Indole Alkaloids: XXIV Unusual behaviour of voachalotine and derivatives in the von braun degradation

The cyanamides obtained by the von Braun degradation of voachalotine (VIII) and derivatives are all bromine free compounds, formed by a “benzylic” cleavage of the C3N4 bond. The alternatively conceivable allylic cleavage of the C21N4 bond was not observed. The derivatives I, III and VI (not containing an OH group) yielded cyanamides comprising a Δ 3,14 double bond conjugated to the indole system. This was not observed in the degradation of voachalotine (VIII): the corresponding cyanamide IX contained a cyclic ether, formed by an intramolecular attack of the alcohol group on C3. The cleavage of the C3-N4 bond can also be assisted by an inter-molecular process involving alcohols added to the solvent (chloroform). In the latter conditions indeed good yields of the 3-alkoxycyanamides XII, XIII, XIV and XV were obtained.