Search results for "two-dimensional"
showing 10 items of 350 documents
Anomalous scaling of the critical temperature of unmixing with chain length for two-dimensional polymer blends
2003
The thermodynamics, structure and the chain configurations of symmetrical polymer mixtures confined into ultrathin films are studied by Monte Carlo simulations of the bond fluctuation model. It is shown that the Flory-Huggins–type scaling of the critical temperature (Tc ~ N) with chain length N in the bulk is replaced by a weaker increase, Tc ~ N1/2, in an ultrathin film, and this is interpreted in terms of geometric arguments. The pair-correlation function g(r) of monomers from different chains exhibits a pronounced correlation hole, and the density of intermolecular contacts zc decreases with N as zc ~ N−1/2.
Dracoside, a New Steroidal Saponin fromHelleborus purpurascens
1994
Abstract A new steroidal saponin, 1-O-[6-O-acetyl-β-D-galactopyranosyl]-24-O-[β-D-apiofuranosyl-(1→3)-4-O-acetyl-α-L-rhamnopyranosyl-(1→4)-β-D-xylopyranosyl-(1→3)-α-L-arabinopyranosyl]-dracogenin, was isolated from the roots of Helleborus purpurascens and named dracoside. Structure elucidation was performed by chemical transformations and various spectroscopic methods, mainly 2D NMR techniques (COSY, HMQC, HMBC).
Diastereoselective formation of highly functionalised α-substituted amino acid derivatives via aldol addition
2005
Abstract Highly diastereoselective aldol additions of (2 R ,4 S )-3- tert -butyl 4-methyl 2- tert -butyloxazolidine-3,4-dicarboxylate ( 1 ) are reported. The utility of the highly substituted oxazolidines of type 1 for diastereoselective α-addition of the fully protected amino acid l -serine with achiral and chiral carbonyl compounds is demonstrated and the relative and absolute configuration of the aldol products are discussed on the basis of NOESY data and solid state structures of selected examples. The aldol products represent highly useful intermediates in the syntheses of sphingosine-related metabolites.
New Triterpene Saponins fromAcanthophyllum pachystegium
2004
Four new triterpenoid saponins, pachystegiosides A (1), B (2), C (3), and D (4), were isolated from the roots of Acanthophyllum pachystegium K. H. Their structures were elucidated by means of a combination of homo- and heteronuclear 2D-NMR techniques (COSY, TOCSY, NOESY, HSQC, and HMBC) and by FAB-MS. The new compounds were characterized as 3-O-{O-β-D-galactopyranosyl-(12)-O-[β-D-xylopyranosyl-(13)]-β-D-glucuronopyranosyl}quillaic acid 28-{O-β-D-xylopyranosyl-(13)-O-β-D-xylopyranosyl-(14)-O-α-L-rhamnopyranosyl-(12)-O-[3,4-di-O-acetyl-β-D-quinovopyranosyl-(14)]-β-D-fucopyranosyl}ester (1), 3-O-{O-β-D-galactopyranosyl-(12)-O-[β-D-xylopyranosyl-(13)]-β-D-glucuronopyranosyl}quillaic acid 28-{O-…
Acylated Triterpene Saponins from Atroxima libericaStapf
2011
The four new acylated triterpene saponins 1–4, isolated as two pairs of isomers and named libericosides A1/A2 and B1/B2, one pair of isomers 5/6, the (Z)-isomer libericoside C2 (5) being new, one new sucrose ester, atroximoside (7), and eight known compounds were isolated from the roots of Atroxima liberica by repeated MPLC and VLC on normal and reversed-phase silica gel. Their structures were elucidated on the basis of extensive 1D- and 2D-NMR studies (1H- and 13C-NMR, DEPT, COSY, TOCSY, NOESY, HSQC, and HMBC) and mass spectrometry as 3-O-β-D-glucopyranosylpresenegenin 28-{O-α-L-arabinopyranosyl-(13)-O-β-D-xylopyranosyl-(14)-O-α-L-rhamnopyranosyl-(12)-4-O-[(E)-3,4-dimethoxycinnamoyl]-β-D-f…
Triterpenoid saponins from Piptadeniastrum africanum (Hook. f.) Brenan
2013
One new triterpenoid saponin, named piptadeniaoside (1), along with two known saponins (2–3) have been isolated from the stem bark of Piptadeniastrum africanum. After previous isolation of flavone derivatives from this plant, new phytochemical investigations were performed for its saponin content. Their structures were established by direct interpretation of their spectral data, mainly HRESIMS, 1D NMR (1H, 13C NMR, DEPT) and 2D NMR (COSY, NOESY, HSQC and HMBC), and by comparison with the literature data.
Cycloartane Glycosides from Three Species of Astragalus (Fabaceae)
2011
Nine cycloartane-type glycosides were isolated from three species of the genus Astragalus (Fabaceae): From the aerial parts of A. cicer L., two new saponins, cicerosides A and B (1 and 2, resp.), i.e., a tetradesmosidic and tridesmosidic cycloartane-type glycosides besides one known compound, from the roots of A. sempervirensLam., one known saponin, and from the roots of A. ptilodesBoiss. var. cariensisBoiss., five known compounds. Their structures were established mainly by 600-MHz 2D-NMR techniques (1H,1H-COSY, TOCSY, NOESY, HSQC, and HMBC) and mass spectroscopy.
Foetidissimosides C–F, Novel Glycosides from the Roots ofCucurbita foetidissima
2004
Two novel echinocystic acid (=(3β,16α)-3,16-dihydroxyolean-12-en-28-oic acid) glycosides, foetidissimosides C (1), and D (2), along with new cucurbitane glycosides, i.e., foetidissimosides E/F (3/4) as an 1 : 1 mixture of the (24R)/(24S) epimers, were obtained from the roots of Cucurbita foetidissima. Their structures were elucidated by means of a combination of homo- and heteronuclear 2D-NMR techniques (COSY, TOCSY, NOESY, ROESY, HSQC, and HMBC), and by FAB-MS. The new compounds were characterized as (3β,16α)-28-{[O-β-D-glucopyranosyl-(13)-O-β-D-xylopyranosyl-(14)-O-6-deoxy-α-L-mannopyranosyl-(12)-α-L-arabinopyranosyl]oxy}-16-hydroxy-28-oxoolean -12-en-3-yl β-D-glucopyranosiduronic acid (1…
Bafouoside C, a new triterpenoid saponin from the roots of Cussonia bancoensis Aubrev. & Pellegr.
2014
Abstract A new triterpenoid saponin named bafouoside C 3-O-β- d -glucopyranosyl-(1 → 4)-[β- d -galactopyranosyl-(1 → 2)]-β- d -glucuronopyranosyloleanolic acid 28-O-β- d -glucopyranosyl ester; (1), together with five known compounds 3-O-β- d -galactopyranosyl-(1 → 2)-β- d -glucuronopyranosyloleanolic acid (2), 23-hydroxyursolic acid (3), 28-O-α- l -rhamnopyranosyl-(1 → 4)-O-β- d -glucopyranosyl-(1 → 6)-O-β- d -glucopyranosyl-23-hydroxyursolic acid (4), 3-O-β- d -glucopyranosyl-23-hydroxyursolic acid (5), and 3-O-α- l -arabinopyranosyl-23-hydroxyursolic acid (6), were isolated from the roots of Cussonia bancoensis Aubrev. & Pellegr. Their structures were established on the basis of 1D- and 2…
Spirostane and cholestane glycosides from the bulbs of Allium nigrum L
2011
Abstract A phytochemical investigation of the fresh bulbs of Allium nigrum L. led to the isolation of new spirostane-type glycosides as two inseparable isomer mixtures, nigrosides A1/A2 (1a/1b) and nigrosides B1/B2 (2a/2b), two new cholestane-type glycosides, nigrosides C and D (3 and 4), together with the known compounds, 25(R,S)-5α-spirostan-2α,3β,6β-trio1-3-O-β- d -glucopyranosyl-(1 → 2)-O-[β- d -xylopyranosyl-(1 → 3)]-O-β- d -glucopyranosyl-(1 → 4)-β- d -galactopyranoside (5a/5b) and 25(R,S)-5α-spirostan-2α,3β,6β-trio1 3-O-β- d -glucopyranosyl-(1 → 2)-O-[4-O-(3S)-3-hydroxy-3-methylglutaryl-β- d -xylopyranosyl-(1 → 3)]-O-β- d -glucopyranosyl-(1 → 4)-β- d -galactopyranoside (6a/6b), isola…