Allylphenol-neolignans

An unusual Cp—Car coupling was observed on anodic oxidation, in acetic and trifluoroacetic acids, of a mixture of ( )-isosaffole (289) and the allylphenol (290).Various interesting products were characterized, among them the cyclohexadienone (291). Although only formed in 4% yield, this product was readily converted by successive alkali and acid reactions, into the neolignan derivative denudatin A (292). 

Lignans are found throughout the plant kingdom and it is not possible to make generalizations about families in which they may occur. However, the neolignans appear to be confined to Magnoliales and the related Piperales, which also contain monomeric propenyl- and allylphenols. ... 

In addition to the biphenyls 4.2a and b only two further allyl- + allylphenol derived neolignans, asatone and isoasatone, are known. 

Chart 25. Schematic representation of coupling processes which lead from propenylphenol (88) and allylphenol (89) derived radicals, either by dimerization (x 2) or by addition of radical 90 [+(90)] to the indicated (in brackets) numbers of neolignans belonging to the... 

The fourth and last example selected refers to a structurally surprising group of neolignans generated by the coupling of allylphenols plus allylphenols (Fig. 7.3.13). The process is initiated by oxidative methoxylation of the precursor 46 to 47. Diels-Alder additions of 47 (as dienophile) -I- 47 (as diene) or of 46 (as dienophile) -l- 47 (as diene) lead respectively to 48 (asatone), and hence to 49, and to 51 (164). Biosynthesis of the sesquineolignan 50 involves both these alternative cycloadditions. Biomimetic synthesis by anodic oxidation of 46 in methanol led to asatone (161). 

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