321 research outputs found
N-Allenyl Amides and O-Allenyl Ethers in Enantioselective Catalysis
No full textNowadays asymmetric catalysis is an established re- ality in the organic synthesis community. Nevertheless, the range of applicability both in organic and in organometallic transformations is so large that seeking for new developments seems unstoppable. In this direction, from time to time a new class of “silent” organic compounds rises to prominence and creates new perspectives in the field. Allenyl amides and allenyl ethers are eminent examples of chemical “perturbation” of the current asymmetric synthetic scenario, with recent applications in metal-based and metal-free catalytic enantioselective manip- ulations. Besides their recent rediscovery in an asymmetric and catalytic context, this family of electron-rich allenes has already gained credit in terms of synthetic flexibility and the latest achievements in the field are summarized in this article
New opportunities in the stereoselective dearomatization of indoles
No full textThe regio- and stereoselective dearomatization of indoles is realized for the first time by combining readily available indolyl precursors and electron-rich allenes, namely allenamides and aryloxyallenes. Inter- as well as intramolecular condensations were realized under gold and Brønsted acid catalysis providing a range of densely functionalized indoline and indolenine cores in high yields and excellent stereochemical outcome. Chemodivergent reaction profiles (Micheal-type addition vs. [2 + 2]-cycloaddition) were realized by a tailored design of both reaction conditions and functionalization of the reaction partners
Manganese-Catalyzed C−H Alkynylation: Expedient Peptide Synthesis and Modification
No full textManganese(I)-catalyzed C-H alkynylations with organic halides occurred with unparalleled substrate scope, and thus enabled step-economical C-H functionalizations with silyl, aryl, alkenyl, and alkyl haloalkynes. The versatility of the manganese(I) catalysis manifold enabled C-H couplings with haloalkynes featuring, among others, fluorescent labels, steroids, and amino acids, thereby setting the stage for peptide ligation as well as the efficient molecular assembly of acyclic and cyclic peptides. A plausible catalytic cycle was proposed
Spin-Labelled Pillar[5]arene as Paramagnetic Host for Supramolecular Assemblies
No full textThe synthesis and characterization of a new member of the pillar[5]arene family carrying a paramagnetic side-arm based on the 2,2,6,6-tetramethylpiperidine N-oxide (TEMPO) moiety has been achieved by the Huisgen-type CuAAC reaction. The possibility of exploiting the proposed pillar[5]arene as a host in paramagnetic rotaxane-like structures was tested in the presence of charged guests and confirmed by NMR spectroscopy
Supramolecular Control of Spin Exchange in a Spin-Labelled [2]Rotaxane Incorporating a Tetrathiafulvalene Unit
No full textThe EPR properties of a novel triradical obtained
by single-electron oxidation of a nitroxide-spin-labelled rotaxane
containing a tetrathiafulvalene unit and cyclobis-
(paraquat-p-phenylene) ring is reported. Rotaxanation is
proved to have a dramatic effect on through-space magnetic
interactions between radical fragments. Analysis of
the EPR spectra by a three-jump model, allowed us to
obtain structural information on the interlocked structure
Stereoselective Organocatalytic Addition of Nucleophiles to Isoquinolinium and 3,4-dihydroisoquinolinium Ions: A Simple Approach for the Synthesis of Isoquinoline Alkaloids
No full textThe frequent occurrence of chiral 1-substituted-1,2,3,4-tetrahydroisoquinoline ring systems in a large number of alkaloids, possessing a broad spectrum of biological and pharmaceutical properties, has prompted out a considerable interest in their synthesis. Economical and valuable stereoselective processes based on organocatalytic transformations represent a new avenue for approaching isoquinoline alkaloids with efficiency and creativity
Spin-Labelling of Host-Guest Assemblies with Nitroxide Radicals
No full textWithin the developing interface between free radicals and supramolecular chemistry, nitroxides play a special role because they are stable and easy to functionalize. This Focus Review summarizes recent progress in the design and synthesis of paramagnetic host-guest complexes characterized by the presence of a nitroxide label covalently linked to the organic macrocycle. Examples include the synthesis of open-shell, mechanically interlocked molecules (MIMs), such as rotaxanes, in which the dumbbell, the wheel, or both are tagged with nitroxide radicals, and how electron spin resonance (ESR) spectroscopy methods have been used for detecting and identifying the assemblies and for clarifying their structure and properties
Metal-Free Enantioselective Electrophilic Activation of Allenamides: Stereoselective Dearomatization of Indoles
No full textThe effective and unprecedented chiral BINOL
phosphoric acid catalyzed (1–10 mol%) dearomatization of
indoles through electrophilic activation of allenamides (ee up
to 94%), is documented. Besides the synthesis of 3,3-disub-
stituted indolenine cores, a dearomatization/hydrogen transfer
cascade sequence is also presented as a new synthetic shortcut toward highly enantiomerically enriched indoline
From QCA (Quantum Cellular Automata) to Organocatalytic Reactions with Stabilized Carbenium Ions
No full textWhat do quantum cellular automata (QCA), "on water" reactions, and SN1-type organocatalytic transformations have in common? The link between these distant arguments is the practical access to useful intermediates and key products through the use of stabilized carbenium ions. Over 10 years, starting with a carbenium ion bearing a ferrocenyl group, to the 1,3-benzodithiolylium carbenium ion, our group has exploited the use of these intermediates in useful and practical synthetic transformations. In particular, we have applied the use of carbenium ions to stereoselective organocatalytic alkylation reactions, showing a possible solution for the "holy grail of organocatalysis". Examples of the use of these quite stabilized intermediates are now also considered in organometallic chemistry. On the other hand, the stable carbenium ions are also applied to tailored molecules adapted to quantum cellular automata, a new possible paradigm for computation. Carbenium ions are not a problem, they can be a/the solution
Properties and Reactivity of Conformationally Constrained Bicyclic Diarylprolinol Silyl Ethers as Organocatalysts
No full textBicyclic silyl ether derivatives 1, which are derived from simple
chemical manipulations of trans-4-L-hydroxyproline,
were recently proposed, by us, as conformationally constrained
analogues of Jørgensen–Hayashi’s catalysts 2. Despite
the structural similarities, 1 displays remarkable performance in iminium chemistry with respect to 2, but much
lower reactivity in enamine chemistry. The peculiar structural
features of 1 confer a pattern of reactivity between
Jørgensen–Hayashi’s catalysts and MacMillan’s imidazolidinones
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