26 research outputs found
Chemical and isotopic compositions in acid residues from three meteorites
Chemical compositions of acid residues obtained from three types of meteorites, 1) Canyon Diablo (IA), 2) Allende (CV3) and 3) Nuevo Mercurio (H5) were determined. Refractory elements were generally enriched in these acid residues. The results match the features that acid residues contain primary condensates from the cooling solar gas as well as extra-solar grains. In the cases of Allende and Canyon Diablo, the contents of a series of refractory siderophile elements such as W, Re, Os, Ir, Mo, Ru and Pt were determined by INAA and AAS. Particularly, in acid residue of Allende, enrichment factors of these elements relative to CI are generally higher (7-20 times/CI), whereas in the cases of W and Mo, they are slightly depleted compared with those of Canyon Diablo. Since both elements would be the first metals to be oxidized under high oxygen fugacity, acid residue of Allende should contain fractions that were produced under oxidizing conditions. In these samples, the isotopic compositions of Ru have been measured by thermal ionization mass spectrometry. So far, in the measurements of acid residue of Allende, all Ru isotope ratios were found to be indistinguishable from terrestrial values within the experimental errors, though errors were large because of small Ru ion beam intensities
Structure-Activity Relationships of the Antitumor C5-Curcuminoid GO-Y030
1,5-Bis(4-hydroxy-3-methoxyphenyl)-1,4-pentadiene-3-one (2) was isolated from Curcuma domestica as a curcumin (1)-related compound, which we named C5-curcumin. Intrigued by the potent antitumor activity of C5-curcumin (2)-related 1,5-bisaryl-1,4-pentadiene-3-ones [bis(arylmethylidene)acetones, termed C5-curcuminoids], we previously conducted a structureâactivity relationship study of C5-curcuminoids and showed that highly active GO-Y030 [1,5-bis(3,5-bis(methoxymethoxy)phenyl)-1,4-pentadiene-3-one (4)] is the most promising antitumor compound. In this study, a panel of C5-curcuminoids based on GO-Y030, consisting of 30 new and 10 known compounds, was synthesized to elucidate in detail which moiety of GO-Y030 is significant for antitumor activity. The results confirmed that both the cross-conjugated dienone moiety and the 3,5-bis(methoxymethoxy) substituent are important for the antitumor activity
Total Syntheses of Marrubiin and Related Labdane Diterpene Lactones
Total syntheses of the labdane diterpene lactones marrubiin, marrulibacetal, desertine, marrulibacetal A, marrubasch F, cyllenine C, marrulanic acid, and marrulactone are described. The trans-decalin moiety of these molecules was constructed in a stereoselective manner by a Pauson-Khand reaction, and the resultant cyclopentenone was oxidatively cleaved for formation of the lactone ring. Elongation of the side chain at C9 was achieved by an epoxide-opening reaction with a variety of nucleophiles, and the functional group manipulations completed the syntheses of these natural products. Stereochemistries of desertine could be established by the transformations
Stereoselective Synthesis of the CDE Ring System of Antitumor Saponin Scillascilloside Eâ1
A stereoselective synthesis of the
CDE ring portion of the antitumor
saponin scillascilloside E-1 has been achieved, utilizing an IrelandâClaisen
rearrangement to construct the contiguous tetrasubstituted stereocenters
at C13 and C17 simultaneously and intramolecular nitrile oxide cycloaddition
to form the five-membered ring as key steps
Synthesis of Chiral Building Blocks for Oxygenated Terpenoids through a Simultaneous and Stereocontrolled Construction of Contiguous Quaternary Stereocenters by an IrelandâClaisen Rearrangement
Methods for highly
stereocontrolled syntheses of chiral building
blocks with a triad of contiguous stereocenters, including two quaternary
ones, have been developed. IrelandâClaisen rearrangement of
the (<i>Z</i>)-silyl ketene acetal generated stereoselectively
from the (<i>R</i>)-3-methylcyclohex-2-enyl ester derived
from an acyclic carboxylic acid proceeded through a chairlike transition
state to give the rearranged product with an <i>S</i> configuration
at the position α to the carboxyl group. Introduction of a cyclic
conformational constraint in the acid component completely switched
the transition state of the rearrangement to a boatlike one, leading
to the predominant formation of a product with an <i>R</i> configuration, from which pseudodiastereomeric α-hydroxy esters
were obtained in a four-step sequence. The enyne obtained through
a base-mediated double eliminative ring-opening reaction was successfully
converted into advanced intermediates for the synthesis of 9-oxygenated
labdane diterpenoids through a Heck reaction and a regioselective
transformation of the resultant diene
Synthesis of Chiral Building Blocks for Oxygenated Terpenoids through a Simultaneous and Stereocontrolled Construction of Contiguous Quaternary Stereocenters by an IrelandâClaisen Rearrangement
Methods for highly
stereocontrolled syntheses of chiral building
blocks with a triad of contiguous stereocenters, including two quaternary
ones, have been developed. IrelandâClaisen rearrangement of
the (<i>Z</i>)-silyl ketene acetal generated stereoselectively
from the (<i>R</i>)-3-methylcyclohex-2-enyl ester derived
from an acyclic carboxylic acid proceeded through a chairlike transition
state to give the rearranged product with an <i>S</i> configuration
at the position α to the carboxyl group. Introduction of a cyclic
conformational constraint in the acid component completely switched
the transition state of the rearrangement to a boatlike one, leading
to the predominant formation of a product with an <i>R</i> configuration, from which pseudodiastereomeric α-hydroxy esters
were obtained in a four-step sequence. The enyne obtained through
a base-mediated double eliminative ring-opening reaction was successfully
converted into advanced intermediates for the synthesis of 9-oxygenated
labdane diterpenoids through a Heck reaction and a regioselective
transformation of the resultant diene