1,721,041 research outputs found
Evidence for an NIH shift as the origin of the apparently anomalous distribution of deuterium in estragole from Artemisia dracunculus
No full textFeeding experiments of [2-H-2]- and [4'-H-2]phenylalanine in Artemisia dracunculus validate the hypothesis that the marked difference in deuterium content at the natural abundance level between the aromatic carbons of estragole (1) is due to an NIH shift during the hydroxylation of the benzene ring
Rhodium(II)-Catalyzed Decomposition of 1-Diazo-4-(2-naphthyl)butan-2-one : Direct Chemical Evidence for the Formation of the Norcaradiene System in the Intramolecular Buchner Reaction
No full textThe Michael reaction of N-cinnamoylazoles with phenols. A simple synthesis of 4-arylchroman-2-ones and 1-arylbenzo[f]chroman-3-ones
No full text4-Arylchroman-2-ones 3 and 1-arylbenzo[f]chroman-3- ones 6 have been prepared in moderate to good yields by reaction of dihydric or trihydric phenols with p-substituted N-cinnamoylazoles in dichloromethane under reflux in the presence of DBU
Studies on aloe. Part 6. Conformation and absolute configuration of aloins A and B and related 10-C-glucosyl-9-anthrones
No full textThe conformation and absolute configuration of the two diastereoisomeric aloins A (2a) and B (2b) were proven on the basis of shielding and deshielding effects and specific NOE (nuclear Overhauser effect) associations in 1H NMR spectroscopy. Previous 1H NMR assignments were corrected and 13C NMR spectra recorded for the first time. Taking into account analogies in CD spectra as well as in NOE effects, the absolute configuration of C-10 was suggested for the epimeric pairs of naturally occurring 1,8-dihydroxy-10-C-β-D-glucopyranosyl-9-anthrones
New guanosine 5’-phosphate derivatives as flavor enhancers
No full textA number of N2 -alkyl and N2 -acyl derivatives of guanosine 5’-phosphate (2 and 3) have been synthesized as potential flavor enhancers and tested for their synergistic effect with monosodium-L-glutamate (MSG), the prototypical substance imparting umami taste to savory-based foods
Stereoselective TiCl4-promoted nucleophilic substitution at C-2 of (4S,5S)-2-alkyl-4-methyl-5-trifluoromethyl-1,3-dioxolanes
No full textTiCl4-mediated nucleophilic ring-opening reactions of chiral acetals derived from (2S,3S)-1,1,1-trifluorobutane-2,3-diol proceed in a completely regioselective manner, leading to the break of the O1-C2 bond accompanied by a high degree of stereoselectivity. The use of triethylsilyl deuteride or allyltributyltin as nucleophiles gives access, after removal of the chiral auxiliary, to stereoselectively deuterated primary alcohols or homoallylic secondary alcohols, respectively, with high enantiomeric excesses
Stereochemistry and fate of hydrogen atoms in the diol-dehydratase-catalyzed dehydration of meso-butane-2,3-diol
No full textThe transformation of meso-butane-2,3-diol into butan-2-ol by a strain of Lactobacillus brevis occurs through a diol-dehydratase-catalyzed conversion of the diol into butan-2-one, which is then reduced to the secondary alcohol by dehydrogenases. Experiments performed with deuterated meso-butane-2,3-diols showed that the dehydration reaction brings about an inversion of configuration at the (R)-configured C-atom of mesobutane-2,3-diol as a consequence of the substitution of the OH group by a H-atom; at the same time, the H-atom already bound to the (R) C-atom is retained in the resulting methylene group. The H-atom replacing the OH group was assessed to come from the medium since the H-atom at the (S)-configured C-atom was completely lost. By contrast, in the case of the conversion of (RS)-propane-1,2-diol into propan-1-ol under the same fermentation conditions, an extensive H-transfer (ca. 80%) from the primary-alcohol function to the adjacent C-atom was observed. This fact is taken as an indication of different modes in which the two substrates are processed by the enzyme (despite the same stereochemical outcome). A speculative hypothesis is presented to interpret such a dissimilarity
New syntheses of optically active vitamin E side chain by chemicoenzymatic approach
No full textTwo independent syntheses of enantiomerically pure ( 3R, 7R) -3,7,11-tri-methyldodecan-1-ol (), the C15 tocopherol side chain, are described. Both reaction sequences start from (R)-citrohellol, obtained by baker's yeast reduction of geraniol. One strategy is based on the coupling between (R)-citronellyl bromide and a C5 optically active unit derived from chemical degradation of the common starting template; the other one uses an achiral isoprenic unit and a C10, building block containing two asymmetric carbon atoms generated by microbial reactions
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