The formation of MeP(O)(OPh) 2 by reductive elimination from L 2 PdMe(P(O)(OPh) 2 ) species has been investigated. The electronic and steric effects of the supporting ligands were investigated by studying reductive elimination reactions from a series of discrete complexes containing nitrogen-and phosphorus-based ligands. The P(O)-C(sp 3 ) bond-forming reaction is slow when the intermediate species contains bidentate nitrogen ligands or small basic monodentate phosphines. Analogous complexes bearing large bite angle diphosphines such as dppf and Xantphos undergo reductive elimination at ambient temperature. The rate of MeP(O)(OPh) 2 formation by reductive elimination from (dppf)PdMe(P(O)(OPh) 2 ) is not affected by the identity or concentration of added ligand (excess dppf or PPh 3 ), suggesting that the reductive elimination occurs from a four-or three-coordinate intermediate. When the rate of reductive elimination is slow, protonolysis reactions between L 2 PdMe(P(O)(OPh) 2 ) intermediates and HP(O)(OPh) 2 leads to the formation of bis-phosphonate complexes. The protonolysis reaction can be circumvented by the use of large bite angle phosphines such as dppf and Xantphos, which lead to rapid rates of P(O)-C(sp 3 ) bond formation. These results demonstrate that the formation of P(O)-C(sp 3 ) bonds by reductive elimination from L 2 PdRP-(O)(OR) 2 complexes is quite sensitive to the steric bulk of the supporting ligand and the presence of excess hydrogen phosphonate.
Continuing investigation of fractions from a supercritical fluid extract of Chinese licorice (Glycyrrhiza uralensis) roots has led to the isolation of 12 phenolic compounds, of which seven were described previously from this extract. In addition to these seven metabolites, four known components, 1-methoxyerythrabyssin II (4), 6,8-diprenylgenistein, gancaonin G (5), and isoglycyrol (6), and one new isoflavan, licorisoflavan C (7), were characterized from this material for the first time. Treatment of licoricidin (1) with palladium chloride afforded larger amounts of 7 and also yielded two new isoflavans, licorisoflavan D (8), which was subsequently detected in the licorice extract, and licorisoflavan E (9). Compounds 1-9 were evaluated for their antibacterial activities against the cariogenic Streptococcus mutans and the periodontopathogenic Porphyromonas gingivalis. Licoricidin (1), licorisoflavan A (2), and 7-9 showed antibacterial activity against P. gingivalis (MICs of 1.56-12.5 μg/mL). The most potent activity against S. mutans was obtained with 7 (MIC of 6.25 μg/mL), followed by 1 and 9 (MIC of 12.5 μg/mL). This study provides further evidence for the therapeutic potential of licorice extracts for the treatment and prevention of oral infections.
Stenocarpella maydis causes a fungal dry-rot of maize ears and is associated with diplodiosis, a neuromycotoxicosis in cattle grazing harvested maize fields in southern Africa and Argentina. There have been no reports of Stenocarpella metabolites in maize crop residues. Chemical investigations of S. maydis-infected grain from ears exhibiting different levels of ear rot severity following a 2010 field outbreak of Stenocarpella ear rot in Illinois led to the detection of diplodiatoxin and chaetoglobosins M and O as major components in the ethyl acetate extracts by LC-MS. Following post-harvest moist incubation of the S. maydis-infected grain, the amounts of each compound increased (approx. tenfold) and chaetoglobosin K was detected as a dominant toxin. In separate (1)H NMR-based analyses, the neurotoxin diplonine was detected as a minor component in methanol extracts of S. maydis-infected grain as well as cultures of S. maydis isolates from Midwest corn. Proline betaine (=stachydrine) and glycine betaine were also detected in these extracts as major components. This constitutes the first report of chaetoglobosin M, chaetoglobosin O, proline betaine, or glycine betaine from S. maydis, and the first record of diplodiatoxin, diplonine, proline betaine, glycine betaine, or chaetoglobosins M, O, or K being associated with a natural field outreak of S. maydis ear rot.
The (Z)‐selective Ru‐catalyzed anti‐Markovnikov addition of aromatic carboxylic acids to alkynes is followed by epoxidation with dimethyldioxirane, prepared in situ.
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