Kehrt, marsch! Ein von einer Aminosäure abgeleiteter Alkohol und ein Arylamid wurden zu einem Kehrenbaustein für Peptidkonformationen kombiniert. Cyclopropananaloga von Tetra‐ und Hexapeptiden bilden parallele Faltblattstrukturen, die durch CH⋅⋅⋅O‐Wasserstoffbrücken stabilisiert werden (siehe Schema). NMR‐spektroskopische Studien bestätigen das Vorliegen von Wasserstoffbrücken.
Enantioselective total synthesis of (+)‐yangambin was achieved. The key transformation is one‐pot conjugate addition/aldol reaction that involves an enantioenriched benzyl tert‐butyl sulfoxide, an enone, and gaseous formaldehyde to construct the bis(phenylpropanoid) backbone with excellent stereoselectivity and in good yield. Reduction of the ketone with diisobutylaluminium hydride and acid‐catalysed cyclisation in EtOH furnished (+)‐yangambin in good yield. The resulting synthesis is short, efficient and highly selective. Formation of 2,5‐diaryltetrahydrofuran lignans was observed as a side reaction in the final step of yangambin synthesis. Acid‐catalysed cyclisation of the aldol intermediate gave a completely different outcome. Dehydration to give an enone was followed by two electrophilic aromatic substitution reactions to furnish a derivative of the lignan lirionol.
The present work is related to predicting the pKa values of organosulfur
compounds through Density Functional Theory (DFT). In this study 22
organo-sulfur compounds have been considered to calculate theoretical pKa
values. Main emphasis has been given on the substitution of different groups
on the Sulfur atom. The computations were performed in the presence of
Dimethyl sulfoxide (DMSO) as solvent. Experimentally the order of increase
of acidity is; Sulfides < < Sulfoxides < Sulfones. Our computed pKa values
also follow the same order. The theoretical pKa values are computed using
the DFT method B3LYP, with the basis sets 6-31G(d), 6-31+G(d,p) and IEFPCM
bulk solvation model. The majority of computed pKa values are in excellent
agreement with the experimental ones through the diffuse function basis set.
Hence this computational approach, B3LYP/6-31+G(d,p)/IEFPCM, could be
utilized to predict the pKa values of these types of organosulfur compounds.
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