1,720,993 research outputs found
Asymmetric Total Syntheses of (+)-Cheimonophyllon E and (+)-Cheimonophyllal
No full textThe highly enantiocontrolled total syntheses of natural (+)-cheimonophyllon E (5) and (+)-cheimonophyllal (6), biologically intriguing oxygenated bisabolane-type sesquiterpenoids, have been
completed. The present synthetic strategy featured the use of an asymmetric aldol-type reaction
for preparing in the first synthetic step an optically active 6-C-substituted 3-methyl-2-cyclohexenone
derivative. Thus, a Mukaiyama aldol reaction of 1-methyl-3-silyloxy-1,3-cyclohexadiene 31 with
α,β-unsaturated aldehyde 11 in the presence of a chiral (acyloxy)borane (CAB)-type Yamamoto
catalyst 33 proceeded with high levels of both diastereo- and enantioselectivities. The predominant
aldol adduct, syn-9, was transformed into γ,δ-epoxy allylic alcohol 8 by a nine-step sequence,
including the substrate-controlled 1,2-reduction of enone, syn-12, also the epoxidation of allylic
alcohol 15. Epoxy-alcohol 8 underwent 5-exo-cyclization in a high regioselective manner under acidic
conditions to produce a bicyclic key intermediate (+)-7, which was eventually efficiently converted
to (+)-cheimonophyllon E (5) or (+)-cheimonophyllal (6)
Asymmetric Total Syntheses of (+)-Cheimonophyllon E and (+)-Cheimonophyllal
No full textThe highly enantiocontrolled total syntheses of natural (+)-cheimonophyllon E (5) and (+)-cheimonophyllal (6), biologically intriguing oxygenated bisabolane-type sesquiterpenoids, have been
completed. The present synthetic strategy featured the use of an asymmetric aldol-type reaction
for preparing in the first synthetic step an optically active 6-C-substituted 3-methyl-2-cyclohexenone
derivative. Thus, a Mukaiyama aldol reaction of 1-methyl-3-silyloxy-1,3-cyclohexadiene 31 with
α,β-unsaturated aldehyde 11 in the presence of a chiral (acyloxy)borane (CAB)-type Yamamoto
catalyst 33 proceeded with high levels of both diastereo- and enantioselectivities. The predominant
aldol adduct, syn-9, was transformed into γ,δ-epoxy allylic alcohol 8 by a nine-step sequence,
including the substrate-controlled 1,2-reduction of enone, syn-12, also the epoxidation of allylic
alcohol 15. Epoxy-alcohol 8 underwent 5-exo-cyclization in a high regioselective manner under acidic
conditions to produce a bicyclic key intermediate (+)-7, which was eventually efficiently converted
to (+)-cheimonophyllon E (5) or (+)-cheimonophyllal (6)
Total Synthesis of (±)-Mycoepoxydiene, a Novel Fungal Metabolite Having an Oxygen-Bridged Cyclooctadiene Skeleton
No full textThe first total synthesis of (±)-mycoepoxydiene has been accomplished. A ring-closing olefin metathesis (RCM) approach was employed for
the construction of the oxygen-bridged eight-membered bicyclic skeleton. The RCM product was converted to the target natural product
featuring the oxidative rearrangement of a furfuryl alcohol introduced as the side chain and the stereoselective 1,2-reduction of a δ-keto-β,γ-unsaturated α-lactol intermediate
Total Synthesis of Natural PI-091, a New Platelet Aggregation Inhibitor of Microbial Origin
No full textThe total synthesis of a new platelet aggregation-inhibiting
γ-lactam PI-091 (1) gave a 1:1
diastereomeric mixture at the γ-ketal carbon. The high-yielding
aldol reaction of an appropriately
protected 1,3,4-trihydroxy-4-methyldecan-2-one 42, prepared
from d-glucose, with the kinetically
generated enolate of 3-methyl-2-butanone provided 43.
The resulting diastereomeric mixture of
the aldol adduct 43 was converted to a 2,4-alkylated furan
45 via an intramolecular ketalization
followed by dehydration. The addition of a singlet oxygen to the
α-trimethylsilylated furan 48
derived from 45 under photochemical conditions
efficiently provided an α,γ-dialkylated γ-hydroxy
γ-lactone 47. The transformation of methyl ketal
52 prepared from 47 into γ-hydroxy γ-lactam
53
was achieved by exposure to liquid ammonia in MeOH. The total
synthesis of 1 was achieved
from 52 through the Dess−Martin periodinane oxidation of
the secondary hydroxy group in the
side chain. The present total synthesis revealed that the
stereogenic carbon center in the side
chain in natural 1 is S
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
Total Synthesis of Macquarimicins Using an Intramolecular Diels−Alder Approach Inspired by a Biosynthetic Pathway
No full textA total synthesis of the macquarimicins A−C (1−3), novel natural products with intriguing tetra-
or pentacyclic frameworks, has been achieved. The synthesis features an extensive investigation of the
biosynthesis-based intramolecular Diels−Alder (IMDA) reactions of (E,Z,E)-1,6,8-nonatrienes. Considering
possible biosynthetic sequences, four types of substrates were synthesized, and their IMDA reactions were
examined. From one of the four substrates, the total synthesis was achieved via a transannular Diels−Alder reaction, which led to the stereoselective construction of the unique molecular framework. The
convergent and efficient synthetic pathway afforded (+)-1 in 27 linear steps with 4.3% and 9.9% overall
yields from readily available ethyl (2E,4S)-4,5-(isopropylidene)dioxy-2-pentenoate (22) and (R)-epichlorohydrin (30), respectively. Furthermore, efficient syntheses of 2, 3, and the 9-epi-cochleamycins A (57) and
B (58) were accomplished. Additionally, the present work established the absolute stereochemistry of
macquarimicins and revised the C(2)−C(3) geometry of 1
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
Total Synthesis of (−)-Mniopetal E, a Novel Biologically Intriguing Drimane Sesquiterpenoid
No full textWe have achieved the total synthesis of (−)-mniopetal E, a drimane sesquiterpenoid which inhibits
the reverse transcriptase of human immunodeficiency virus (HIV)-1. Our enantiospecific total
synthesis of this target molecule in naturally occurring form commenced with a known 2,3-anhydro-d-arabinitol derivative, which was prepared using the Sharpless asymmetric epoxidation strategy.
The key steps of our total synthesis were as follows: (1) a combination of highly stereocontrolled
inter- and intramolecular Horner−Emmons carbon elongations for construction of a butenolide
tethering a 1,2,4,9-functionalized nona-5,7-diene moiety at the β-carbon, (2) stereoselective thermal
intramolecular Diels−Alder reaction of the thus-formed trienic compound, providing preferentially
an endo-cycloadduct with the desired π-facial selection, and (3) efficient transformation of the
γ-lactone moiety in the major cycloadduct to the γ-hydroxy-γ-lactone part in mniopetal E. Our
total synthesis of (−)-mniopetal E established the unsettled absolute stereochemistry of the
antibiotic
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