646
R. H. THOMSON
found in the fresh leaves and immature fruits of this shrub is the
diketone (XLVII), the only example of the natural occurrence of such
a "ß-hydronaphthoquinone" (32). Another pigment found in the
Diospyros genus is the orange-red o-quinone, diosquinone (XLIV) isolated from the bark of D. tricolor Hiern (33). An orange isomer,
juglone (XLV), is similar in structure to plumbagin and related
quinones but confined apparently to members of the Juglandaceae (I,
34). It exists in a reduced form, as the 4-/?-D-glucoside of a-hydrojuglone (1,4,5-trihydroxynaphthalene) (35, 36), in most parts of the
walnut tree, subject to seasonal variation (37). The highest concentration (< 8%) occurs in male catkin buds in the dormant winter state, but
the quinone is most conveniently obtained from green walnut shells. A
phenoloxydase has been isolated from the husks, and when this is
added to an aqueous solution of hydrojuglone glucoside, juglone soon
appears (35). It has often been observed that walnut trees are poisonous
to surrounding plants, which sometimes creates problems in horticulture
(38). Juglone is generally considered to be the toxic principle, and the
experiments of Bode (39) indicate that it is carried down from the
leaves onto neighboring plants by rain. Walnut shells and the fruits of
various Diospyros species have been used to poison fish, D. ebenum
has been suspected of poisoning livestock in Australia, and the sap of
D. mariti,ma Blume is vesicant. All these plants contain juglone or Cmethyljuglones, either as such or in a reduced form.
The only other chemically related group of naphthoquinones consists of lapachol (XLI), lomatiol (XLII), and the o-quinone dunnione
(XLIII); in the latter, which is isomeric with lapachol, the isopentenyl
side chain has become part of a five-membered ring. [There is a third
o-naphthoquinone, biflorin, which occurs in the leaves and petals of
Capraria biftora Linn. (Scrophulariaceae) (40). Its structure has not
yet been elucidated.] These three quinones have been isolated from
different parts of quite unrelated plants. The yellow pigment lapachol
occurs principally in the heartwoods of the Bignoniaceae [7.6% in
Tecoma araliacea DC. (41)], whereas dunnione is found as an orangered deposit on the leaves, stems, and flowers of Streptocarpus Dunnii
Mast., the very large leaves bearing as much as 2 gm. of pigment (42).
Lomatiol forms the yellow outer layer of the seeds of a number of (but
not all) Lomatia spp. (43). In L. longifolia R.Br. it is found also in
the ripe fruits (3.5%) but more abundantly in the seeds (12%), and it
may be present in the bark of L. ilicifolia, which has been used for
dyeing purposes. Several naphthoquinone-containing plants have been
used as natural dyes, but none ever attained the importance of the
anthraquinones. Lawsone (XXXIX) is the dyeing principle of henna,
R. H. THOMSON
found in the fresh leaves and immature fruits of this shrub is the
diketone (XLVII), the only example of the natural occurrence of such
a "ß-hydronaphthoquinone" (32). Another pigment found in the
Diospyros genus is the orange-red o-quinone, diosquinone (XLIV) isolated from the bark of D. tricolor Hiern (33). An orange isomer,
juglone (XLV), is similar in structure to plumbagin and related
quinones but confined apparently to members of the Juglandaceae (I,
34). It exists in a reduced form, as the 4-/?-D-glucoside of a-hydrojuglone (1,4,5-trihydroxynaphthalene) (35, 36), in most parts of the
walnut tree, subject to seasonal variation (37). The highest concentration (< 8%) occurs in male catkin buds in the dormant winter state, but
the quinone is most conveniently obtained from green walnut shells. A
phenoloxydase has been isolated from the husks, and when this is
added to an aqueous solution of hydrojuglone glucoside, juglone soon
appears (35). It has often been observed that walnut trees are poisonous
to surrounding plants, which sometimes creates problems in horticulture
(38). Juglone is generally considered to be the toxic principle, and the
experiments of Bode (39) indicate that it is carried down from the
leaves onto neighboring plants by rain. Walnut shells and the fruits of
various Diospyros species have been used to poison fish, D. ebenum
has been suspected of poisoning livestock in Australia, and the sap of
D. mariti,ma Blume is vesicant. All these plants contain juglone or Cmethyljuglones, either as such or in a reduced form.
The only other chemically related group of naphthoquinones consists of lapachol (XLI), lomatiol (XLII), and the o-quinone dunnione
(XLIII); in the latter, which is isomeric with lapachol, the isopentenyl
side chain has become part of a five-membered ring. [There is a third
o-naphthoquinone, biflorin, which occurs in the leaves and petals of
Capraria biftora Linn. (Scrophulariaceae) (40). Its structure has not
yet been elucidated.] These three quinones have been isolated from
different parts of quite unrelated plants. The yellow pigment lapachol
occurs principally in the heartwoods of the Bignoniaceae [7.6% in
Tecoma araliacea DC. (41)], whereas dunnione is found as an orangered deposit on the leaves, stems, and flowers of Streptocarpus Dunnii
Mast., the very large leaves bearing as much as 2 gm. of pigment (42).
Lomatiol forms the yellow outer layer of the seeds of a number of (but
not all) Lomatia spp. (43). In L. longifolia R.Br. it is found also in
the ripe fruits (3.5%) but more abundantly in the seeds (12%), and it
may be present in the bark of L. ilicifolia, which has been used for
dyeing purposes. Several naphthoquinone-containing plants have been
used as natural dyes, but none ever attained the importance of the
anthraquinones. Lawsone (XXXIX) is the dyeing principle of henna,