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ACID-CATALYZED AND ENZYMATIC-HYDROLYSIS OF TRANS-2-METHYL-3,4-EPOXYTETRAHYDROPYRAN AND CIS-2-METHYL-3,4-EPOXYTETRAHYDROPYRAN
No full textWhereas the acid-catalyzed hydrolysis of trans- and cis-3,4-epoxy-2- methyltetrahydropyran gives the corresponding diols by opening both at C-4 and C-3 (64% attack at C-4 in the trans-epoxide, 86% in the cis-epoxide), the hydrolysis of the same substrates catalyzed by rabbit liver microsomal epoxide hydrolase is entirely regiospecific and involves in both cases exclusive attack at C-4. The racemic cis-epoxide reacts at a faster rate than the trans. The 2R,3R,4S-enantiomer of the latter epoxide reacts at a much faster rate than its antipode to yield the (-)-(2R,3R,4R)-diol which is isolated at least 98% optically pure up to almost 50% conversion when starting from the racemic substrate. A reference sample of optically pure (+)-(2S,3S,4S)-diol was prepared from L-rhamnose. The enantiomeric excess of the (-)-(2R,3R,4R)-diol was also determined more precisely through g.l.c. analysis of diastereoisomeric MTPA esters. The present results confirm previous hypotheses on the topology of the hydrolase active site and emphasize the overriding importance of the orientation of a lipophilic substituent near the oxirane ring
Efficient differentiation of the hydroxyl groups of 3,4-O-isopropylidene-D-galactopyranosides by lipase catalyzed esterification and de-esterification
No full textThe Pseudomonas sp. (LPS) promoted acyl transfer from vinyl acetate to selected 3,4-O-isopropylidene-D-galactopyranosides takes place in a completely selective manner giving in high yield the corresponding 6-O-acetates. The acetylation rate is strongly dependent on the type and the orientation of the aglycon, varying from a maximum of reactivity for the 1-deoxy derivative, 1,5-anhydro-3,4-O-isopropylidene-D-galactitol (1d), to a minimum for beta configurated alkyl glycosides and showing a complete loss of reactivity for 3',4':2,3:5,6-tri-O-isopropylidenelactose dimethyl acetal (1e). The latter compound is, however, selectively 6'-O-esterified in good yield by lipase from Candida Antarctica and vinyl acetate. Also the course of the enzymatic hydrolysis of 2,6-di-O-acetyl-3,4-O-isopropylidene-D-galactopyranosides 2 is dependent on the type of the aglycon, both for the reaction rate and the selectivity. The 2-O-acetates 4 are selectively obtained in good yields with porcine pancreatic lipase (PPL) promoted hydrolysis in the case of beta- and alpha-methyl, and 1-deoxy derivatives (2a, 2b and 2d), while for beta-benzyl (2c) and lactose (2e) analogues satisfactory results are obtained with lipase from Mucor miehei (IM20)
Verso l'umanizzazione del latte vaccino: preparazione di derivati selettivamente protetti del lattosio.
No full textREGIOSELECTIVITY AND ENANTIOSELECTIVITY OF THE ENZYMATIC-HYDROLYSIS OF ISOBUTYL 3,4-ANHYDRO-2,6-DIDEOXY-BETA-DL-RIBO-HEXOPYRANOSIDE
No full textALKALINE AND ENZYMATIC-HYDROLYSIS OF ISOBUTYL 3,4-ANHYDRO-2,6-DIDEOXY-DL-HEXOPYRANOSIDES - PREPARATION OF ENANTIOMERIC BOIVINOPYRANOSIDES THROUGH A HIGHLY EFFICIENT KINETIC RESOLUTION
No full textRacemic isobutyl 3,4-anhydro-2,6-dideoxy-β-ribo-hexopyranoside and -lyxo-hexopyranoside have been prepared starting from the cycloadduct between 3-buten-2-one and isobutyl vinyl ether. Alkaline hydrolysis converted the lyxo isomer exclusively into isobutyl 2,6-dideoxy-β-DL-xylo-hexopyranoside (isobutyl β-DL-boivinopyranoside), the ribo isomer into a 57:43 mixture of the same glycoside, and its arabino diastereomer (isobutyl β-DL-olivopyranoside). Rabbit microsomal epoxide hydrolase similarly converted the racemic lyxo epoxide into the xylo diol in a regiospecific way and exhibited a high degree of enantioselectivity: when the enzymatic reaction was stopped at 50% conversion, isobutyl β-L-boivinopyranoside and isobutyl 3,4-anhydro-2,6-dideoxy-β-D-lyxohexopyranoside were obtained, both with an enantiomeric excess of at least 96%
UNUSUAL STERIC COURSE OF THE EPOXIDE HYDROLASE CATALYZED-HYDROLYSIS OF (+/-)3,4-EPOXYTETRAHYDROPYRAN - A CASE OF COMPLETE STEREO-CONVERGENCE
No full textThe steric course of the hydrolysis of racemic 3,4-epoxytetrahydropyran promoted by rabbit liver microsomal epoxide hydrolase has been investigated. Both enantiomers of the epoxide give (-)-(R,R)-trans-tetrahydropyran-3,4-diol at practically equal rates, the optical purity being at least 96% at any stage of conversion. These results can be justified by assuming that both enantiomers react in the 1,2-M conformation, nucleophilic attack by water taking place exclusively at position 3 of the 3S,4R enantiomer and at position 4 of the 3R,4S one, in contrast with the nonenzymic ring opening in which a high preference for attack at C(4) has been found
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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