Unsymmetrical covariance processing of COSY or TOCSY and HSQC NMR data to obtain the equivalent of HSQC‐COSY or HSQC‐TOCSY spectra

Artifacts observed in the indirect covariance NMR spectrum of HSQC‐TOCSY data have recently been analyzed and a method for their elimination proposed. More recently, unsymmetrical covariance processing has been applied HSQC and HMBC spectral data to afford long‐range carbon‐carbon correlation information equivalent to that obtained from n, 1‐, 1, n‐ and m,n‐ADEQUATE spectra. We now wish to describe the results obtained through the application of unsymmetrical covariance processing of HSQC and COSY or TOCSY data, which affords the equivalent of HSQC‐COSY and HSQC‐TOCSY data in a fraction of the time required to record these spectra directly and with considerably higher sensitivity.     

Fast catalytic hydroxylation of hydrocarbons with ruthenium porphyrins

Ruthenium porphyrin complexes such as carbonylruthenium(II) tetrakispentafluorophenylporphyrin [Ru(II)(TPFPP)(CO)] were found to be efficient catalysts for the hydroxylation of alkanes in the presence of 2,6-dichloropyridine N-oxide as the oxidant under mild, nonacidic conditions. Up to 14 800 turnovers (TO) and rates of 800 TO/min were obtained for the hydroxylation of adamantane. The hydroxylation of cis-decalin afforded cis-9-decalol and cis-decalin-9,10-diol, exclusively, thus, excluding a long-lived radicals mechanism. The kinetics of product evolution in a typical catalytic oxygenation showed an initial induction period followed by a fast, apparently zero-order phase with maximum rates and high efficiencies. Deuterium isotope effects (kH/kD) in the range of 4.2-6.4 were found for the hydroxylation of alkanes. A Hammett treatment of the data for the oxidation of para-substituted toluene derivatives showed a linear correlation with a highly negative rho+ value of -2.0. On the basis of kinetic and spectroscopic evidence, Ru(VI)(TPFPP)(O)2, Ru(II)(TPFPP)(CO), and Ru(IV)(TPFPP)Cl2 observed during catalysis were ruled out as candidates for the active catalyst responsible for the high efficiencies and turnover rates in the oxidation reactions. The fastest rates of adamantane hydroxylation with 2,6-dichloropyridine N-oxide were achieved by the reductive activation of Ru(IV)(TPFPP)Cl2 with a zinc amalgam. This redox activation is consistent with the formation of an active Ru(III) intermediate in situ by a one-electron reduction of the Ru(IV) porphyrin. EPR spectra characteristic of Ru(III) have been observed upon the reduction of Ru(IV)(TPFPP)Cl2 with a zinc amalgam. In the adamantane oxidation mediated with Ru(III)(TPFPP)(OEt), it was found that, during this process, the Ru(III) porphyrin was gradually converted to a dioxoRu(VI) porphyrin. Concomitant with this conversion, the reaction rates decreased. Catalyst activation was also stimulated by autoxidation of the solvent CH2Cl2. On the basis of these data, a mechanism is proposed that incorporates a "fast" cycle involving metastable Ru(III) and oxoRu(V) intermediates and a "slow" oxidation cycle, mediated by oxoRu(IV) and trans-dioxoRu(VI) porphyrin complexes.     

Coherent control of harmonic generation in superlattices: single-mode response

We study the harmonic generation spectrum in a semiconductor superlattice (a quantum-dot array) at slow relaxation. The effect of a single-mode response in an alternating rectangular (meander) electric field is demonstrated: For certain values of field parameters, the extremely wide discrete output spectrum with slowly decaying tails (multiharmonic generation) shrinks to one single harmonic (single-harmonic generation). Similarly, the effect is manifested in the transient continuous spectrum by diminishing the divergencies (peaks) at all odd harmonics but one. Substantial control over the spectrum is demonstrated.     

Polyketides from the marine-derived fungus Ascochyta salicorniae and their potential to inhibit protein phosphatases

Chemical investigation of the marine fungus Ascochyta salicorniae led to the isolation of two new epimeric compounds, ascolactones A (1) and B (2), in addition to the structurally-related polyketides hyalopyrone (3), ascochitine (4), ascochital (5) and ascosalipyrone (6). The absolute configurations of the epimeric compounds 1 and 2 were assigned as (1R,9R) and (1S,9R), respectively, through simulation of the chiroptical properties using quantum-chemical CD calculations, and chiral GC-MS subsequent to oxidative cleavage (Baeyer-Villiger oxidation) of the side chain. In silico screening using the PASS software identified some of the A. salicorniae compounds (1-6) as potential inhibitors of protein phosphatases. Compound was found to inhibit the enzymatic activity of MPtpB with an IC(50) value of 11.5 microM.     

Dynamic Cross‐Exchange in Halophosphonium Species: Direct Observation of Stereochemical Inversion in the Course of an SN2 Process

The complex fluxional interconversions between otherwise very similar phosphonium bromides and chlorides R₃PX⁺X^? (R=Alk, Ar, X=Cl or Br) were studied by NMR techniques. Their energy barriers are typically ca. 11 kcal mol^?1, but rise rapidly as bulky groups are attached to phosphorus, revealing the importance of steric factors. In contrast, electronic effects, as measured by Hammett analysis, are modest (ρ 1.46) but still clearly indicate negative charge flow towards phosphorus in the transition state. Most significantly, detailed analysis of the exchange pathways unequivocally, and for the first time in any such process, shows that nucleophilic attack of the nucleophilic anion on the tetrahedral centre results in inversion of configuration.     

Radical 1,4-aryl transfer in arylcarboxamides leading to phthalimides, biaryls and enantiomerically enriched β-arylethylamines

5-exo Cyclisation of vinyl-, aryl- and alkyl-radicals onto the aryl group of arylcarboxamides is followed by β-scission of the resulting spirocyclohexadienyl radicals with ejection of a carbamoyl radical. The fate of this radical depends on the substrate but, in the cases studied, either 5-endo cyclisation or direct reduction follows to give phthalimides, biaryls or β-arylethylamines.