A theoretical approach to substituent effects. A comparison of the isoelectronic BH,CH3, and NH groups and their interaction with substituents in disubstituted benzenes

Ab initio molecular orbital theory with the STO -3G basis set is used to examine both charge and energy interactions in a series of meta- and para-substituted phenylborate anions and toluenes. Comparison of the results is made with data for substituted anilinium cations. It is concluded that whereas NH is a powerful σ acceptor, with essentially no π interaction, BH is primarily a π donor, and, to a slight extent only, a π donor. CH₃ is indicated to be both a weak σ and π donor. Energies of interaction of BH and NH with a series of substituents are an order of magnitude larger than corresponding values for CH₃. Interaction energies for BH are of opposite sign to those for NH. The results may be understood qualitatively using perturbation molecular orbital (PMO ) theory.     

Synthesis of 1,3,4,14b-tetrahydro-2H,10H-pyrazino[1,2-α]- pyrrolo[2,1-c][1,4]benzodiazepines

1,3,4,14b-Tetrahydro-2H,10H-pyrazino[1,2-α]pyrrolo[2,1-c] [1,4]benzodiazepines (1a-e) were synthesized to investigate their potential CNS activity. The key step in the synthesis was the formation of the 10,11-dihydro-5H-pyrrolo[2,1-c][1,4]benzodiazepine (13) by reduction and concomitant cyclization of the nitroketone (11). Biological evaluation of 1a-e revealed interesting properties for 1b (CGS 7525A) [2].     

Aureomycin nitrate,its preparation and identification

Summary Aureomycin hydrochloride does not show any promise as a reagent for qualitative detection or quantitative estimation of any of the common cations or aidons. The well developed crystals of the rather insoluble nitrate may be used for the identification of aureomycin. The low solubility of the nitrate may give a better stability as compared with the hydrochloride, which would be of interest to the pharmacist.

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Zusammenfassung Versuche mit Aureomycinhydrochlorid gaben keine Anhaltspunkte für eine Eignung dieser Verbindung als Reagens für die Auffindung oder Bestimmung der häufig angetroffenen Kationen oder Anionen. Die gut ausgebildeten Kristalle des wenig löslichen Nitrats können für die Erkennung von Aureomycin benützt werden. Die geringe Löslichkeit des Nitrats erhöht dessen Beständigkeit im Vergleich zum Hydrochlorid, was für Pharmazeuten von Interesse sein könnte.

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Résumé Le chlorhydrate d'auréomycine ne montre aucune propriété intéressante comme réactif de recherche qualitative ou d'estimation quantitative des cations et anions courants. On peut utiliser les cristaux bien développés de son nitrate assez peu soluble pour identifier l'auréomycine. La faible solubilité de ce nitrate peut lui conférer une meilleure stabilité que celle du chlorure ce qui est digne d'intérêt pour les pharmaciens.

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Dedicated to Prof. Dr.Hans Lieb at his 70th birthday.     

The structure of propadienone (CH2CCO)

A complete r_o structure has been obtained for propadienone (CH₂CCCO) with the aid of ab initio molecular-orbital calculations. The effect of electron correlation has been investigated using third order Møller-Plesset perturbation theory. The molecule is found to be planar bent (CCC145°) and the calculated structure yields rotational constants which are in good agreemant with experimental values.     

The geometric and electronic structures of oxocarbons. An ab initio molecular orbital study

Ab initio molecular orbital calculations with the STO-3G and 4-31G basis sets have been carried out for the neutral oxocarbons C_nO_n (n = 3, 4, 5, 6 and 7), the dianions C_nO_n^2- (n = 3, 4, 5, 6 and 7), the monoanions C_nO_nH^? (n = 3 and 4) and the related acids C_nO_nH₂ (n = 3 and 4). Fully optimised geometries have been obtained for all species. The geometries, stabilities and acidities are discussed.     

The bimolecular hydrogen-deuterium exchange behavior of protonated alkyl dipeptides in the gas phase

As part of an ongoing characterization of the intrinsic chemical properties of peptides, thermal hydrogen-deuterium exchange has been studied for a series of fast-atom-bombardment-generated protonated alkyldipeptides and related model compounds in the reaction with D₂O, CH₃OD, and ND₃ in a Fourier transform ion cyclotron resonance mass spectrometer. Despite the very large basicity difference between the dipeptides and the D₂O and CH₃OD exchange reagents, efficient exchange of all active hydrogen atoms occurs. From the kinetic data it appears that exchange of the amino, amide, and hydroxyl hydrogens proceeds with different efficiencies, which implies that the proton in thermal protonated dipeptides is immobile. The selectivity of the exchange at the different basic sites is governed by the nature of both the dipeptide and the exchange reagent. The results indicate that reversible proton transfer in the reaction complexes, which effectuates the deuterium incorporation, is assisted by formation of multiple hydrogen bonds between the reagents. Exchange is considered to proceed via the intermediacy of different competing intermediate complexes, each of which specifically leads to deuterium incorporation at different basic sites. The relative stabilization of the competing intermediate complexes can be related to the relative efficiencies of deuterium incorporation at different basic sites in the dipeptide. For all protonated dipeptides studied, the exchange in the reaction with ND₃ proceeds with unit efficiency, whereas all active hydrogen atoms are exchanged equally efficiently. Evidently specific multiple hydrogen bond formations are far less important in the reversible proton transfers with the relatively basic ammonia, which allows effective randomization of all active hydrogen atoms in the reaction complexes.     

The use of comet assay data with a simple reaction mechanism to evaluate the relative effectiveness of free radical scavenging by quercetin, epigallocatechin gallate and N-acetylcysteine in UV-irradiated MRC5 lung fibroblasts

Comet assay data (tail DNA %) have been gathered for the concentration dependent role of three antioxidants (AOs); quercetin (Q), epigallocatechin gallate (EGCG) and N-acetylcysteine (NAC) in reducing UV-induced damage to DNA in normal fetal lung fibroblasts (MRC5). All three compounds demonstrate a concentration dependent reduction maximum with a pro-oxidant effect at higher (though not cytotoxic) concentrations. Manipulation of a simple 4-step reaction mechanism for free radical (FR) scavenging by AOs produced rate constant ratios which allowed the relative effectiveness (Q > EGCG > NAC) of the AOs to be evaluated.     

Interpretation of CALUX results in view of the EU maximal TEQ level in milk

Analyses of dioxins in food have become increasingly important since the European Commission has enforced maximal toxic equivalent concentration (TEQ) levels in various food and feed products. Screening methodologies are usually used to exempt those samples that are below the maximum permitted limit and that can, therefore, be released to the market. In addition, one needs to select those samples that require confirmation of their dioxin TEQ level. When bioassays are used as screening tools, the interpretation of the obtained results should consider the higher variability and uncertainty associated with them. This paper explores the use of CALUX data as quantitative screening results. The validation of the method for the polychlorinated dibenzo-p-dioxins (PCDD)/F TEQ determination in milk samples is described with emphasis on the decision limit (CC) and the precision of the method. The decision limit amounts to 4.53 pg TEQ/g fat. Repeatability and within-lab reproducibility coefficients of variation are below 30%. The newly introduced parameter CC^* of 1.47 pg TEQ/g fat delimits with CC a range of suspicious results. These data are not significantly different from the maximum limit of 3 pg TEQ/g fat and should be confirmed by a confirmatory analytical method such as HRGC–HRMS.     

Rhodium(I) catalysis of cycloprop[a]acenaphthylene isomerizations

Cycloprop [a] acenaphthylene was found to rearrange to phenalene in the presence of rhodium dicarbonyl chloride dimer. Deuterium labeling of this molecule at C(7) (both exo and endo), at C(8), at C(7) (exo) and C(8) and at C(7) (exo), C(8), and C(8') indicated that the C(7) (exo) deuterium was migrating stereospecifically. Furthermore, all of the isotopic label present in the cycloprop[a]acenaphthylene was found statistically distributed over positions 1, 3, 4, 6, 7, and 9 of the phenalene product. Control experiments established the need of the catalyst, the inability to achieve rearrangement of the exo-7-methyl derivative, and the susceptibility of a monodeuterated phenalene for extensive isotopic scrambling in its own right when exposed to rhodium(I). These results have been interpreted on the basis of oxidative addition by rhodium(I) into the central bond of cycloprop[a]acenaphthylene from above the “flap”, followed by shifting of the C(7) (exo) hydrogen (or deuterium) to give a η³-allylrhodim-(III) complex. The experimental data further support a mechanism involving subsequent rearrangement of this intermediate around the periphery of the phenalene ring. This process which is otherwise degenerate is thought to be facilitated by the special electronic features of the phenalenyl system.