Über die absolute Konfiguration der Visnagane

Abstract

18. I V . 69).S~rwmary. ( + )-cis-Khellactone methyl ether (4) and ( -)-traizs-klicllactonc methyl ethcr (6) had earlier been assigncd the absolute configurations 3'-S; 4'-S and 3'-S; 4'-H, respectively, on thc basis of the FREUDENBERG rule. Both compounds together with thcir defunctionalised derivatives (-1.7 and ( + ) -8 ( = (+)-lomatin), obtained from a mixture of (+)-visnadin (1) and (+)-san~idin( 2 ) , were investigated by the HOREAU method. A conformational analytical study showed that the optical yield should rise in the order 4 < 6 < 7, 8. This order was found and the cc-phenylhutyric acid libcrated was always dextrorotatory. The centre 3' of the khellactones and their derivatives must be R-chiral and not S. Treatment of ( -)-6 with pyridinium pcrbromide gave ( -)-trans-3bromokhellactone methyl ether (11) as orthorhombic crystals. The X-ray crystal structure dctermination was made using the anomalous scattering of thc Mo-Kcc radiation by Br. The result,ccntre 3' R-chiral (fig. g) -showed that the HOREAU mcthod was corrcct. Die natiirlich vorkommenden Visnagane stellen Diester des 3', 4'-Dihydroxy-3', 4'dihydro-seselins = Khellacton dar. Als Sauren sind Essigsaure, Isovaleriansaure, cc-Methylbuttersaure, Seneciosaure und Angelicasaure angetroffen worden. Auf Grund cheinischer und NMR.-spektroskopischer Evidenz folgt, dass alle bisher isoIierten Visnagane, mit einer Ausnahme, Derivate des (+)-cis-Khellactons (3) l) darstellen. ES sind dies: (-1-)-Dihydrosamidin [l], (+)-Visnadin (1) [l] [ 2 ] , Suksdorfin [ 3 ] , Pteryxin 131 141, 3'-Acety1-4'-senecioyl-cis-khellacton 14 a], (-t)-Samidin (2) 111 (21, Isopteryxin [4 b], Anomalin [4 b] [4 c] [S], Calipteryxin [4 b] und 3', 4'-Di-senecioyl-cis-khellacton [4 a]. Die Ausnahme reprasentiert das Peuformosin = 3'-Angeloyl-4'-senecioyl-ciskhellacton [4b] [5 a], das sich vom (-)-cis-Khellacton ableitet und den Antipoden des Calipteryxins darsteilt.

Thallium(III) nitrate oxidation of chromens: cis-dihydroxylation and dialkoxylation

“…These clearly indicate that the ring adopts the expected halfchair conformation. They also confirm the relative stereochemistry of the substituents to this ring with the acetyl groups cis to each other and trans to the methyl group, as shown in (8).…”

“…However because of the uncertainties in this system over conformational preferences and the effect of oxygens on .Ivic, a single-crystal X-ray diffraction study of the diacetate ( 8) was undertaken.' The diacetate (8) formed needles from ethanol, m.p. 129-1 3 1 "C. The structure was determined by X-ray crystallography with refinement converging at R 4.57% over 1640 independent observed reflections.…”

Coumarins fromPeucedanum wulongense

“…The compounds were identified by spectral analysis and optical activity as (þ )-cis-khellactone (7) and (2 )-trans-khellactone (8) as a minor artefact arising from epimerization at C-4 0 because of the benzyl effect and S N 2 reaction mechanism [4]. The absolute configurations of 7 and 8 were described previously as 3 0 R, 4 0 R and 3 0 R, 4 0 S by chemical methods and X-ray diffraction analysis [5,6] and, accordingly, the absolute configuration of 1 was established as 3 0 R, 4 0 R. The chemical structure of 1 was finally elucidated as 3 0 (R)-angeloyloxy-4 0 (R)-isovarloyloxy-3 0 ,4 0 -dihydroseselin. Its enantiomer, named Pd-III ([a ] D þ 25.9), was isolated from the root of same genus, Peucedanum praeruptorum [7].…”