摘要:

IntroductionIsolation and structural elucidation of lawsonicin (rac-trans-1), a natural product ofLawsonia alba, was reported by one of us in 20031(Fig. 1). Although the assignment of a core 2-aryl-(3′,4′-substituted)-2,3-trans-dihydrobenzo[b]furan is unambiguous, the 5,6-substitution pattern of the benzofuran ring was not conclusively established. Lawsonicin is of unknown biosynthesis;2however, its formula, C20H24O6, implies that the molecule may derive from dimerisation of coniferyl alcohol (C10H12O3) and belong to a class of known 2-aryl-2,3-dihydrobenzo[b]furan lignans.Structures of racemic 2,3-transepimers of lawsonicin (1) and dihydrodehydrodiconiferyl alcohol (2).Lignans constitute a large group of plant secondary metabolites whose biosynthesis involves the dimerisation of phenylpropenes.3Resonance stabilisation of a radical formed from a (phenylpropene) monolignol facilitates oxidative coupling of radical partners (or electrophilic attack of a single radical upon a second monolignol molecule),4to give structurally diverse products, of which 8-8′-linked lignans are the most common. The established biosynthesis of dehydrodiisoeugenols,viabimolecular coupling of phenoxy radicals derived from laccase-catalysed oxidation of 2-methoxy-4-trans-propenylphenol (isoeugenol),5,6is widely applied to 8-5′-linked lignan (neolignan) biosynthesis and can be delineated for dimerisation of coniferyl alcohol (3) (Fig. 2).6,7Schematised biosynthesis of dihydrodehydrodiconiferyl alcoholvia8-5′ radical coupling.8-5′-Dimerisation leads to an intermediatep-quinone methide,4,8,9intramolecular cyclisation of which installs the dihydrobenzo[b]furan core, prior to an enzymatic allylic alcohol reduction.10Dihydrodehydrodiconiferyl alcohol,2, is furnished in this overall oxidoreductive process. Added structural classes of lignan may be defined, featuring an 8-O4′, 8-3′, 8-2′ or 8-1′ linkage and potentially involving further post-dimerisation modifications. However, the ascribed connectivity of lawsonicin (rac-trans-1) cannot be rationalised within these established pathways for monolignol dimerisation, and we herein propose structural revision of lawsonicin, to the known neolignan, dihydrodehydrodiconiferyl alcohol11–14(rac-trans-2,15Fig. 1).

ISSN:1477-0520    

DOI:10.1039/b918179b

出版商:RSC    

年代:2009

数据来源: RSC