An equilibrium mixture of the isomeric depsides para- and meta- scrobiculin has been extracted from the lichen Lobaria scrobiculata. Acetylation of this mixture led to the isolation of two isomeric triacetates, the structures of which were confirmed by unambiguous synthesis. This equilibration provides compelling evidence that m-depsides arise biogenetically by C-hydroxylation of the B-ring of a p-depside followed by an intramolecular rearrangement.

1,8-Dihydroxy-3,6-dimethoxy-9H-xanthen-9-one ( 1 ) has been isolated from a lichen for the first time and its structure verified by unambiguous synthesis.

The total synthesis of the xanthones 5,7-dichloro-8-hydroxy-2-methoxy-1,3-dimethyl-9H-xanthen-9-one(10),5,7-dichloro-2,8-dihydroxy-1,3-dimethyl-9H-xanthen-9-one(11),2-dechloro-8-O-methylthiomelin (6), 2-dechlorothiomelin (8), 4-dechloro-8-O-methylthiomelin (7), 4-dechlorothiomelin (5) and northiomelin (9) has been achieved. The compounds (5)-(7) and (10) were shown to be identical to the corresponding xanthones isolated from the lichen Rinodina thiomela, while chromatographic comparisons confirmed that the biosynthetically related xanthones (8), (9) and (11) were minor constituents of this species. This is the first reported natural occurrence of the xanthones (8) and (11).

The structure of the toxic kaurene aminoglycoside bifloratoxin (3) from Melanthera biflora (L.) Willd ( syn. Wedelia biflora DC.) has been determined, and complete 1H and 13C n.m.r. spectral assignments achieved. The aglycon unit of bifloratoxin (3) is identical to carboxyatractyligenin (11), the aglycon of carboxyatractyloside (8).

The structures of butlerin A (1) (3′,4,4″,6′-tetramethoxy[1,1′:4′,1″-terphenyl]-2′-yl acetate), butlerin B (2) (3′,4,4″,5′-tetramethoxy[1,1′:4′,1″-terphenyl]-2′-yl acetate) and butlerin C (5) (3′,4,4″-trimethoxy[1,1′:4′,1″-terphenyl]-2′,6′-diyl acetate), three para-terphenyls from the lichen Relicina connivens, have been established by X-ray analysis of crystals of butlerins A and B and spectroscopic comparisons. Crystals of a mixture of butlerins A and B are orthorhombic, Pbcn, a 22.388(3), b 13.525(2), c 7.082(3) Ǻ; Z 4. Refinement of 1049 data with I > 3σ(I) gave conventional R factors of 0.043 and 0.042.