NMR spectroscopic investigations of substituted dineolignans from Manglietia garrettii

As part of the search for novel lignan compounds from Magnoliaceae with potential pharmacological activity and with chemotaxonomic interest, the leaves of Manglietia garrettii CRAIB (Thai name montha doi) grown in the temperate house of the botanical garden in Graz were phytochemically examined. In addition to the four known neolignans magnolol, honokiol, 5′-methylhonokiol, and obovatol two new isomeric neolignans, garrettilignan A and garrettilignan B, were isolated from the dichloromethane extract and their structures were determined by NMR spectroscopy and MS. Each of the two new compounds comprised 54 carbon atoms in six phenyl propane units, three terminal 4-allylphenol subunits, two 4-allyl-1,2,6-trihydroxyphenyl subunits together with a 4-trihydroxypropyl-1,2,6-trihydroxyphenol. As five subunits were connected via arylether bridges, a constitutional analysis with HMBC experiments was not possible. Instead, a combination of chemical modification (acetylation), which revealed the positions of three phenolic hydroxyl groups, HMBC and selective NOE experiments was used to determine the connections of the subunits in garrettilignans A and B. Common feature of both compounds is a core built up of two obovatol units, one of them is substituted by two additional 4-allylphenol moieties. The difference between these isomeric compounds is found in the linkage of the two obovatol units. In conclusion, garrettilignans A and B belong to the rare class of substituted dineolignans.

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