The derivatization of organocatalysts with functional appendages suitable to anchor onto solid supports is usually achieved by stepwise syntheses. As an alternative to such a strategy, this work describes a one‐pot approach to silylated prolyl‐peptide catalysts by a multicomponent reaction that enables the simultaneous incorporation of the catalytic and the heterogenizable (triethoxysilane) moieties. A microreactor with high catalytic efficacy and reproducibly in the conjugate addition of aldehydes to nitroolefins was obtained by grafting onto HPLC‐grade silica (10 μm) and packing into a column with a selected catalyst. A 3 D continuous‐flow system that includes the on‐line monitoring of the reaction outcome was set up. For that, the microreactor was coupled to a chromatographic column for the separation of the remaining substrates from the Michael adduct in the second dimension, followed by a chiral polysaccharide column for the analysis of conversion and stereoselectivity. This approach represents a new instrumental setup that combines the advantages of multidimensional chromatography and flow catalysis.
The racemic pterocarpanquinone LQB-118 is active, in mice and hamsters, against tegumentary and visceral leishmaniasis. This compound also presents antiinflammatory and antineoplastic activity in mice. The low level of toxicity observed in these studies makes LQB-118 a promising drug candidate. In order to conduct further biological testing to investigate enantioselectivity in the above-mentioned activities, a multimilligram amount of each enantiomer of LQB-118 was produced. Furthermore, vibrational circular dichroism (VCD) and Density Functional Theory (DFT) calculations were used to determine unambiguously their absolute configurations. The comparison of experimental and calculated VCD data led to the assignment of (-)-LQB-118 as 7aR,12aR and, consequently, (+)-LQB-118 as 7aS12aS.
One of the most widely used methods to assess the stereochemistry of chiral 2(5H)-furanones is an empirical electronic circular dichroism (ECD) helicity rule. In the present work, an extensive experimental and theoretical investigation of the scope of the above-mentioned empirical rule for acetogenins with a hydroxyl group substituted at C-4 revealed a possible exception to this rule. The underlying causes for this observation are discussed with respect to side chain substitutions, conformational requirements, chromophore handedness as well as a qualitative orbital analysis. Further investigation using vibrational circular dichroism (VCD) spectroscopy led to the identification of spectral markers that seem to be more localized and less affected by side chain substitutions. As the presence of a [capital Upsilon]-lactone ring and a hydroxyl group at C-4 is a very common structural feature of Annonaceous acetogenins, we recommend the combined use of ECD and VCD spectroscopy, along with quantum chemical computations, for the stereochemical analysis of structurally related molecules.
The multicomponent synthesis of prolyl pseudo-peptide catalysts using the Ugi reaction with furfurylamines or isocyanides is described. The incorporation of such a polymerizable furan handle enabled the subsequent polymerization of the peptide catalyst with furfuryl alcohol, thus rendering polyfurfuryl alcohol-supported catalysts for applications in heterogeneous enamine catalysis. The utilization of the polymer-supported catalysts in both batch and continuous-flow organocatalytic procedures proved moderate catalytic efficacy and enantioselectivity, but excellent diastereoselectivity in the asymmetric Michael addition of n-butanal to β-nitrostyrene that was used as a model reaction. This work supports the potential of multicomponent reactions towards the assembly of catalysts and their simultaneous functionalization for immobilization.
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