Alstonine, tetrahydro

Alloanodendrine, K22 Alloaromadendrene, T26 Allodevardarool, T34 Allohimachalol, T12 Allohydroxycitric acid, A30 Alloisocitric lactone, A30 Alloisoleucine, A27 Allomatrine, K24 Allose, Cl Allosedamine, K18 Allosedridine, K19 Allothreonine, A24 Alloxanthin, T55 Alloyohimbane, KIO Allulose, C2 Alnusene, T42 Alolycopine, K25 Alopecurine, K26 Alstonine, tetrahydro-, K9 Altritol, Cl Altrose, Cl... 

For the biosynthetic conversion of cathenamine (76) to the 19- and 20-epi derivatives, an equilibrium should exist between the enamine and imi-nium forms of cathenamine (i.e., 76 and 97) (767). This was examined (209) with deuterium labeling studies of incorporation into tetrahydro-alstonine (75), whereupon C-21 was labeled from both the enamine and iminium forms. When the enamine form was present, a second deuterium was incorporated (presumably at C-20) on reduction with NaBD DjO. Sulfate was effective in pushing the equilibrium toward the iminium species (209). 

The transformation to the 1,2,3,4-tetrahydro derivatives is best accomplished by hydrogenating the 2-alkyl-p-carboline salts in methanol adjusted to pH 10 with potassium hydroxide, as shown for alstonine hydrochloride (37) (eq. 12.65),124 or hydrogenating the anhydronium base in methanol, as shown with 2-methylharman (38) (eq. 12.66).125... 

Alkaloids of the ajmalicine type, which differ from the tetrahydro-alstonine type only a trans D/E ring junction, show quite different spectra. Instead of the fragments of mass 251 [202] and 223 [203], they exhibit an intense peak at mass 225, thus differentiating these stereoisomeric types.49... 

Pleiocarpamine has been found in the bark of Alstonia vitiensis (Seeman) var. novo ebudica monachino, and 10-methoxygeissoschizol, a-yohimbine, and alstonine in the leaves and stems of Rauwolfia obscura the stems also contain tetrahydroal-stonine and an anhydronium base which, unfortunately, decomposed in the mass spectrometer and was obtained in insufficient amount for structure elucidation. 

The u.v. spectrum of serpentinine corresponds to an addition of the absorption of serpentine (244) or alstonine (245) to that of yohimbine (246). In accordance with this, serpentinine in aqueous dimethylformamide exhibits two pK a values of 6.0 (tetrahydro-)5-carboline part) and 10.6 (anhydronium base part) pK of... 

The partial synthesis of all four isomers (45-48) from tetrahydro-alstonine (49) renders secure the postulated stereochemistry at positions 15, 19, and 20 48). Since in uncarine-C (45) and uncarine-E (46) 19,20 = 11 Hz, the hydrogen atoms at positions 19 and 20 must be trans diaxially oriented similarly the magnitude of the corresponding coupling constant in the spectra of uncarines-D (47) and -F (48) ( 19,20 = 1-5 Hz) indicates that these hydrogen atoms are trans diequatorially oriented. This obviously indicates a conformational inversion in the isomerization of the C and E isomers to uncarines-D and -F and is only consistent with a cis D/E ring junction. 

Noteworthy are the total syntheses of akuammigine and tetrahydro-alstonine (3) which appeared too late for detailed discussion in Volume... 

Tetrahydro-alstonine — NADPH 1 81 62 fjiM Iminium — — Partial - 223... 

The addition of a six-membered ring between C-14 and Na in tetrahydro-alstonine (120) was achieved by oxidation with mercuric acetate, which gave the unstable 3,14-enamine (121) via the corresponding C-3, Nb-immonium salt. The reaction of (121) with methyl chloroacetate resulted in alkylation at C-14 and cyclisation, with formation of the crystalline hexacyclic enamine (122), reduction of which afforded the 3,14-dihydro-derivative (123) (Scheme 12). 

One further vital point emerges from this study. When strictosidine (139) was allowed to react with /3-glucosidase and sodium cyanoborohydride, tetrahydro-alstonine (120) was obtained, but no akuammigine. Hence no epimerization at C-3 is required, and it would seem that strictosidine acts as precursor for the 3/3 -H alkaloids and vincoside for the 3a-H series. ... 

Indole alkalod biosynthesis Tryptamine, sarpagine, ajmaline, vinblastine, vindoline, catharanthine, serpentine, tetrahydro alstonine, ajmalicine... 

---