1,720,971 research outputs found
Organocatalytic Upgrading of Biomass Derived Building Blocks
No full textThe depletion of finite primary fossil fuels we are facing makes necessary a deep metamorphosis in fundamental parts of the chemical industry. A progressive transition from petro-based starting materials toward renewable biomass-derived sources will have to take place in the synthesis of added-value chemicals, important for our everyday life, such as pharmaceuticals, polymers, agrochemicals etc. Moreover, greener processes, carried out under friendlier reaction conditions, must be designed to address current concerns about the climate change and the resulting pressing need to reduce the environmental footprint of chemical processes. To this end, organocatalysis could offer a valuable opportunity for upgrading biomass-derived platform molecules in line with the principles of Green Chemistry. This review presents some of recent and remarkable advancements in this emerging area. Organocatalysis has proven to be an efficient tool to transform low value bio-based renewable platform building blocks into new high value bio-based chemicals, with potential applications as synthetic intermediates, innovative materials and pharmaceutically active compounds
Valorization of Waste: Sustainable Organocatalysts from Renewable Resources
No full textOne of the greatest challenges facing our society is to reconcile our need to develop efficient and sophisticated chemical processes with the limited resources of our planet and its restricted ability to adsorb pollution. Organocatalysis has allowed many issues to be addressed in the development of sophisticated, but less polluting, processes. However, minimizing waste also means an efficient utilization of raw and renewable materials. Waste biomass represents an alternative to conventional petroleum-based chemical manufacturing and is a highly attractive renewable resource for the production of chemicals and high-value-added organocatalysts. Recent achievements in the use of renewable biomass feedstocks for the synthesis of organocatalysts are presented. Their application in synthetic methodologies, including multicomponent reactions, which are performed under solvent-free conditions or in eco-friendly reaction media, as well as recycling and reusing the organocatalysts, is illustrated. A few pioneering examples that demonstrate the potential of these promoters in asymmetric synthesis have also been documented. In particular, this review covers examples on the use of hetero- and homogeneous organocatalysts derived from 1) waste biopolymers, such as chitosan, alginic acid, and cellulose; ii) renewable platform molecules, such as levoglucosenone, isosorbide, mannose, d-glucosamine, and lecithin; 3) terpenes and rosin, such as pinane, isosteviol, and abietic acid; and iv) natural proteins (gelatin, bovine tendons, silk fibroin proteins)
Unravelling the mechanism of the organocatalyzed aminolytic kinetic resolution of α-nitroepoxides: A theoretical study
No full textThe aminolytic kinetic resolution of 2-methyl-2-nitro-3-phenyl oxirane with aniline was investigated by quantum chemical computations. It has been assessed that the reaction proceeds via a concerted ring opening/elimination of nitrous acid mechanism. The results are in agreement with the experimental findings when using the Takemoto's catalyst, which is estimated to perform in a highly efficient manner in the kinetic resolution of this class of epoxides in the absence of any background reaction. H-Bonding, π-π and CH/π interactions appear to make a significant contribution to the formation of a ternary complex and a transition state, which preferentially facilitates the reaction of the (2R,3S)-nitroepoxide to the α-amino ketone, leaving (2S,3R)-nitroepoxide unreacted
Diaryl Prolinols in Stereoselective Catalysis and Synthesis: An Update
No full textDiscovered more than one decade ago, α,α-diaryl prolinols, most of them commercially available compounds, demonstrated to be distinctive organocatalysts in asymmetric synthesis. Their application successfully spanned across different carbon-carbon or carbon-heteroatom bond forming reactions and cascade processes, exploiting covalent and noncovalent activation of the reagents. In this minireview, the advances from 2013 up to the end of 2018 in α,α-diaryl prolinols promoted stereoselective catalysis and synthesis of biologically active compounds, are illustrated
Magnesium Monoperphthalate (MMPP): a Convenient Oxidant for the Direct Rubottom Oxidation of Malonates, β-Keto Esters, and Amides
No full textA mild and convenient protocol for the α-hydroxylation of α-substituted malonates, β-ketoesters, and β-ketoamides is disclosed. Cheap and stable magnesium monoperphthalate (MMPP) was effective for a first direct α-hydroxylation protocol of the Rubottom oxidation, providing tartronic esters, cyclic α-hydroxy β-ketoesters and amides in good to high yields, when working at room temperature and in ethanol as the solvent. The protocol has been successfully scaled up to gram quantity
Thiourea-amine catalysed asymmetric synthesis of functionalised epoxides
No full textBi-or multifunctional thiourea-amines are highly popular organocatalysts used for the C-C, C-N, and C-S bonds formation and in cascade reactions to prepare several heterocyclic compounds. More recently, thiourea-amines displayed useful catalytic activity to prepare functionalised epoxides via asymmetric epoxidation of electron-poor alkenes and kinetic resolution of racemic epoxides. In this personal account we present the results achieved by using this class of organocatalysts, which enabled the synthesis of a variety of functionalised epoxides in good to high yield, diastereo- and enantioselectivity. The optically active epoxides proved to be useful synthetic intermediates for further derivatization to heterocyclic or acyclic derivatives, bearing tertiary and quaternary stereocenters
Acid-Catalyzed Rearrangement of Epoxides
No full textThis chapter illustrates recent developments of acid-catalyzed Meinwald and semipinacol rearrangements of epoxides. The oxiranes can be elaborated under different reaction conditions, involving the use of Brønsted acids, Lewis acids derived from the main group, transition metal complexes, including heterogeneous Lewis acids. More challenging asymmetric rearrangement of epoxides and selected examples of this transformation occurring in the synthesis of bioactive and natural products will be highlighted
Diastereoselective Synthesis of Functionalized 5-Amino-3,4-Dihydro-2H-Pyrrole-2-Carboxylic Acid Esters: One-Pot Approach Using Commercially Available Compounds and Benign Solvents
No full textA novel three-step four-transformation approach to highly functionalized 5-amino-3,4-dihydro-2H-pyrrole-2-carboxylic acid esters, starting from commercially available phenylsulfonylacetonitrile, aldehydes, and N-(diphenylmethylene)glycine tert-butyl ester, was developed. The one-pot strategy delivered this class of amidines bearing, for the first time, three contiguous stereocenters, in good to high yield and diastereoselectivity. The entire sequence was carried out using diethyl carbonate and 2-methyl tetrahydrofuran as benign solvents, operating under metal-free conditions. The process could be conveniently scaled-up, and the synthetic utility of the products was demonstrated
Catalytic Asymmetric Approach to 1,3,4,5-Tetrahydro-1,4-benzodiazepin-2-ones in One-Pot
No full textHerein we illustrate a first asymmetric synthesis of medicinally attractive tetrahydro-1,4-benzodiazepin-2-ones performed under catalytic conditions and one-pot fashion. The process relies on a sequential Knoevenagel reaction/asymmetric epoxidation/domino ring-opening cyclization (DROC) using commercially available aldehydes, phenylsulfonylacetonitrile, cumyl hydroperoxide, 2-(aminomethyl)aniline and a readily available quinine-derived urea as the catalyst. The heterocycles have been isolated with good regioselectivity, satisfactory to good yield and up to 98 % ee. The protocol proved also to be suitable for the preparation of previously undescribed 1,5-dihydro-4,1-benzoxazepin-3(2H)-ones with up to 86 % ee.A first catalytic and asymmetric synthesis of medicinally relevant tetrahydro-1,4-benzodiazepin-2-ones and unprecedented 1,5-dihydro-4,1-benzoxazepin-3(2H)-ones, has been developed. The telescoped approach relies on the use of commercially available reagents, recyclable organocatalyst and a single solvent. The heterocycles were isolated in good overall yield (30-69 %) and satisfactory to high enantioselectivity (up to 98 % ee). imag
Bi-functional and mono-component organocatalysts for the ring-opening alternating co-polymerisation of anhydride and epoxide
No full textBi-functional organocatalysts constituted by a (thio-)urea moiety and an iminophosphorane moiety were synthesized and optimised for the ring-opening alternating co-polymerisation of phthalic anhydride with three different epoxides: cyclohexene oxide, propylene oxide and butylene oxide. The most effective catalyst featured a cyclohexyl urea moiety, an iminophosphorane moiety with three 2,4-dimethyl-3-methoxy phenyl substituents, and a short spacer length of two carbon atoms between them. All tested epoxides reached quantitative conversion within 24 hours with ester-selectivities up to >97%. NMR and DFT experiments reveal that the catalysts exist in solution as dimers that dissociate during the initiation of the polymerisation. During the polymerisation, the catalyst is coordinated to the growing chain and further modulates its reactivity through reversible protonation/deprotonation suppressing transesterification side reactions even at prolonged polymerisation times without the need for a co-catalyst. The rate-determining step of the polymerisation is the ring-opening of the epoxide by the carboxylate chain end, and accordingly, higher temperatures (up to 150 degrees C) and higher concentrations of epoxide and catalyst increase polymerisation rates
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