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

Total Synthesis of Decarboxybetalaines by Photochemical Ring Opening of 3‐(4‐Pyridyl)alanine A 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 (2 S )‐17‐decarboxybetanidine ( 9 , red, 34%), (2 S )‐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 (2 S , 15 S ) and that of 9b as (2 S , 15 R ), 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 .