Glycosides of marine invertebrates. XVI. Cucumariogenin from glycosides of the holothurianCucumaria fraudatrix
A new genin of the holostane series has been isolated by the acid hydrolysis of the glycosidic fraction of the holothurianCucumaria fraudatrix. It has been shown that this genin is the native alycone of glycoside G1 from the same holothurian. The structure of the genin has been established as 16β-acetoxyholosta-7,24-dien-3β-ol.
Conformational studies on pyranoid sugar derivatives by NMR spectroscopy. Correlations of observed proton—proton coupling constants with the generalized Karplus equation†
Proton spin-spin coupling values measured for a wide range of tetrasubstituted tetrahydropyran ring-systems obtained from aldopentopyranose sugars have been correlated with values calculated from a generalized version of the Karplus equation which takes into account the electronegativities of the various atoms in the molecule and the configurations of these substituents. Minor discrepancies between observed and calculated values can be attributed to slight flattening of the tetrahydropyran ring as a result of steric interactions of the substituents. The magnitude of the geminal coupling J5e,5a of these aldopentopyranose derivatives provides a rough measure of the conformational population; when H-4 is axial [Cl(D) or 1C(L) conformation] J5e,5a is ∼10.9 Hz, and is ∼13.4 Hz when H-4 is equatorial [1C(D) or Cl(L) conformation].
Studies of synthesis and conformation in the D-ribopyranose series
Abstract Reactions of tribenzoyl-α- and β- D -ribopyranosyl bromides with mercuric cyanide yielded 2,3,4-tri-O-benzoyl-β- D -ribopyranosyl cyanide. Its structure and configuration were proved by reduction with LAH to 6-amino-1,5-anhydro-6-deoxy- L -allitol, deamination of which gave 1,5-anhydro- L -allitol. The latter was identified by comparison with its D -enantiomer prepared from D -allose. The ring conformation of twelve 2,3,4-tribenzoyl- D -ribopyranose derivatives, and of two related acetyl compounds have been determined by PMR spectroscopy at 60 or 100 Mc/s. The operation, or non-operation of the anomeric effect appears to be the principal factor determining which particular chair conformation (Cl or 1C) predominates. The chemical shift and coupling constant differences in these derivatives and in their conformations are discussed. The anomeric proton in tribenzoyl β- D -ribopyranosyl iodide is unusually deshielded.