Intramolecular 1,5- versus 1,6-hydrogen abstraction reaction promoted by alkoxyl radicals in pyranose and furanose models

The primary alkoxyl radical generated by reaction of 1-(2-hydroxyethyl)-glycosides with (diacetoxyiodo)benzene (DIB) and iodine can undergo regioselective intramolecular hydrogen abstraction (IHA) reactions to furnish four different dioxabicyclic systems derived from carbohydrates. The results strongly suggest that the regiocontrol and feasibility of the cyclisation are dependant not only on geometric and stereoelectronic factors, but also on polar and thermochemical factors. The correct selection of the substituents at the precursors can favour the 1,6-IHA against the 1,5-IHA pathway.     

IRREVERSIBLE PHOTO IN ACTIVATION OF NITRATE REDUCTASE DURING THE FLAVIN-SENSITIZED PHOTOCHEMICAL ASSAY OF ITS CATALYTIC ACTIVITY

Abstract— The classic photochemical system in which flavin and EDTA act as photosensitizer and electron donor, respectively, has been employed for assaying in vitro the catalytic activity of Ankistrodesmus braunii nitrate reductase. When the photochemical assay is performed under air, but not in anaerobiosis, a considerable decay in the nitrite photoproduction rate is observed after 10–15 min. which is accompanied by a decrease of reduced methyl viologen-nitrate reductase activity. The first enzyme activity, i.e. diaphorase is photoinactivated even more quickly and does not present any initial lag phase. Some oxygen species (superoxide and/or hydrogen peroxide) are probably involved in the photoinactivating mechanism, which appears to be non-specific and irreversible.
The use of flavin/EDTA photosystems is therefore, very practical for the in vitro assay of nitrate reductase activity, although anaerobic conditions are required for optimum results. Since inactivation of the terminal activity is, however, relatively slow, anaerobiosis would not be required for assay times shorter than 10 min.     

Fragmentation of carbohydrate anomeric alkoxyl radicals: a new synthesis of 1,1-difluoro-1-iodo alditols

[reaction: see text]. The beta-fragmentation of 2,2-difluoro-saccharide anomeric alkoxyl radicals, generated under oxidative condition by treatment of the respective alcohols with (diacetoxyiodo)benzene (DIB) and iodine, afforded 1,1-difluoro-1-iodo alditols in high yield. The reactivity of the fluorinated radical generated by rupture of the C-I bond has been preliminarily assessed by reductive deiodination with tributyltin hydride/AIBN and intermolecular allylation using the Keck reaction.     

Synthesis,tautomerism and diels-alder reactions of 1, 4-dihydroxy-9, 10-anthraquinon-9-imines

The ammonolysis of quinizarin and simple derivatives thereof affords 9-imino-derivatives, the tautomeric forms of which act as dienophiles in Diels-Alder reactions to yield tetracyclic compounds of potential value as anthracyclinone precursors.     

Regiospecificity in deuterium labeling determined by mass spectrometry

Several mass spectrometry methods were explored to determine the regiospecificity of deuterium substitutions in hydrocarbon mixtures. The case investigated in this work was that of ethane mixtures obtained by catalytic HD exchange between either C(2)H(6) and D(2) or C(2)D(6) and H(2) over platinum surfaces. A total of ten isotopologs are possible, and were indeed detected in all cases. Deconvolution of low-resolution mass spectra was found sufficient to determine the composition of the gas mixtures in terms of the total number of deuterium substitutions, but not to identify symmetric versus asymmetric substitutions in the C(2)D(2)H(4), C(2)D(3)H(3), and C(2)D(4)H(2) products. High-resolution mass spectrometry allowed the separation of the intensities due to C(2)X(4)(+) fragments from those from molecular C(2)X(6)(+) signals (X = H or D), and with that for a more accurate determination of the composition of the mixtures. Relative probabilities were determined for the symmetric versus asymmetric removal of X(2) from C(2)X(6)(+) ions and for isotope scrambling in the mass spectrometer, and with that information fairly good cracking patterns were then calculated for the C(2)X(4)(+) fragments produced by each individual pure C(2)X(6) isotopologue. However, total deconvolution of all ten components in the ethane mixtures obtained by HD exchange catalysis was beyond the experimental accuracy of the measurements. Tandem mass spectrometry/collision-induced decomposition mass spectrometry (MS/CID-MS) proved more useful for this task. In particular, it was possible to determine the proportion of symmetric versus asymmetric double HD exchange in samples for which the total ethane-d(2) (in the case of C(2)H(6) + D(2)) or ethane-d(4) (with C(2)D(6) + H(2)) amounted to only approximately 3% on the ethane mix. A comparison with other analytical methods, NMR in particular, is provided.     

Structural studies of spiroarsoranes derived from 2-aminophenols

This paper describes the structural studies of 2-phenyl-9,9′-dimethyl-2,2′-spirobis(1,3,2-λ⁵-benzoxazarsoline) 5, 2-phenyl-8,8′-dimethyl-2,2′-spirobis(1,3,2-λ⁵-benzoxazarsoline) 6 by ¹H,¹³C,¹⁵N NMR in [²H₆]DMSO and CDCl₃. The solid state studies were made by X-ray experiments. Infrared spectroscopy was obtained in CDCl₃ and the vibrational signals were assigned using DFT calculations. The nature of the As–N, As–C and As–O bonds in these compounds was established by NBO studies.     

11,16 Oxetane lactones. Spectroscopic evidences and conformational analysis

Sesquiterpene lactones constitute a wide group of compounds with several biological activities, including allelopathic. The naturally occurring sesquiterpene lactones dehydrocostuslactone and cynaropicrin have been modified in three different ways: preparation of 11,13-oxetane lactones, addition of a second Michael acceptor and reduction of the α-methylene-γ-lactone in order to future structure-activity relationship (SAR) studies. We have obtained all oxetane lactones stereoisomers at C-11 and C-16 positions. This fact has allowed us to establish some correlations between experimental data, derived by NMR and X-ray analysis, and the configuration at C-11 and C-16, which could be a useful tool to establish the stereochemistry as well as to confirm the presence of an oxetane ring on similar compounds. Comparative conformational analyses as a key aspect in the biological behaviour of those compounds in future structure-activity relationship (SAR) studies are presented.     

Synthetic studies on stevastelins. 2. Synthesis of lipidic- and peptidic-modified analogues

[Chemical reaction: See text] The synthesis of a series of stevastelin analogues with modification of the susbstituent at the C-2 position of the stearic acid chain (compounds 28 and 31), variation of the amino acids (compounds 41, 42, 73, and 78), or lacking the lipidic chain (compound 91) is described. The replacement of L-valine and L-threonine with other amino acids proceeded without difficulties for the synthesis of analogues 41 and 42; however, the substitution of L-serine with simple amino acids, such as glycine or L-alanine, proved to be elusive, which was adscribed to factors of conformational flexibility. Finally, the substitution with L-valine or L-threonine proceeded without difficulties to provide the analogues 73 and 78 respectively.