Rapid Syntheses of Dehydrodiferulates via Biomimetic Radical Coupling Reactions of Ethyl Ferulate

Abstract

Dehydrodimerization of ferulates in grass cell walls provides a pathway toward cross-linking polysaccharide chains limiting the digestibility of carbohydrates by ruminant bacteria and in general affecting the utilization of grass as a renewable bioresource. Analysis of dehydrodiferulates (henceforth termed diferulates) in plant cell walls is useful in the evaluation of the quality of dairy forages as animal feeds. Therefore, there has been considerable demand for quantities of diferulates as standards for such analyses. Described here are syntheses of diferulates from ethyl ferulate via biomimetic radical coupling reactions using the copper(II)-tetramethylethylenediamine [CuCl(OH)-TMEDA] complex as oxidant or catalyst. Although CuCl(OH)-TMEDA oxidation of ethyl ferulate in acetonitrile produced mixtures composed of 8-O-4-, 8-5-, 8-8- (cyclic and noncyclic), and 5-5-coupled diferulates, a catalyzed oxidation using CuCl(OH)-TMEDA as catalyst and oxygen as an oxidant resulted in better overall yields of such diferulates. Flash chromatographic fractionation allowed isolation of 8-8- and 5-5-coupled diferulates. 8-5-Diferulate coeluted with 8-O-4-diferulate but was separated from it via crystallization; the 8-O-4 diferulate left in the mother solution was isolated by rechromatography following a simple tetrabutylammonium fluoride treatment that converted 8-5-diferulate to another useful diferulate, 8-5-(noncyclic) diferulate. Therefore, six of the nine (5-5, 8-O-4, 8-5-c, 8-5-nc, 8-5-dc, 8-8-c, 8-8-nc, 8-8-THF, 4-O-5) diferulic acids that have to date been found in the alkaline hydrolysates of plant cell walls can be readily synthesized by the CuCl(OH)-TMEDA catalyzed aerobic oxidative coupling reaction and subsequent saponification described here.

Grass cell wall feruloylation: distribution of bound ferulate and candidate gene expression in Brachypodium distachyon

“…Quantitation of FA and p CA was by integration of peak areas at 280nm with reference to calibrations made using known amounts of pure compounds. Peaks of the major FA dimers were identified by comparison of retention times with pure standards kindly supplied by Professor John Ralph (Lu et al, 2012; 5-5, 8- O -4, 8-5-benzofuran) or by comparison of spectrum with that in Waldron et al (1996) (8-5). Dimer quantitation was achieved relative to FA monomer using areas of these peaks and the response factors for dimers and FA monomer in Waldron et al (1996).…”

Feruloylation and structure of arabinoxylan in wheat endosperm cell walls from RNAi lines with suppression of genes responsible for backbone synthesis and decoration

“…Quantification of FA was by integration of peak areas at 280 nm with reference to calibrations made using known amounts of pure compounds. Peaks of the major diFAs were identified by comparison with retention times and absorption spectra with pure standards kindly supplied by Professor John Ralph (Lu et al, 2012). Values reported for diFAs are the sum of amounts of 8-5 0 benzofuran (BF), 8-O-4 0 , 5-5 0 and 8-5 0 diFAs.…”

A Multi-Step Chromatographic Approach to Purify Radically Generated Ferulate Oligomers Reveals Naturally Occurring 5-5/8-8(Cyclic)-, 8-8(Noncyclic)/8-O-4-, and 5-5/8-8(Noncyclic)-Coupled Dehydrotriferulic Acids

“…Several methods have been described to synthesize DFA in the past (Ralph et al, 1994 , 1998 ; Yamamoto et al, 1999 ; Schatz et al, 2006 ; Lu et al, 2012 ). In contrast, synthetic routes to obtain TriFA have been sparsely investigated (Bunzel et al, 2008b ; Mouterde et al, 2013 ).…”

“…To generate ferulate oligomers, the catalytic procedure developed by Lu et al ( 2012 ) was used in a slightly modified way as described earlier (Waterstraat et al, 2016 ). In brief, ethyl ferulate (888 mg, 4 mmol) was added to a solution of CuCl (40 mg, 0.4 mmol) and TMEDA (60 μL, 0.4 mmol) in acetonitrile (200 mL).…”