New Pyrrolo[1,2-a]azepine Type Alkaloids from Stemona and Stichoneuron (Stemonaceae)

Summary. Three new pyrroloazepine type alkaloids, stichoneurines A and B and 6-hydroxycroomine were isolated from the lipophilic root extracts of Stichoneuron caudatum and Stemona tuberosa collected in Thailand together with the already known croomine, tuberostemonine, and tuberostemonine A. The structures were elucidated by spectroscopic methods including H/H-COSY, HMQC, and HMBC. Information on the relative stereochemistries and conformational behaviour was obtained by analysis of the NOESY spectra. The formation of pyrroloazepine alkaloids in the genus Stichoneuron is reported for the first time and supports its affiliation to the family Stemonaceae. The occurrence of two different types of alkaloids, of the tuberostemonine and croomine series, in different geographical provenances of Stemona tuberosa is of special chemosystematic interest and may contribute to a more natural species delimitation within that complex group.     

X-ray structural analysis of tetrahydrofurofuran lignans

The crystal structures of two diastereomeric tetrahydrofurofuran lignans, diasesartemin (1) and episesartemin-B (2), were determined. The former one is of the classical axial-axial substitution type (the two aryl moieties of the 1,4-diaryl-tetrahydro-1H,3H-furo[3,4-c]furan in ax-ax position), the latter one belongs to the axial-equatorial type. Analysis of the X-ray results revealed that in both cases the substituents are attached in pseudo-equatorial positions due to a flip of the (slightly twisted) envelope conformation of the oxolane five-rings. In1 the oxygen atoms of the two five rings point against each other (overall boat conformation of the tetrahydrofurofuran system orendo-endo topology), in2 one ring is folded towards and the other one is folded away from the second ring (overall boat-chair conformation of the tetrahydrofurofuran system orendo-exo topology): as a consequence,all aryl substituents adopt a pseudo-equatorial position. The terms ax-ax, ax-eq, and eq-eq used throughout in the literature are therefore misleading as far as the true conformations are concerned. Analysis of the aromatic short wavelength CD couplet of1 by means of the coupled oscillator model shows that the X-ray derived geometry is compatible with the CD data in solution.

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Röntgenstrukturanalyse von Tetrahydrofurofuran-Lignanen

Zusammenfassung Es wurde die Kristallstruktur von zwei diastereomeren Tetrahydrofurofuran-Lignanen, Diasesartemin (1) und Episesartemin-B (2), bestimmt. Ersteres gehört dem klassischen axial-axial-substituierten Typ an (dabei sind die zwei Arylsubstituenten des 1,4-Diaryl-tetrahydro-1H,3H-furo[3,4-c]furans in ax-ax-Position), das zweite gehört zum axial-equatorial-Typ. Die Röntgenstrukturanalyse zeigte, daß die Aryl-Substituenten immer in pseudo-equatorialen Positionen stehen, da die Oxolan-Fünfringe eine entsprechende (leicht getwistete) Briefumschlag-Konformation einnehmen. In1 zeigen die Sauerstoffatome der beiden Ringe zueinander (insgesamt eine Boot-Konformation des Tetrahydrofurofuransystems bzw.endo-endo-Topologie), in2 ist ein Ring in Richtung zum, der andere Ring jedoch weg vom jeweils zweiten gegenüberliegenden gefaltet (insgesamt eine Boot-Sessel-Anordnung in der Tetrahydrofurofuran-Einheit bzw.endo-exo-Topologie): Eine Folge davon ist, daß alle Arylsubstituenten eine pseudo-equatoriale Position einnehmen. Die in der Literatur durchgehend verwendete Bezeichnung ax-ax, ax-eq oder eq-eq ist daher bezüglich der tatsächlichen Konformation mißverständlich. Die Analyse der kurzwelligen aromatischen CD-Banden von1 mittels des Modells der gekoppelten Oszillatoren zeigte, daß die Konformation aus der Röntgenstrukturanalyse mit den CD-Daten in Lösung in Übereinstimmung ist.

     

2-Hydroxy-4-methoxy-trans-cinnamic acid as a precursor of herniarin inArtemisia dracunculus

The isolation of (E)-2-hydroxy-4-methoxycinnamic acid (1) fromArtemisia dracunculus L. supports strongly the assumption that this compound is an intermediate in the biosynthesis of 7-methoxycoumarin (herniarin,3). The structure of the UV-unstable compound1 was derived from¹H-NMR data and by comparison of the stable dimethylated derivative with synthetic (E)-2,4-dimethoxycinnamic acid methyl ester (2).Dedicated to Prof. Dr.Kurt L. Komarek on the occasion of his 60th birthday.     

Linear Relaxations and Reduced-Cost Based Propagation of Continuous Variable Subscripts

In hybrid solvers for combinatorial optimisation, combining Constraint (Logic) Programming (CLP) and Mixed Integer Programming (MIP), it is important to have tight connections between the two domains. We extend and generalise previous work on automatic linearisations and propagation of symbolic CLP constraints that cross the boundary between CLP and MIP. We also present how reduced costs from the linear programming relaxation can be used for domain reduction on the CLP side. Computational results comparing our hybrid approach with pure CLP and MIP on a configuration problem show significant speed-ups.     

Corrected Structure of Aglalactone Isolated from Aglaia elaeagnoidea (Meliaceae)

Summary.  The structure of aglalactone, a dihydrobenzofuranone from Aglaia elaeagnoidea, was revised on the basis of 2D NMR data and lanthanide induced shifts (LIS). Received June 29, 2000. Accepted July 11, 2000     

Lateral diffusion studied by pulsed field gradient NMR on oriented lipid membranes

This mini-review focuses on the utilization of pulsed magnetic field gradients to measure diffusional motion in systems of macroscopically oriented lipid bilayers. The NMR diffusion technique is proposed to have appreciable potential for future biophysical investigations in the field of membrane biology. Topics such as transport of molecules both across and in the plane of the membrane can be successfully studied, and the formation of lipid domains and their intrinsic dynamics can also be scrutinized. First, a short introduction to the NMR technique is given together with a brief discussion on methods of obtaining a good bilayer orientation. Then, a number of recent results on biophysical/biological membrane systems of great interest is presented, in which some unique conclusions on so-called 'raft membranes' are reached. It is shown for systems with large two-phase areas of liquid disordered and liquid ordered phases that lipid lateral diffusion is faster in the former phase and has a smaller apparent activation energy. Further, on the time-scale of the experiments (50-250 ms), exchange between the two phases is fast in the phospholipid-cholesterol-water ternary system, whereas it is slow in the sphingomyelin-dioleoylphosphatidylcholine-cholesterol-water quaternary system.     

Albartin—a new sesquiterpene-coumarin ether fromArtemisia alba

A new sesquiterpene isofraxidin ether (isofraxidin=7-hydroxy-6,8-dimethoxy-coumarin) was isolated from the roots ofArtemisia alba and characterized by means of¹H-nmr,¹³C-nmr, ms, uv, and ir. Albartin is an exomethylene isomer of 3-acetyloxy-drimartol isofraxidin ether with -OAc axial and -CH₂OAr equatorial. The chemosystematic significance of this class of compounds within the genusArtemisia is briefly discussed.

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Albartin — ein neuer Sesquiterpen-Cumarin-Ether ausArtemisia alba (Kurze Mitteilung)

Zusammenfassung Aus den Wurzeln vonArtemisia alba wurde ein neuer Sesquiterpen-Isofraxidin Ether (Isofraxidin = 7-Hydroxy-6,8-dimethoxy-cumarin) isoliert und mittels¹H-NMR,¹³C-NMR, MS, UV und IR charakterisiert. Albartin ist ein exo-Methylen-Isomer von 3-Acetyloxy-drimartol-isofraxidin-ether mit-OAc axial und -CH₂OAr equatorial. Die chemosystematische Bedeutung dieser Verbindungsklasse innerhalb der GattungArtemisia wird kurz diskutiert.

     

New unsymmetrically substituted tetrahydrofurofuran lignans from artemisia absinthium : Assignment of the relative stereochemistry by lanthanide induced chemical shifts

The isolation of thirteen tetrahydrofurofuran lignans from the roots of A. absinthium and six closely related species is reported. A new group of four lignans, all stereoisomers of 1- [(3,4-methylenedioxy-5-methoxy) phenyl]-4-(3,4,5-trimethoxyphenyl)-tetrahydro-1H,3H-furo[3,4-c]furan, was found. The compounds, named sesartemin, episesartemin A, episesartemin B, and diasesartemin, were characterized by ¹H NMR (including lanthanide induced shifts), UV, IR and MS. A fifth new lignan of the sesamin type (the eq/eq isomer of fargesin) could be identified as a minor constituent. The relative configurations of two further products (fargesin and epiashantin) could be confirmed by the lanthanide induced shift technique.