12. QUINONES: STRUCTURE AND DISTRIBUTION
707
ment of this type might account for the formation of the two physcion
anthrones and for other anthraquinones which appear to be acetate
derived but have a "wrong" substitution pattern in one ring.
The isotopic studies referred to above, and many others, seem to
indicate that the acetate route is the principal one used by fungi in
the biosynthesis of aromatic compounds. Most fungal quinones fit into
this scheme [6-methyl-l,4-naphthoquinone (XCIII) and boletol (CXII)
may be exceptions], but other modes of biogenesis are, of course, possible. Tanenbaum and Bassett (343) have found that Penicillium patulum
produces a variety of acetate-derived aromatic compounds (e.g., 6methylsalicyclic acid) and also pyrogallol, which appears to arise by the
shikimate pathway. The biosynthesis of quinones by the latter route is
obviously possible, as we have seen already. It will be recalled that
tetrahydroxybenzoquinone is formed by bacterial oxidation of myoinositol (p. 675), which suggests other possibilities.
Little experimental evidence is available on the biogenesis of
quinones other than those formed by fungi, but there is structural
evidence which suggests that many other quinones are built up from
acetate units. The fact that nearly all natural derivatives of naphthalene,
and more particularly anthracene, are quinones is perhaps a pointer in
this direction, since the acetate pathway leads directly to polyphenolic
derivatives (especially 9-anthrones) which are readily convertible to
quinones without the aid of enzymes. It can be assumed that lichen
anthraquinones (pp. 665-666) are formed from acetate precursors; this
route appears to operate in bacteria. The pyrromycinones and related
pigments produced by Streptomycetes, are striking examples of aromatic
compounds formed by head-to-tail linkage of acetate units (CLXXVI)
(166), and the structure determinations of Brockmann
(164),
Prelog (165), and Ollis (166) were based, in part, on the acetate
theory. c-Pyrromycinone containing a hydroaromatic ring with all
the oxygen atoms still in position, is presumably an intermediate
stage en route to the fully aromatic 77-pyrromycinone. The tracer
studies of Ollis and co-workers (344) have established that the carbon skeleton of 6-pyrromycinone is of acetate origin but the C-ethyl
group (see CLXXVI) is actually derived from propionate. This is the
first clear demonstration of mixed acetate-propionate biogenesis and it
exemplifies a further modification of the simple acetate theory.
In higher plants, the occurrence of anthraquinones of the emodin
type in the Rhamnaceae, Polygonaceae, and other families, implies that
these compounds originate in the same way as in the fungi, and the
frequent occurrence of the corresponding anthrones supports this view.
Birch (345) has pointed out that both anthraquinones and phenanthrene-
707
ment of this type might account for the formation of the two physcion
anthrones and for other anthraquinones which appear to be acetate
derived but have a "wrong" substitution pattern in one ring.
The isotopic studies referred to above, and many others, seem to
indicate that the acetate route is the principal one used by fungi in
the biosynthesis of aromatic compounds. Most fungal quinones fit into
this scheme [6-methyl-l,4-naphthoquinone (XCIII) and boletol (CXII)
may be exceptions], but other modes of biogenesis are, of course, possible. Tanenbaum and Bassett (343) have found that Penicillium patulum
produces a variety of acetate-derived aromatic compounds (e.g., 6methylsalicyclic acid) and also pyrogallol, which appears to arise by the
shikimate pathway. The biosynthesis of quinones by the latter route is
obviously possible, as we have seen already. It will be recalled that
tetrahydroxybenzoquinone is formed by bacterial oxidation of myoinositol (p. 675), which suggests other possibilities.
Little experimental evidence is available on the biogenesis of
quinones other than those formed by fungi, but there is structural
evidence which suggests that many other quinones are built up from
acetate units. The fact that nearly all natural derivatives of naphthalene,
and more particularly anthracene, are quinones is perhaps a pointer in
this direction, since the acetate pathway leads directly to polyphenolic
derivatives (especially 9-anthrones) which are readily convertible to
quinones without the aid of enzymes. It can be assumed that lichen
anthraquinones (pp. 665-666) are formed from acetate precursors; this
route appears to operate in bacteria. The pyrromycinones and related
pigments produced by Streptomycetes, are striking examples of aromatic
compounds formed by head-to-tail linkage of acetate units (CLXXVI)
(166), and the structure determinations of Brockmann
(164),
Prelog (165), and Ollis (166) were based, in part, on the acetate
theory. c-Pyrromycinone containing a hydroaromatic ring with all
the oxygen atoms still in position, is presumably an intermediate
stage en route to the fully aromatic 77-pyrromycinone. The tracer
studies of Ollis and co-workers (344) have established that the carbon skeleton of 6-pyrromycinone is of acetate origin but the C-ethyl
group (see CLXXVI) is actually derived from propionate. This is the
first clear demonstration of mixed acetate-propionate biogenesis and it
exemplifies a further modification of the simple acetate theory.
In higher plants, the occurrence of anthraquinones of the emodin
type in the Rhamnaceae, Polygonaceae, and other families, implies that
these compounds originate in the same way as in the fungi, and the
frequent occurrence of the corresponding anthrones supports this view.
Birch (345) has pointed out that both anthraquinones and phenanthrene-