An Aryldihydronaphthalene Lignan with a Novel Type of Ring System and Further New Lignans from Linum perenne L.

Abstract

From the dichloromethane extract of aerial parts of Linum perenne L. (Linaceae), in addition to three lignans of the arylnaphthalene type (justicidin B, isojusticidin B and retrohelioxanthin), we isolated four lignans of the aryldihydronaphthalene type. The major constituent contains a novel type of ring closure, formed by additional cyclisation between rings A and D of the aryldihydronaphthalene skeleton via an oxymethylene bridge, yielding a novel 2,8-dihydro-3H-benzo[e]naphtho[1,8-bc]oxepine ring system hitherto unreported for a lignan or any other natural product. We named this novel unusual lignan linoxepin. In addition, three further aryldihydronaphthalenes (one new and two known) were isolated. The absolute stereochemistry at C-8 of linoxepin and the related dihydronaphthalene lignans was established as R on grounds of CD spectra which were in good agreement with results of time dependent density functional (TDDFT) quantum mechanical simulations. The occurrence of aryldihydronaphthalenes in the genus Linum or the Linaceae has not been reported before.

Key words

Arylnaphthalene lignan - aryldihydronaphthalene lignan - Linum perenne L. - Linaceae - CD spectroscopy - TDDFT-CD simulation

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