Synthese von Betenamin und von Betalain‐Modellsubstanzen

Synthesis of Betenamine and of Betalaine Model Substances For comparisons of color, spectroscopic properties, p K a values, and stabilities, a number of model substances containing the 1,7‐diazaheptamethinium chromophore 8 of the yellow and red betalaine plant pigments were prepared by the thermal or photolytic ring opening of simple pyridine derivatives (such as 2 and 14‐16 ), followed by the introduction of amines. Among the novel compounds prepared were betenamine perchlorate ( 5 ), the ‘naked’ ring system of the beet‐pigment betanine ( C ) as well as two 1,7‐diazaheptamethinium salts 25 and 27 with terminal amono acids. The synthesis of 5 started with 4‐(2‐aminoethyl)pyridine ( 1 ) and proceeded via 2 , ring opening with indoline to 4 , saponification, and intramolecular amine replacement ( Scheme 1 ). The syntheses of 25 and 27 involved only one step, namely ring opening of γ‐picoline using ( S )‐cyclodopa ( 24 ) and ( S )‐proline ( 26 ), respectively ( Scheme 3 ).