Pre-anthraquinones

The larger and structurally more complex group of pre-anthraqui-nones includes octaketides in which the anthraquinone moieties are incompletely developed. Two groups are known the so-called monomeric pre-anthraquinones which have structures based on either a 3,4-dihydroanthracen-l(2//)-one or a 1,2,3,4-tetrahydroanthraquinone nucleus and dimeric pre-anthraquinones which may be formed by phenolic coupling between two dihydroanthracenone sub-units. The latter group is discussed in detail in Section 3.5.3. 

Among toadstools belonging to Cortinarius subgenus Phlegmacium the principal pigments are derivatives of 3,4-dihydroanthracen-l (2//)-one and, in contrast to the situation in Dermocybe, anthraquinones themselves play only a subordinate role (617). The pivotal biosynthetic intermediate atrochrysone (321) is present in Cortinarius atrovirens and C. odoratus (Table 28) where it is accompanied by the 4-hydroxy derivatives (322) and (323) of varying stereochemistry. The dimethyl ether 

The bright red and yellow colours of the cap and stipe base, respectively, of an Australian dermocybe are due to the presence of the tetra-hydroanthraquinones (330) and (331) (2SS). These novel quinones, which must be biogenetically closely related to torosachrysone, exhibit strong antibacterial and antifungal activity (24). The structures followed principally from the electronic and H-n.m.r. spectra and from chemical correlation with the acetyl derivatives of austrocortinin (301) [from (330)] and 6-methylxanthopurpurin-3-0-methyl ether (300) [from (331)]. 

It is interesting to note the 5,8-quinone formulation for (330). The predominance of this tautomer was evident from the H-n.m.r. chemical shift of the C-7 proton (5 6.21) and was further consistent with the C-n.m.r. spectrum in which C-5 gave rise to a doublet (8 177.9, /=7.3 Hz) due to three bond coupling with 7-H (5 5). 

The tetrahydroanthraquinone (332) and the corresponding derivative of austrocortilutein have also been isolated and characterised from the red Australian dermocybe (283a) and have subsequently been detected chromatographically in D. splendida and D. umbonata (402). 

---

Besides leaves, roots are also a storage site for the accumulation of many interesting secondary metabolites such as anthraquinones, pre-anthraquinones, anthrones, chromones, alkaloids, flavonoids, conmarins. 

Stereochemical Studies on Pre-Anthraquinones and Dimeric Anthraquinone Pigments... 

In recent years atrochrysone (4) and several other pre-anthraquinones have been isolated from fungi and higher plants (Scheme 2). The parent compound 4 occurs in the toadstools Cortinarius atrovirens and . odoratus (ref. 3). The monomethyl ethers torosachrysone (7) (ref. 4) and asperflavin (8) (ref. 5) are produced by Cassia torosa and Aspergillus flavus, respectively. Torosa-chrysone-8-O-methyl ether (9) has been found in fruitbodies of several toadstools along with its trans-4-hydroxy derivative 10 (ref. 3). Further compounds of this type, e.g. vismione A (11) and B (12), have been isolated from Vismia species (ref. 6). With the only exception of 7, the absolute configuration of these pre-anthraquinones remains unknown. 

Scheme 2. Pre-anthraquinones from fungi and higher plants...

The (3 )-configuration of torosachrysone (7) has been determined by application of the exciton chirality method (ref. 12) to its 3-0-benzoate (ref. 13). In order to obtain rigorous proof of the absolute configuration of the pre-anthraquinones, we have carried out a synthetic correlation with (-)-quinic acid (20) (Scheme 5). 

The largest and most important chemical class within this category comprises the anthraquinone and pre-anthraquinone pigments found in great variety in toadstools belonging to the genera Dermocybe, Cor-tinarius, Tricholoma and Leucopaxillus. 

Scheme 60. Possible relationships between monomeric pre-anthraquinones and anthraqui-...

Dimeric pre-anthraquinones probably are formed by initial phenolic coupling of two dihydroanthracenone units. The sites in the respective aromatic nucL i at which this coupling occurs form the basis for the classification of pigments followed below. 

The stereochemical complexity and chemical sensitivity of many dimeric pre-anthraquinones makes them less amenable to chromatographic identification than the anthraquinones themselves. Consequently, they have proved somewhat more difficult to use for the taxonomy of Phlegmacium (118, 265, 371, 509, 515, 617) than have the anthraquinones to the systematics of Dermocybe. Chemical instability of many pigments causes marked colour changes when sporophores of certain Phlegmacium species are stored in the herbarium (617),... 

By far the most diverse group of dimeric pre-anthraquinones in regard to number and distribution are the flavomannin derivatives (Table 33). Interestingly, compounds of this type are not restricted to Cor-tinarius, subgenus Phlegmacium, but also occur in Dermocybe and in yellow Tricholoma species. Whereas Cortinarius and Dermocybe are closely related, the systematic relationship to Tricholoma is less obvious. 

Of paramount importance to structure elucidation of the flavoman-nin derivatives and of the other groups of dimeric pre-anthraquinones discussed below was the use of n.m.r. spectroscopy. Of special significance in this regard have been the shifts arising as a consequence of the anisotropic influence of the biaryl linkages and the position of... 

Besl and Bresinsky 86) discovered in Leucopaxillus tricolor (Tri-cholomataceae), besides endocrocin (303), two yellow pigments which bore superficial resemblances to asperflavin (329) and to phlegmacin (371). Closer chemical investigations, however, have revealed that these substances represent a new type of dimeric pre-anthraquinone 94,515). 

The chromatographic properties of the dimeric pre-anthraquinones are given in Table 37 (575, 617). 

Cortinarius, especially Subgenus Phlegmacium Yellow - green, red, violet Pre-anthraquinones (Section 3.5.3)... 

---