel MO method. The data were found to fit a straight line of the form y=ax+b. The effects of hydrogen-bonding between pyridinol molecules on the correlation have been considered, and they have been estimated quantitatively for 6-methyl-3-pyridinol via Extended Hückel Calculations on a hydrogen-bonded model.

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Zusammenfassung Die Kernresonanzspektren von fünf 3-Pyridinolen in Dioxan und Dimethylsulfoxyd wurden mit den Elektronendichten aus dem erweiterten Hückelverfahren korreliert, und zwar ergibt sich eine lineare Beziehung. Der Einfluß von intermolekularen Wasserstoffbrücken ist für einen Fall (die 6-Methyl-Verbindung) durch EHT-Rechnungen festgestellt und in den übrigen Fällen geschätzt worden.

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Résumé Les spectres NMR d'une série de cinq 3-pyridinols dans le dioxane et le diméthyle-sulfoxyde ont été corrélés avec les densités électroniques calculées par la méthode de Hückel Etendue. Les données se placent correctement sur une droite y=ax+b. Les effets de la liaison hydrogène entre les molécules de pyridinol ont été envisagés et estimés quantitativement pour le 6-méthyl-3-pyridinol par des calculs EHT sur un modèle à liaison hydrogène.

     

Synthesis of Pyranthiones

A general method for the preparations of pyranthiones is particularly useful for preparing substituted xanthiones which are unavailable by established procedures. Reaction of a xanthone with p-tosyl isocyanate or with trichloroacetyl isocyanate produces the corresponding imino derivatives, which upon treatment with n-butylamine followed by hydrogen sulfide gives the desired xanthione in good yield.     

Steric buttressing in the Pauson-Khand reactions of aryl enynes

A variety of aryl enynes have been constructed from o-iodophenol derivatives containing ortho-tert-butyl groups via O-alkylation and a Sonogashira cross-coupling reaction. These substrates undergo efficient thermal and oxidative intramolecular Pauson-Khand reactions leading to the formation of sterically congested cyclopentenones, as well as the formation of medium-sized rings, although in the latter case with unusual regioselectivity. Incorporation of a TMS moiety on the alkyne group in a higher homolog led to cyclization via the normal mode, albeit in relatively low yield.     

Structure activity relationship by NMR and by computer: a comparative study

There has recently been considerable interest in using NMR spectroscopy to identify ligand binding sites of macromolecules. In particular, a modular approach has been put forward by Fesik et al. (Shuker, S. B.; Hajduk, P. J.; Meadows, R. P.; Fesik, S. W. Science 1996, 274, 1531-1534) in which small ligands that bind to a particular target are identified in a first round of screening and subsequently linked together to form ligands of higher affinity. Similar strategies have also been proposed for in silico drug design, where the binding sites of small chemical groups are identified, and complete ligands are subsequently assembled from different groups that have favorable interactions with the macromolecular target. In this paper, we compare experimental and computational results on a selected target (FKBP12). The binding sites of three small ligands ((2S)1-acetylprolinemethylester, 1-formylpiperidine, 1-piperidinecarboxamide) in FKBP12 were identified independently by NMR and by computational methods. The subsequent comparison of the experimental and computational data showed that the computational method identified and ranked favorably ligand positions that satisfy the experimental NOE constraints.     

Substituent effects in π-(tricarbonylchromium)arenes: V. Thermodynamic dissociation constants of some substituted π-(tricarbonylchromium)benzoic acids

The thermodynamic dissociation constants of a series of 38 substituted π-(tricarbonylchromium)benzoic acids in 50% aqueous ethanol at 25°C have been determined. The results require revision of some literature values.The pK_a^*-values of the π-(tricarbonylchromium)benzoic acids were correlated with the electronic substituent parameters in terms of the Yukawa-Tsuno equation. The reaction constant (ρ) decreases from 1.4 for the benzoic acids to 0.8 for the π-(tricarbonylchromium)benzoic acids, reflecting the decreased ability of the complexed aromatic system to transmit electronic substituent effects. For the alkylsubstituted π-(tricarbonylchromium)benzoic acids, conformational effects of the Cr(CO)₃ group can account for some of the anomalies observed. The substituent parameters, σ_meta and σ_para, of the π-(Cr(CO)₃)phenyl group as a substituent were derived from the dissociation constants of the complexed phenylbenzoic acids.     

Preparation of chiral triarylphosphines by Pd-catalysed asymmetric P-C cross-coupling

Enantioselective C-P cross-coupling of diarylphosphines and ortho-substituted aryl iodides has been achieved with >90%ee, using an in situ catalyst prepared from Et,Et-FerroTANE, Pd(2)(dba)(3) x CHCl(3) and LiBr.     

A new ring transformation: Synthesis of 5,5-dimethyl-3-oxo-1-pyrroline 1-oxides from 6,6-dimethyl-4-oxo-5,6-dihydro-1,2,4h-oxazines

The nitrosation of some γ,δ-unsaturated β-diketo compounds affords the 3-substituted 4-oxo-5,6-dihydro-1,2,4H-oxazines. These compounds are converted to the isomeric 3-oxo-1-pyrroline 1-oxides by a facile thermal rearrangement.