1,720,970 research outputs found
Nuovi N-ossidi chirali quali catalizzatori organici per l'allilazione stereoselettiva di aldeidi aromatiche
No full textIn the context of a rapidly growing interest in enantioselective organic (i.e. metal-free) catalysis, catalytic amounts of often structurally complex pyridine and amine N-oxides have been recently used by some research groups as organic Lewis bases capable of promoting the enantioselective allylation of aldehydes. In this thesis a research work aimed to develop simpler and easier-to-prepare chiral N-oxides is described. Three new families of chiral pyridine and amine N-oxides were synthesized and tested in the reaction of aromatic aldehydes with allyltrichlorosilane affording the corresponding homoallylic alcohols:
* 2-picolinic and 2,6-pyridinedicarboxylic acid derivatives, prepared in one or two steps from cheap and commercially available starting products (up to 50% e.e. in the allylation of benzaldehyde);
* 2-(2,6-dihydroxyphenyl)-3-methylpyridine derivatives, obtained by an efficient four-steps synthesis from commercially available products (up to 84% e.e. in the allylation of benzaldehyde);
* trans-2,5-diphenylpyrrolidine derivatives, synthesized from 1,4-diphenylbutane-1,4-dione, whose development is still in progress
Chiral (salen)Co(III)(N-benzyl-L-serine) derived phosphites : monodentate P-ligands for enantioselective catalytic applications
No full textReaction of the (S,S)-salen-cobalt(III)-N-benzyl-L-serine complex 2 with four diol-derived chlorophosphites
afforded phosphites 3a–3d in moderate yields (37–72%). Structural studies of these monodentate phosphite ligands and of their Rh-complexes were performed in solution by 1H and 31P NMR spectroscopy.
The ligands were screened in several enantioselective catalytic applications, showing good activity and moderate enantioselectivity in the palladium-catalyzed desymmetrization of meso-cyclopenten-2-ene-1,4-diol biscarbamate
Approaches to the Stereoselective Total Synthesis of (-)-Dictyostatin
No full textThe sponge-derived macrolide (-)-dictyostatin (1) has been reported to exhibit paclitaxel-like effects on cellular microtubules and to inhibit human cancer cell proliferation (even of paclitaxel-resistant cancer cell lines) at low nanomolar concentrations, with activity somewhat superior to the already very active discodermolide. Although four total syntheses of (-)-dictyostatin were recently completed, the development of a practical and flexible synthesis remains an important goal, particularly as its natural supply is extremely scarce.
Here we report a highly stereoselective synthesis of the C1–C26 (2), the C10-C26 (3) and the C1–9 (4) fragments of (-)-dictyostatin. Work is in progress to complete the total synthesis of (-)-dictyostatin
A Practical Synthesis of the C1-C9 Fragment of Dictyostatin
No full textA stereoselective synthesis of the C1–C9 fragment of (–)-dictyostatin has been achieved by use of a titanium(IV) chloride
mediated chelation-controlled Mukaiyama aldol reaction and two modified Horner–Wadsworth–Emmons olefinations (Roush–
Masamune and Still–Gennari)
Supramolecular Catalysts : Investigation of Arene π-π and Acid-Base Interactions to Promote the Assembly of Monodentate P-Ligands
No full textIn recent years, monodentate phosphorus ligands have held center stage in asymmetric catalysis, thanks to their outstanding activity, selectivity and convenient, fast and practical synthesis. Bidentate ligands are of course still very important, even though their synthesis is usually more tedious and expensive. Supramolecular bidentate ligands, based on the assembly of monodentate ligands possessing complementary functionalities, combine the essential features of bidentate ligands (preorganization and rigidity) with the advantages of monodentate ligands (practicality). Metal-ligands interactions (Reek, van Leeuwen, Takacs), hydrogen bonds (Breit, Reek, Nishibayashi), and ionic interactions (van Leeuwen) have been so far the main non-covalent forces used to promote the recognition of monodentate ligands.1
Here we report two new approaches for the assembly of monodentate phosphites in Rh-complexes: i) arene - interactions, using a binary combination of an electron-poor (with a perfluroarene or dinitroarene unit) and an electron-rich ligand (with a methoxyarene unit); ii) acid-base interactions, using a binary combination of an acidic (bearing a carboxylic acid) and a basic (bearing a tertiary amine) chiral binaphtholic phosphite.
The preferential formation of the Rh-heterocomplexes was investigated by 31P-NMR and the ligand binary combinations were also tested in the Rh-catalyzed asymmetric hydrogenation of functionalized olefins (methyl acetamidoacrylate and dimethyl itaconate). In both cases, enhanced ee’s (up to 99 %) were obtained using selected heterocobinations with respect to the corresponding homocombinations.
We thank MiUR for a postdoctoral fellowship (to L. Pignataro, PRIN prot. 2006030449), the European Commission [RTN Network (R)Evolutionary Catalysis MRTN-CT-2006-035866] for financial support and for a predoctoral fellowship (to B. Lynikaite) and a postdoctoral fellowship (to J. Cvengroš). C. Gennari gratefully acknowledges Merck Research Laboratories for the Merck's Academic Development Program Award
PhthalaPhos : chiral supramolecular ligands for enantioselective hydrogenation reactions
No full textRhodium-catalyzed asymmetric hydrogenation of olefins with phthalaphos, a new class of chiral supramolecular ligands
No full textA library of 19 binol-derived chiral monophosphites that contain a phthalic acid diamide group (Phthala- Phos) has been designed and synthesized in four steps. These new ligands were screened in the rhodium-catalyzed enantioselective hydrogenation of prochiral dehydroamino esters and enamides. Several members of the library showed excellent enantioselectivity with methyl 2-acetamido acrylate (6 ligands gave >97% ee), methyl (Z)-2- acetamido cinnamate (6 ligands gave >94% ee), and N-(1-phenylvinyl)acetamide (9 ligands gave >95% ee), whilst only a few representatives afforded high enantioselectivities for challenging and industrially relevant substrates N-(3,4-dihydronaphthalen-1- yl)-acetamide (96% ee in one case) and methyl (E)-2-(acetamidomethyl)-3- phenylacrylate (99% ee in one case). In most cases, the new ligands were more active and more stereoselective than their structurally related monodentate phosphites (which are devoid of functional groups that are capable of hydrogen-bonding interactions). Control experiments and kinetic studies were carried out that allowed us to demonstrate that hydrogen-bonding interactions involving the diamide group of the PhthalaPhos ligands strongly contribute to their outstanding catalytic properties. Computational studies carried out on a rhodium precatalyst and on a conceivable intermediate in the hydrogenation catalytic cycle shed some light on the role played by hydrogen bonding, which is likely to act in a substrate-orientation effect
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
- …
