Dicarboxylic acid lignans

Lignans of another type, dicarboxylic acid lignans (p-hydroxycinnamate dimers), were isolated from liverworts (Hepaticopsida) and some vascular plants [11, 64]. Little is known about the biosynthesis of this class of lignans, but a feeding experiment demonstrated the conversion of caffeic acid to optically pure epiphyllic acid in a liyerwoTt Lophocolea heterophylla (Fig. 12.9) [65]. 

An optically active dicarboxylic acid lignan (caffeic acid dimer), named epiphyllic acid was isolated from liverwort as well as its derivatives, which do not belong to any typical lignan-producing subgroups. They also occur in some vascular plants, even as a triterpene ester of epiphyllic acid in Rhoipte-lea chiliantha [37]. The conversion of caffeic acid to epiphyllic acid was demonstrated by Tazaki et al. [38]. However, little is known about the biosynthesis of this type of lignan in vascular plants yet. 

A convenient route to lignans of this class is apparent from the known transformation of the phenylpropiolic acid to the anhydride of phenylnaphthalene-2,3-dicarboxylic acid by the action of acetic anhydride (Scheme 19). This reaction was discovered about a century ago (94) the interesting early history has been briefly reviewed (95) and a wide generality of the reaction demonstrated (96). It is particularly useful for products bearing identical substituents in rings A and C. Thus, it has been long known that 3,4-methylenedioxyphenylpropiolic acid (1011 on... 

A recent and succinct approach to 2-aryltetrahydrofuran precursors to lignan natural products involves the cycloaddition of 2-aryM-phenyloxazoles with di-methylacetylene dicarboxylate. For example, 2,4-diphenyloxazole 273 (R = H) reacts with dimethylacetylene dicarboxylate in refluxing xylenes in the presence of sodium carbonate and hydroquinone to provide 2-phenyl-3,4-furandicarboxylic acid dimethyl ester 274 in 98% yield (Fig. 3.82). Not surprisingly, 4,5-diphenylox-azole and 2,4,5-triphenyloxazole gave lower yields of the corresponding furans... 

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