Totalsynthese von Decarboxybetalainen durch photochemische Ringöffnung von 3‐(4‐Pyridyl)alanin

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Abstract
Total Synthesis of Decarboxybetalaines by Photochemical Ring Opening of 3‐(4‐Pyridyl)alanineA photochemical approach is presented for the total synthesis of the decarboxybetalaines, which were previously known from the mild decarboxylation of the natural plant colorants, the betalaines: Irradiation of rac‐3‐(4‐pyridyl)alanine (1) yielded the rac‐2‐decarboxybetalamic‐acid‐imine (4, 86%), presumably via a Dewar pyridine 2, a cyclic aminal 3 and an electrocyclic ring opening. The imine‐zwitterion 4 was treated with three amines, namely (S)‐cyclodopa (6), (S)‐proline (7), and indoline (8), to afford three decarboxybetalaines, namely (2S)‐17‐decarboxybetanidine (9, red, 34%), (2S)‐13‐decarboxyindicaxanthine (10, yellow, 56%), and rac‐16‐decarboxyindobetalaine (11, orange, 78%), respectively. The structures of these coloring matters were confirmed by their electrophoretic behavior and their spectroscopic properties. 17‐Decarboxybetanidine 9 was shown to be a ca. 1:1 mixture of two C(15)‐epimers 9a and 9b, separable by chromatography. The configuration of 9a was determined as (2S, 15S) and that of 9b as (2S, 15R), by correlating their optical rotations with those of betanidine (12a) and isobetanidine (12b), respectively. The decarboxybetalaines 9, 10, and 11 did not show the double‐bond isomerism at C(β), (Cγ) of the chromophore which had been found characteristic for the corresponding betalaines 12, 13, and 14.

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