J. Martín Torres-Valencia

Absolute configuration of 7,8-seco-7,8-oxacassane diterpenoids from Acacia schaffneri.

Chemical investigations of Acacia schaffneri led to the isolation of the new diterpenoid (5S,7R,8R,9R,10S)-(-)-7,8-seco-7,8-oxacassa-13,15-diene-7,17-diol (1), together with the known (5S,7R,8R,9R,10S)-(-)-7,8-seco-7,8-oxacassa-13,15-dien-7-ol-17-al (2) and (5S,7R,8R,9R,10S)-(-)-7,8-seco-7,8-oxacassa-13,15-dien-7-ol (3). Compounds 2 and 3 were analyzed by single-crystal X-ray diffraction, while the structure of 1 was determined by 1D and 2D NMR experiments and by chemical correlation with 2. Oxidation of 3 afforded conformationally restricted (5S,8R,9R,10S)-(-)-8-hydroxy-7,8-seco-cassa-13,15-dien-7-oic acid ε-lactone (4), which was studied by vibrational circular dichroism spectroscopy. Comparison of the experimental VCD spectrum of 4 with the DFT//B3PW91/DGDZVP2 calculated spectrum assigned for the first time the absolute configuration of these seco-oxacassane diterpenes.

Antimicrobial activity of three Mexican Gnaphalium species.

The antibacterial activity of the hexane, ethyl acetate and methanol extracts of the flowers, leaves and stems of Gnaphalium oxyphyllum var. oxyphyllum, G. liebmannii var. monticola and G. viscosum was investigated. The hexane extracts showed in all cases the higher inhibitions, G. oxyphyllum flower extract exhibiting the wider spectrum of activity.

Absolute configuration of labdanes and ent-clerodanes from Chromolaena pulchella by vibrational circular dichroism

The aerial parts of Chromolaena pulchella biosynthesize two groups of diterpenes belonging to opposite enantiomeric series, specifically, the furanoid ent-clerodanes (5R,8R,9S,10R)-(-)-hardwikiic acid (1), methyl (5R,8R,9S,10R)-(-)-hardwikiate (2), (5S,8R,9S,10R)-(-)-hautriwaic acid lactone (3), methyl (5R,8R,9S,10R)-(-)-nidoresedate (4) and methyl (8R,9R)-(-)-strictate (5), as well as the labdanes (5S,8R,9R,10S)-(+)-(13E)-labd-13-ene-8,15-diol (6) and (5S,8R,9R,10S)-(+)-isoabienol (7). The absolute configuration of the two groups of diterpenes was unambiguously assigned by comparison of the vibrational circular dichroism spectra of 3 for ent-clerodanes, and of 7 for labdanes with their theoretical spectra obtained by density functional theory calculations. The results support a biogenetic proposal to diterpenes found in the studied botanical species.

ENANTIOSELECTIVE PREPARATION OF (2R, 3R)-(+)- AND (2S, 3S)-(-)-2,3-EPOXY-2-METHYLBUTANOIC ACIDS AND SOME DERIVATIVES

Abstract (2 R ,3 R )-(+)- and (2 S ,3 S )-(−)-2,3-epoxy-2-methylbutanoic acids (epoxyangelic acids) were prepared from ( Z )-2-methyl-2-butenoic acid using the Sharpless asymmetric epoxidation method in combination with the use of (−)- and (+)-menthol as chiral auxiliaries. Both substances, obtained in high enantiomeric excess, were characterized by spectroscopic and optical activity data. Their absolute configuration was determined by correlation with ( R )-(+)-2-methyl-1,2-butanediol.

Preparation of (2R,3S)-(−)- and (2S,3R)-(+)-2,3-epoxy-2-methylbutanoic acids and some of their esters

Abstract Enantiomerically pure (2 R ,3 S )-(−)- and (2 S ,3 R )-(+)-2,3-epoxy-2-methylbutanoic acids 7 and 8 were prepared from 2-methyl-2-butenoic acid 1 ( tiglic acid ). They were characterized by spectroscopic and optical activity data and their absolute configuration was determined by chemical correlation with ( R )-(+)- and ( S )-(−)-2-methyl-1,2-butanediols. The corresponding methyl ( 16 and 17 ), menthyl ( 3 and 4 ), and 9α-angeloyloxy-1-oxolongipin-2-en-7β-yl ( 14 and 15 ) esters were also prepared.

Stereochemical assignment of 2,3‐epoxy‐2‐methylbutanoate esters in natural products

Inspection of the 182 known natural products containing 2,3-epoxy-2-methylbutanoate ester residues revealed a very high tendency in nature for the preferential epoxidation of (Z)-2-methyl-2-butenoate esters (angelates) over (E)-2-methyl-2-butenoate esters (tiglates), since 177 compounds contain epoxyangelate and only five have epoxytiglate ester residues. The absolute configuration of the epoxide group has been determined for 22 compounds, again showing a very high natural stereoselectivity, since 20 compounds have the (2R,3R) and only two the (2S,3S) absolute configuration. In contrast, unconfirmed stereochemical predictions for an additional 21 compounds suggest that 12 have the (2S,3S) and 9 the (2R,3R) configuration. Botanical sources, 1H-NMR data of the epoxide-containing ester residue and molecular structures were compiled, and methodologies are presented to determine or to further support the absolute configuration of the 2,3-epoxy-2-methylbutanoate ester residues, based on a chemical correlation with (R)- or (S)-2-methyl-1,2-butanediol or on partial synthesis. Copyright

The absolute configuration of cuauhtemone and related compounds

Abstract The absolute configuration of cuauhtemone, a eudesmane-type sesquiterpene isolated from Pluchea species (Asteraceae), has been revised from 1 to 2 by chemical correlation with (R)-(+)-2-methyl-1,2-butanediol 3 through the naturally occurring 2,3-epoxy-2-methylbutanoate derivative 4. The relative stereochemistry of 4 was confirmed by X-ray diffraction analysis. The obtained data are also useful for reconsideration of the absolute configurations of a relevant group of natural products, which were elucidated according to the stereochemistry of cuauhtemone.

Silver nanoparticles from AgNO3–affinin complex synthesized by an ecofriendly route: chitosan-based electrospun composite production

Abstract A novel nanofibrous chitosan-based composite containing affinin and silver nanoparticles is obtained by electrospinning. Silver nanoparticles are synthesized by sunlight photoreduction of the metal complex [Ag2–(affinin)](NO3)2 in polymeric solution, via a green one-pot methodology, wherein chitosan and affinin act as reducing, dispersing and stabilizing agent.