Alstonine and Tetrahydroalstonine

The UV-spectrum of tetrahydroalstonine closely resembles that of yohimbine, except that it contains an inflection at 250 m/x.. Hence, tetrahydroalstonine probably contains an indole ring system and an additional chromophore. Reduction of tetrahydroalstonine with sodium and butanol gives a hexahydroalstonol, C20H26N2O2, by conversion of the ester function into a primary alcohol grouping and addition of two 

From these observations no unequivocal conclusion could be reached concerning the identity of the chromophore which absorbs at 250 m/x, nor the function of the third oxygen atom. However, the presence of a /3-carboline system was accepted, in which Nb was substituted and Na was unsubstituted the latter accounted for the presence of one active hydrogen in tetrahydroalstonine. Since alstonine hydrochloride absorbs at higher wavelengths than 2-ethyl /3-carboline hydrochloride, it was assumed to possess greater conjugation, and the partial structure II was therefore proposed for alstonine (49). 

The isolation of /3-carboline derivatives and alstyrine from the degradation of alstonine suggests that the alkaloid contains the ring system IV to this must be added the chromophores, the ester group, and the third, unidentified, oxygen atom. Since the UV-spectrum of alstonine is very 

However, the UV-spectrum of tetrahydroalstonine (maxima at 230 and 290 mp, inflection at 250 mp) indicates that it is composed of an indole chromophore plus additional absorption in the 250 mp region. This spectrum shows quite different characteristics from the summation spectrum of 2,3-dimethylindole and 2,6-dimethyl-3-carbomethoxy-5,6-dihydro-l,2-pyran (VII) the summation spectrum exhibits a pronounced minimum at 250 mp. Hence, formula VI for tetrahydroalstonine 

The IR-data are also consistent with these conclusions. Twin maxima at 1695 and 1613 cm 1 are exhibited by tetrahydroalstonine, coryn-antheine, tetrahydroserpentine, and the model substance VIII, all of which possess the chromophore, ROOC—C=C—OR these are not observed in the spectra of VII and the saturated pyran derivatives (55). The formula X for alstonine also explains all of its apparently anomalous properties. The resistance of the ester group to saponification and of the ring E double bond to hydrogenation are characteristic of VIII, whereas the presumed molecular compound obtained earlier by reaction of alstonine with dinitrophenylhydrazine (54) is probably a derivative of its open-chain carbonyl equivalent (XI) (55). 

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