1,721,020 research outputs found
Synthetic Applications of 2-(Azidomethyl)allyltrimethylsilane
No full textStarting from commercially available 2-(chloromethyl)-allyltrimethylsilane, the corresponding 2-(azidomethyl)allyl silane was prepared through reaction with NaN3. The product was stable upon isolation and storage and could be used for thermal cycloaddition of the azido group with alkenes to give allylsilane-containing triazolines or aziridines. This reaction was not accelerated by microwave (MW) dielectric heating, however, the azide fragment undergoes MW-assisted Cu(I)-catalyzed cycloaddition with a range of alkynes (including ynamides). Lewis acid mediated Hosomi-Sakurai reaction of the allylsilane with aldehydes was also possible. A one-pot transformation into different triazolo-containing homoallyl alcohols was carried out through a sequence of Cu(I)-catalyzed azide cycloaddition under MW dielectric heating and BCl3-mediated reaction with aromatic aldehydes. © Georg Thieme Verlag Stuttgart · New York
Microwave Dielectric Heating for Solvent-free Organic Transformations
No full textThe use of solvents in organic synthesis is today one of the main causes of pollution and risk for humans and the environment. Although many green solvents have been recently developed, the best solvent is still no solvent. The use of microwaves (MWs) for heating reaction mixtures provides an efficient tool to perform organic reactions in the absence of solvents. The combination of solvent-free protocols and microwave-assisted organic synthesis (MAOS) is therefore one the most challenging tools to abide by the 12 green chemistry principles. This overview reports on the recent results, benefits and challenges of the synergy between MW heating and neat reactions. Important milestones of organic chemistry such as oxidation, alkylation, cross-coupling, Knoevenagel, Claisen and Michael addition reactions, together with innovative sustainable organic chemistry reactions such as borrowing hydrogen protocols, are discussed with critical attention to their benefits compared to conventional heating and the use of ‘hidden’ solvents employed during work-up and purification steps
Synthesis of Enantiopure 7-Substituted Azepane-2-Carboxylic Acids as Templates for Conformationally Constrained Peptidomimetics
No full textThe introduction of a cyclic amino acid in a peptide is one of the best methods to rigidify a strand. A general approach towards a new class of seven-membered ring amino acids is described starting from (S)-tribenzyl glutamic acid γ-aldehyde, which reacts with β-keto phosphonates to generate the Horner-Wadsworth-Emmons product. In the presence of H2 and a Pd catalyst, a four-step process occurs involving double-bond hydrogenation, hydrogenolysis of three benzyl protecting groups, imine formation, and reductive amination to produce the 7-substituted azepane carboxylic acid in good overall yield and with good to excellent diastereomeric ratios. An amino function can be introduced in the 7-position as an addnl. orthogonal chem. handle for readily generating diversity on the cyclic amino acid scaffold by using a β-keto phosphonate derived from amino acids. A cyclic RGD (Arg-Gly-Asp) pentapeptide analog contg. this new class of noncoded amino acids was also prepd. by microwave-assisted cyclization, showing a promising activity as αvβ3 integrin inhibitor
Domino reactions triggered by hydroformylation
No full textHF reaction represents a selective method for the synthesis of aldehydes starting from alkenes. Because of versatile aldehydes reactivity, it is possible to perform different domino protocols based on contemporary HF, including Michael’s reaction, reductive amination, cyclopropanation, lactonization, and many others. This overview reports on the last 5 years’ results obtained on this field
Microwave Assisted Hydrogenation of Pyridinecarboxylates: a Convenient Access to Valuable Scaffolds for Diversity Oriented Synthesis
No full textHydrogenation of pyridine under Microwave (MW) dielectric heating is a valuable method for a rapid approach to piperidine synthesis. Starting from easily available pyridinecarboxylate derivatives, aminopipecolic scaffolds were prepared and further decorated with a procedure potentially useful for parallel synthesis. MW hydrogenation on PtO2 proceeded with high stereoselectivity and low catalyst loading under mild conditions (80°C and 850 MPa of H2 pressure). MWs were also effective to perform a Pinner reaction on 2-cyano-nitropyridines to obtain the corresponding methyl nitropyridine carboxylates. A general procedure to decorate the 3-, 4-, and 5-amino-pipecolic derivatives obtained from the pyridine hydrogenation has been also developed
A highly stereo-controlled protocol to prepare pipecolic acids based on Heck and cyclohydrocarbonylation reactions
Full text linkA novel synthetic protocol based on an indium-mediated glyoxylate allylation, Heck coupling and Rh-catalysed cyclohydrocarbonylation (CHC) was established to access enantiomerically pure polysubstituted pipecolic acids. The key steps are the Heck reaction, which is performed exclusively using a phenone-oxime derived palladacycle and the domino hydroformylation-cyclisation of a styryl derivative obtained from the Heck coupling. The reaction scheme, proceeding with good stereoselectivity, is also suitable for the preparation of substituted piperidine derivative
Synthesis of Enantiopure 7-Substituted Azepane-2-carboxylic Acids as Templates for Conformationally Constrained Peptidomimetics
No full textThe introduction of a cyclic amino acid in a peptide is one of the best methods to rigidify a strand. A general approach towards a new class of seven-membered ring amino acids is described starting from (S)-tribenzyl glutamic acid gamma-aldehyde, which reacts with beta-keto phosphonates to generate the Horner-Wadsworth-Emmons product. In the presence of H-2 and a Pd catalyst, a four-step process occurs involving double-bond hydrogenation, hydrogenolysis of three benzyl protecting groups, imine formation, and reductive amination to produce the 7-substituted azepane carboxylic acid in good overall yield and with good to excellent diastereomeric ratios. An amino function can be introduced in the 7-position as an additional orthogonal chemical handle for readily generating diversity on the cyclic amino acid scaffold by using a beta-keto phosphonate derived from amino acids. A cyclic RGD (Arg-Gly-Asp) pentapeptide analogue containing this new class of noncoded amino acids was also prepared by microwave-assisted cyclization, showing a promising activity as alpha(v)beta(3) integrin inhibitor
Pd/C catalysis under microwave dielectric heating
No full textMicrowave-assisted organic synthesis (MAOS) provides a novel and efficient means of achieving heat organic reactions. Nevertheless, the potential arcing phenomena via microwave (MW) interaction with solid metal catalysts has limited its use by organic chemists. As arcing phenomena are now better understood, new applications of Pd/C-catalyzed reactions under MW dielectric heating are now possible. In this review, the state of the art, benefits, and challenges of coupling MW heating with heterogeneous Pd/C catalysis are discussed to inform organic chemists about their use with one of the most popular heterogeneous catalysts
Synthesis of 2-Substitued Indoles via Pd-Catalysed Cyclization in an Aqueous Micellar Medium
Full text linkThe synthesis of 2-substituted indoles starting from the corresponding unprotected 2-alkynylanilines was made possible in 3% TPGS-750-M water using Pd(OAc)2 alone as the catalyst. The reaction was sensitive to the heating mode respect to the nature of the starting material as, in many cases, convectional heating was better than microwave dielectric heating. The MW (microwave) delivery mode had also an influence in the formation of by-products and, consequently, product yields. A tandem Sonogashira-cyclisation reaction was also accomplished using Pd(OAc)2/Xphos in the nanomicellar water environment
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