1,720,977 research outputs found
Facile inversion of configuration of N-Boc-b-aminoalcohols via SN2 cyclization to oxazolidinones.
No full textEpoxyalcohol route to hydroxyethylene dipeptide isosteres: a new synthesis of the diaminoalcohol core of HIV-protease inhibitor ABT-538 (ritonavir).
No full textUnexpected 1,2,3-triazole formation in the reaction of diethylaluminum azide with a’- amino a,b-unsaturated ketones.
No full text4-Acyl-1H-1,2,3-triazoles are formed from diethylaluminum azide and -(N,N-dibenzylamino)-,-unsaturated ketones
by [3+2] cycloaddition of azide, followed by 1,5 hydride transfer to the carbon of the triazoline side chain and fragmentation
of the tertiary amino group promoted by coordination of the latter to the Lewis acid. The structure of a triazole product is
confirmed by X-ray crystallography
Val-Ala Dipeptide Isosteres via Hydrocyanation of alfa'-Amino-alfa,beta-Unsaturated Ketones. Control of Stereoselectivity by the N-Protecting Group.
No full textThree diastereoisomeric hydroxyethylene isosters of the Val-
Ala dipeptide were synthesized from α,β-unsaturated ketones
1 derived from N-Boc- and N,N-dibenzyl-L-valine. The
enones were hydrocyanated with diethylaluminum cyanide
to give the corresponding β-cyano ketones with the stereoselectivity
depending on the protecting group. N-Boc protected
enone 1a gave a 1:1 mixture of anti and syn adducts
4a, 5a while the corresponding N,N-dibenzyl compound 1c
gave a 6:1 mixture of anti, syn adducts 4c, 5c. Borohydride
reduction of the resulting cyano ketones is also controlled by
the protecting group, resulting in opposite stereoselectivities
for N-Boc and N,N-dibenzyl compounds. The cyano alcoholsthus obtained were converted, in several steps, into two series
of enantiomerically pure hydroxyethylene isosters of the
Val-Ala dipeptide. In the first series the hydroxy group and
the N-terminal of the isoster are internally protected through
the formation of an oxazolidine; in the second series the hydroxy
group and the C-terminal are protected as lactone.
Two oxazolidines (28, 29), corresponding to syn,syn and
syn,anti 4-hydroxy-5-amino acid isosters, and three lactones
(23−25), corresponding to syn,syn, syn,anti, and anti,anti isosters
were obtained by this approach
C2-Symmetric Val-Val Dipeptide Isosteres via Diethylaluminium Azide Ring Opening of Epoxyalcohols
Full text linkDiethylaluminium azide has been used as a highly regio- and stereo-selective reagent for the ring opening of valine-derived, sterically hindered epoxyalcohols. This reagent has enabled extending a previously described synthesis of diaminodiol dipeptide isosteres (P1-Ψ[CH(OH)-CH(OH)]-P1') also to isosteres with branched residues in position P1'. The new methodology is compatible with Boe and Cbz proteetion of the starting aminoacid and has been applied to the synthesis of a C2-symmetric diaminodiol Val-Val dipeptide isostere 11, starting from the methyl ester of L-valine, in 25% over-all yield over seven (for Boe proteetion) or six (for Cbz proteetion) passages. A C2-symmetric di-MEM proteeted diaminodiol 17 and a mono-Boc proteeted, desymmetrized derivative 10 have also been obtained by the same approach. Compounds 10, 11 and 17 can be used as core units of C2-symmetric and non-symmetric peptido-mimetic inhibitors of aspartic proteases and as intermediates for the synthesis of cyclic urea inhibitors of HIV-protease
New synthesis of 5-amino-4-hydroxy-2,6-dimethylheptanoic acid, a hydroxyethylene isostere of the Val-Ala dipeptide
No full text
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
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