Etsuji Yamanaka

Structure of DHG, a depolymerized glycosaminoglycan from sea cucumber, Stichopus japonicus

Abstract Structure of DHG, a depolymerized glycosaminoglycan from sea cucumber was determined to be a fucose-branched chondroitin 4,6-sulfate by means of chemical degradation and 2D-NMR spectroscopy.

The active principles of the branchlet and hook of Uncaria sinensis Oliv. examined with a 5-hydroxytryptamine receptor binding assay

Of the alkaloids obtained from Uncaria sinensis Oliv., geissoschizine methyl ether, corynantheine and dihydrocorynantheine decreased specific [3H]5‐HT binding to membrane preparations from rat brain and from in‐vitro experiments on guinea‐pig ileum, these alkaloids were found to be partial agonists for 5‐HT receptors. Therefore, they might be useful in the treatment of diseases resulting from disorders of 5‐HT metabolism.

Transformation of indole alkaloids—I : Conversion of oxindole alkaloids into indole alkaloids

Abstract Chemical conversion of some natural oxindoles (pteropodine, isopteropodine and isorhynchophylline) into the corresponding indole alkaloids has been made by way of a sequence of reactions which include formation of iminoethers of the natural oxindoles with Meerweins reagent, reduction of the iminoethers to 2,3-seco-indoles and cyclization of 2,3-seco-indoles to the desired natural indole alkaloids. Sodium borohydride in acetic acid was found to be a specific reagent for the reduction of oxindole-iminoethers to 2,3-seco-indoles which were the key intermediates in these transformations. Yohimbine-oxindole iminoether was similarly converted to yohimbine and pseudoyohimbine. A number of by-products were obtained and their structures were elucidated.

Biomimetic synthesis of koumine skeleton: partial synthesis of 11-methoxykoumine (Gelsemium-type alkaloid) from 18-hydroxygardnerine

11-Methoxykoumine (13) was derived from 18-hydroxygardnerine (4) through CC intramolecular bond formation by Pdo catalysis between the indole part and allylic cation in an indole alkaloid. This is first example of the proof for the biogenetic route of koumine skeleton.

Introduction of hydroxyl group at C-14 of indole alkaloids: the partial synthesis of 14α-hydroxyrauniticine☆

Abstract Stereoselective hydroxylation at C-1 of the indoloquinolizidine ( 1 ) to the hyroxyl derivatives ( 5 and 6 ) and the partial synthesis of 14α-hydroxyrauniticine ( 11 ) from rauniticine ( 7 ) are described.

Ca2+ channel‐blocking effect of the yohimbine derivatives, 14β‐benzoyloxyyohimbine and 14β‐p‐nitrobenzoyloxyyohimbine

Yohimbine possesses a relatively selective α2‐adrenoceptor antagonistic effect and a weak Ca2+ channel blocking activity, so two derivatives, 14β‐benzoyloxyyohimbine and 14β‐p‐nitrobenzoyloxyyohimbine were compared with yohimbine using those parameters. The two derivatives blocked Ca2+‐induced contraction of the rat perfused mesenteric vascular bed at 3 times 10−6 M, and developed dose‐dependent shifts of the dose‐response curve to the right in concentrations from 3 times 10−6 to 10−5M. Yohimbine showed a weak Ca2+ channel‐blocking effect only at a concentration of 10−5M. On the other hand, both derivatives at 3 times 10−6 M did not affect the dose‐response curve to noradrenaline‐induced vasoconstriction. Furthermore, at 10−8 M they had no effect on the dose‐response curve to clonidine‐induced inhibition of the twitch response of the rat isolated vas deferens evoked by electric stimulation, whereas yohimbine significantly attenuated clonidine‐induced inhibition. These results indicate that introduction of bulky substituents into the 14‐position of yohimbine intensely potentiated the Ca2+ channel‐blocking effects and reduced α‐adrenoceptor antagonistic effects compared with those of yohimbine, and suggest that chemical modifications of yohimbine may lead to structurally new types of Ca2+ channel‐blockers.

Gardneria alkaloids. Part 13. Structure of gardmultine and demethoxygardmultine; bis-type indole alkaloids of Gardneria multiflora Makino

Structures of two bis-type alkaloids, gardmultine (1) and demethoxygardmultine (2), isolated from Gardneria multiflora Makino (Loganiaceae), have been elucidated on the basis of the chemical and spectroscopic evidence.