Die Struktur des Anhydro-isocalebassin-methyläthers; Säurekatalysierte Umlagerungen des Curare-Alkaloids C-Calebassin. 60. Mitteilung über Calebassen-Alkaloide [1]

Treatment of the Calebash alkaloid C-calebassine ( 1 ), C₄₀H₄₈N₄O₂⁺⁺·2X^?, with hot mineral acid yielded the anhydro-isocalebassine acid adduct 3H , with the formula C₄₀H₄₇N₄O⁺⁺⁺·3X^?.H₂O. This was converted into anhydro-isocalebassine methyl ether salts ( 4 ) C₄₁H₄₈N₄O⁺⁺·2X^?·H₂O with alkaline dimethyl sulphate. A total X-ray analysis of the orthorhombic diiodide led to formula 4 . The conversion of C-calebassine ( 1 ) into 4 involves a radical change in the central part of the molecule. The structure of the acid adduct 3H can be deduced from the structure of 4. Treatment of 3 H with warm methanol or treatment of C-calebassine ( 1 ) with acetic acid yielded a pale yellow compound 2 , C₄₀, H₄₄N₄⁺⁺·2X^?·H₂O. The central part of the molecule 2 contains a pyrrole ring, and most probably this is an intermediate in the conversion of 1 into 3 H by mineral acid. The changes of 3 H and 2 in acidic, neutral and basic media can be shown by electronic spectroscopy. Reaction of 3 H with 1 mole of oxygen at pH 4 converted it into a stable blood-red oxidation product C₄₀H₄₄N₄O₂⁺⁺·2X^? containing a merocyanine system. The structure 7 , proposed for this compound, is discussed.