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

摘要

Dehydrodimenzation of ferulates m grass cell walls provides a pathway toward cross linking polysaccharide chains limiting the digestibility of carbohydrates by ruminant bacteria and m 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)-tetramethylefhylenediamine [CuCl(OH)-TMEDA] complex as oxidant or catalyst Although CuCl(OH)- TMEDA oxidation of ethyl ferulate in acetonitnle 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-Dlferulate 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 tetrabutylammomum fluoride treatment that conveted 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 CuCI(OH)—TMEDA catalyzed aerobic oxidative coupling reaction and subsequent saponification described here.