636
R. H. THOMSON
those used as dyes or drugs. As a result the anthraquinones now have
some taxonomic value, but this is hardly the case, as yet, with the
others, and caution is needed in deducing phylogenetic relationships
from the presence of the same, or similar, quinones in different plants.
Anthraquinones for example, occur in the monocotyledonous Liliaceae
as well as in several dicotyledonous families, and there are examples in
the benzoquinone and naphthoquinone series of similar quinones appear
ing in apparently unrelated families. Thus, 2,6-dimethoxybenzoquinone
is found in the Simarubeae and the Meliaceae, which occupy similar
systematic positions, but they have little connection with the Ranunculaceae, another source of the same quinone. Hydroxynaphthoquinones
of similar structure occur in the Droseraceae, Plumbaginaceae, and
Juglandaceae; again the first two families are related, but not the third.
The existence of the same or very similar quinones in quite unrelated
plants suggests that they originate from very simple precursors and
may be of little phytochemical significance; it must also be borne in mind
that unrelated plants may produce the same substance by different biochemical pathways.
(1). Anthraquinones. The distribution of anthraquinones in flowering
plants has been reviewed recently by Hegnauer (4). In practice, they
have usually been isolated from roots and bark, although the pigments
are not confined to these tissues as several histochemical studies have
shown (5). In structure the anthraquinones can be divided roughly into
two groups: those that occur in the Rubiaceae (about 30 quinones) and
those which do not (about 10 quinones). The latter are found mainly
in the Polygonaceae (particularly Rumex and Rheum spp.), Rhamnaceae, and Leguminosae (mainly Cassia spp.). Members of both groups
frequently occur as glycosides, a feature not encountered in the natural
benzoquinones or naphthoquinones. The majority are polyhydroxy
derivatives of 2-methylanthraquinone but the pattern of substitution
differs in the two groups, which may be a consequence of their different biogenetic origins. The ß-methyl group occurs in various states of
oxidation, —CH 3 , —CH 2 OH, —CHO, —C0 2 H, or it may be absent
altogether, but no other side chain is found in the angiospermous
anthraquinones in contrast to many of those produced by lower plants
and animals.
The structures of the rubiaceous quinones (VI-XXIII) and most of
the other anthraquinones (XXIV to XXXIII) are shown; Table II gives
the distribution of the second group. There are two anthraquinones that
lack hydroxyl groups: anthraquinone itself, found in quebracho tannin
extracts (Quebrachia lorentzii Engl.) (6) and possibly also in wattle
extracts (Acacia decurrens Willd) (6), and 2-methylanthraquinone
R. H. THOMSON
those used as dyes or drugs. As a result the anthraquinones now have
some taxonomic value, but this is hardly the case, as yet, with the
others, and caution is needed in deducing phylogenetic relationships
from the presence of the same, or similar, quinones in different plants.
Anthraquinones for example, occur in the monocotyledonous Liliaceae
as well as in several dicotyledonous families, and there are examples in
the benzoquinone and naphthoquinone series of similar quinones appear
ing in apparently unrelated families. Thus, 2,6-dimethoxybenzoquinone
is found in the Simarubeae and the Meliaceae, which occupy similar
systematic positions, but they have little connection with the Ranunculaceae, another source of the same quinone. Hydroxynaphthoquinones
of similar structure occur in the Droseraceae, Plumbaginaceae, and
Juglandaceae; again the first two families are related, but not the third.
The existence of the same or very similar quinones in quite unrelated
plants suggests that they originate from very simple precursors and
may be of little phytochemical significance; it must also be borne in mind
that unrelated plants may produce the same substance by different biochemical pathways.
(1). Anthraquinones. The distribution of anthraquinones in flowering
plants has been reviewed recently by Hegnauer (4). In practice, they
have usually been isolated from roots and bark, although the pigments
are not confined to these tissues as several histochemical studies have
shown (5). In structure the anthraquinones can be divided roughly into
two groups: those that occur in the Rubiaceae (about 30 quinones) and
those which do not (about 10 quinones). The latter are found mainly
in the Polygonaceae (particularly Rumex and Rheum spp.), Rhamnaceae, and Leguminosae (mainly Cassia spp.). Members of both groups
frequently occur as glycosides, a feature not encountered in the natural
benzoquinones or naphthoquinones. The majority are polyhydroxy
derivatives of 2-methylanthraquinone but the pattern of substitution
differs in the two groups, which may be a consequence of their different biogenetic origins. The ß-methyl group occurs in various states of
oxidation, —CH 3 , —CH 2 OH, —CHO, —C0 2 H, or it may be absent
altogether, but no other side chain is found in the angiospermous
anthraquinones in contrast to many of those produced by lower plants
and animals.
The structures of the rubiaceous quinones (VI-XXIII) and most of
the other anthraquinones (XXIV to XXXIII) are shown; Table II gives
the distribution of the second group. There are two anthraquinones that
lack hydroxyl groups: anthraquinone itself, found in quebracho tannin
extracts (Quebrachia lorentzii Engl.) (6) and possibly also in wattle
extracts (Acacia decurrens Willd) (6), and 2-methylanthraquinone