Transformations of seco alkaloids, illustrated by corynantheine

The absolute stereochemistry of the group depended on a number of arguments. For C-3 it is based on a comparison of the molecular rotation differences between the bases and their dehydro and tetradehydro derivatives with analogous yohimbine derivatives. For C-15 on the conversion of dihydrocorynantheol and dihydrocinchonamine into the same quaternary salt. Dihydro-cinchonamine had previously been prepared from cinchonine by a route which did not affect the stereochemistry at C-15. 

Bumamicine. This natural constituent isolated in trace quantities from Hmteria eburnea is included here because of its obvious relationship to corynantheol and because it represents a good example of the use of mass spectrometry in structure deduction. Bumamicine had an ultraviolet absorption maximum typical of a 2-acylindole which changed to that of a typical indole upon either sodium borohydride reduction or solution in acid. After the latter experiment the original chromophore was regenerated in basic solution. From the nuclear magnetic resonance spectrum the four aromatic protons, the ethylidene and the N-methyl were identifiable. The recovered alkaloid upon acetylation had an ultraviolet and nuclear magnetic resonance spectrum consistent for an ind N,0-diacetate (probably primary). 

A synthesis of dihydrobumamicine has been accomplished by re-arranging 7-acetoxy-7H-dihydrocorynantheol methosalt in aqueous acetic acid (cf. general principles outlined in Chart 3.5). 

Some subtle factors must be involved in the success of this reaction since 7-acetoxy-7H-yohimbine gave the derivative. 

Hunterburnine a- and -Methochlorides. Similar to the case of bumamicine, the quaternary salts hunterburnine a- and j8-metho-chlorides were isolated from Hunteria ebumea in amounts which did not lend themselves to chemical examination and they were not volatile enough for mass spectrometric work. By physical methods they were recognized to be 5-hydroxyindoles, to contain a quaternary N-methyl and a vinyl group. Since their iodides were in a form suitable for X-ray analysis this appeared to be the method of choice for their structural elucidation which is discussed for the )3-methiodide. 

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