Synthetic Study of Carthamin: Biomimetic Synthesis of Carthamin Model via Peroxidase‐Catalyzed Oxidative Decarboxylation of Its Precursor Model

Abstract

A chiral carthamin model (3S,3′S)‐1‐[5‐acetyl‐2,6‐diketo‐3‐C‐β‐d‐glucopyranosylcyclohex‐4‐enylidene]‐1′‐[5′‐acetyl‐3′‐C‐β‐d‐glucopyranosyl‐2′,3′,4′‐trihydroxy‐6′‐oxocyclohexa‐1′,4′‐dienyl]methane, in which two cinnamoyl groups were replaced by an acetyl group, was synthesized by the dimerization of (S)‐2‐acetyl‐4‐C‐(per‐O‐acetyl‐β‐d‐glucopyranosyl)cyclohexadienone with glyoxylic acid, followed by peroxidase‐catalyzed oxidative decarboxylation and de‐O‐acetylation, or de‐O‐acetylation and peroxidase‐catalyzed oxidative decarboxylation. The corresponding total yields were 12.5% or 17.1% from 3‐C‐(per‐O‐acetyl‐β‐d‐glucopyranosyl)phloroacetophenone, and the reaction pathway was identical to the biosynthetic pathway.

Biomimetic synthesis of carthamin, a red pigment in safflower petals, via oxidative decarboxylation

“…Recently, we achieved the biomimetic total synthesis of an analog of 1 in a good yield from de‐ O ‐methyl ether of 5 via a precarthamin analog in a weakly basic aqueous solution at 25°C. [ 12 ] Using these mild conditions, the synthesis of 1 was examined. De‐ O ‐methylation of 8 with 10 equiv.…”

“…Formation of 1 from 8 was achieved using the following synthetic protocol of a model compound in which two cinnamoyl residues were replaced with acetyl groups. [ 12 ] The protocol involved the (a) synthesis of a precarthamin‐acetate analog via condensation of two 1,3‐diketones with glyoxylic acid, (b) synthesis of carthamin‐acetate analog via POD‐catalyzed oxidation of the precarthamin‐acetate analog, and (c) synthesis of the carthamin‐analog by final saponification. In the previous method, the final de‐ O ‐acetylation of 4 was conducted using NaOMe as a base; however, 1 was not obtained and purple‐ and blue‐colored products were mainly produced instead.…”