Seventeen new alkaloids (1-17) and 14 known analogues have been isolated from an aqueous extract of the root of Isatis indigotica. The structures and absolute configurations of these compounds were determined by extensive spectroscopic data analysis, including 2D NMR, single-crystal X-ray crystallography using anomalous scattering of Cu Kα radiation, and electronic circular dichroism spectra calculations based on the quantum-mechanical time-dependent density functional theory. Compounds 1, 2, and 3 are the first examples of natural products with unique linkages between a molecule of 2-(4-methoxy-1H-indol-3-yl)acetonitrile and 2-(1H-indol-3-yl)acetonitrile, 2-(4-methoxy-1H-indol-3-yl)acetonitrile, and 4-hydroxyphenylethane, respectively. Compounds (-)-4 and (+)-4 represent the first natural products with the pyrrolo[2,3-b]indolo[5,5a,6-b,a]quinazoline skeleton. Some structural assignments for the new alkaloids suggest that the assignments made for certain previously reported alkaloids require revision. Compounds 1-3 and arvelexin (18) show antiviral activity against the influenza virus A/Hanfang/359/95 (H3N2), with IC(50) values of 3.70-12.35 μM, and 17 inhibits Coxsackie virus B3 replication with an IC(50) of 6.87 μM.
Five new mexicanolide-type tetranortriterpenoids, tigloylseneganolide A (1), 2'R-methylbutanoylproceranolide (2), 2'S-methylbutanoylproceranolide (3), 2'R-cipadesin A (4), and 2'R-cipadesin (6), as well as the known 2'S-epimers of 4 and 6 (5 and 7), together with six other known limonoids, were isolated from the seeds of Cipadessa baccifera. The structures of these compounds were elucidated on the basis of spectroscopic analyses and chemical methods. 1H NMR-based conformational analysis was applied to establish the absolute configuration of the sterically hindered 2-methylbutanoyl in three epimeric pairs (2-7). A general rule for the determination of the absolute configurations of 2R- and 2S-methylbutanoyl groups at C-3 of a limonoid in a mixture is proposed.
Stimuli-responsive unimolecular chirality switching is a highly intriguing topic because the molecular structure as well as its function can be adjusted simultaneously by a switching process. Herein, a novel acid/base-tunable unimolecular chirality switching system based on a pillar[5]azacrown pseudo[1]catenane is reported. The bicyclic pillar[5]azacrown pseudo[1]catenane PN4 is synthesized through fusing an azacrown ring onto one repeating unit of a pillar[5]arene. Protonation and deprotonation can reversibly regulate the conformational transformations of PN4 between selfinclusion and self-exclusion structures, which results in the chiroptical inversions of the pseudo[1]catenane. NMR spectra, circular dichroism spectra, and single-crystal structures demonstrate these processes. This pseudo[1]catenane is a novel pillararene-based unimolecular chirality switching system driven by acid/base responsiveness and reveals a new perspective on the supramolecular chirality chemistry of macrocycles.
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