Identification and synthesis of new ferulic acid dehydrodimers present in grass cell walls

Seven isomeric dehydrodimers of ferulic acid (4-hydroxy-3-methoxycinnamic acid) have been synthesized and identified in extracts of saponified cell walls of cocksfoot, switchgrass, and suspension-cultured corn. Dehydrodimers (E, E) -4, 4′-dihydroxy-5, 5′-dimethoxy-3, 3′-bicinnamic acid, trans-5- (E) -2-carboxyvinyl-2- (4-hydroxy-3-methoxyphenyl) -7-methoxy-2, 3-dihydrobenzofuran-3-carboxylic acid, (Z) -β-4- (E) -2-carboxyvinyl-2-methoxyphenoxy-4-hydroxy-3-methoxycinnamic acid, (E) -3-4- (E) -2-carboxyvinyl-2-methoxyphenoxy-4-hydroxy-5-methoxycinnamic acid, (E, E) -4, 4′-dihydroxy-3, 5′-dimethoxy-β, 3′-bicinnamic acid, 4, 4′-dihydroxy-3, 3′-dimethoxy-β, β′-bicinnamic acid, and trans-7-hydroxy-1- (4-hydroxy-3-methoxyphenyl) -6-methoxy-1, 2-dihydronaphthalene-2, 3-dicarboxylic acid, all arise from oxidative coupling of ferulate esters in cell walls and represent products of 8–5, 8–8, 8–O–4, 4–O–5, and 5–5 radical coupling. Prior literature has acknowledged only the presence of the 5–5-coupled dehydrodimer (E, E) -4, 4′-dihydroxy-5, 5′-dimethoxy-3, 3′-bicinnamic acid. Consequently, by measuring only a single dehydrodimer and assuming inappropriate response factors, ferulate dehydrodimers have been underestimated by factors of up to 20. Synthetic routes to all seven isomers have been developed to provide structural authentication and determination of GC response factors.