12. QUINONES: STRUCTURE AND DISTRIBUTION
649
alia from R. ambigua Lav. (67) and S. vernicifera Hayata (67) have a
Ci 7 side chain. Thitsiol is a catechol alkylated (C 17 ) at position 4 (67).
Several simple benzoquinones are known only in a reduced form,
usually as a glucoside, but there are scattered references (68) to the
occurrence of hydroquinone, and also catechol, in the free state, in the
leaves of several plants. These are possibly derived from gentisic (69)
and protocatechuic (68) acids, respectively, which occur widely. Arbutin (LV) (hydroquinone monoglucoside) occurs in several families,
more especially in the Ericaceae, sometimes accompanied by its methyl
ether (methylarbutin). Usually it is present in the leaves, but in Geum
rivali Bourg. it occurs also in the roots (70), and in Rhododendron
pontica Linn., in the flowers, leaves, and stems (71). The genus Pyrola
is of particular interest; P. rotundifolia Linn, produces arbutin (72),
homoarbutin (LVI) (73), and the naphthoquinone chimaphilin
(XXXVIII), and pyrolatin (LVII) has been isolated from the leaves of
P. japonica Sieb. (74). The latter has a Ci 0 terpenoid side chain.
2,6-Dimethoxybenzoquinone has been found in several dicotyledonous
plants, but in wheat it is present in a reduced form. Both 2-methoxyand 2,6-dimethoxyquinol occur in wheat germ and wheat bran (75);
the former has been isolated from the germ as its monoglucoside (76).
The most interesting and most important benzoquinones produced
by higher plants are the ubiquinones (coenzymes Q) and plastoquinone (76a), which may occur generally in green leaves. Plastoquinone (2,3-dimethyl-5-solanesyl-l,4-benzoquinone, previously Q 25 4)
(LXVII) was first isolated by Kofler (78) from alfalfa and from horse
chestnut leaves and was also identified spectroscopically in the leaves of
oats, ivy, and stinging nettle. Recent investigations in the Merck (79)
and Hoffman-La Roche (80, 81) laboratories have established the structure of this quinone, which has a C 45 polyisoprenoid side chain and is
closely related to the ubiquinones. Kofler and co-workers (80) have
isolated plastoquinone from the leaves of some twenty plants (oak, ash,
beech, sycamore, willow, fig, sunflower, etc.), and Crane (82), using
spinach, has shown that it is concentrated in the chloroplasts. In general, it has been detected in the photosynthetic tissues of all higher
plants examined but is not limited exclusively to chlorophyll-containing
tissues. The work of Crane (82) revealed that plastoquinone is present
in the variegated leaves of the monocotyledon Pandanus vetchii
(Pandanacea), but the amount present in the green portions (0.0173 mg.
per gram fresh weight) is ten times that in the white portions
(0.0017 mg. per gram). Small amounts also exist in corn roots (Zea
mays) and cauliflower buds, which are capable of developing chloroplasts, and the quinone is possibly present in the proplastids of these
649
alia from R. ambigua Lav. (67) and S. vernicifera Hayata (67) have a
Ci 7 side chain. Thitsiol is a catechol alkylated (C 17 ) at position 4 (67).
Several simple benzoquinones are known only in a reduced form,
usually as a glucoside, but there are scattered references (68) to the
occurrence of hydroquinone, and also catechol, in the free state, in the
leaves of several plants. These are possibly derived from gentisic (69)
and protocatechuic (68) acids, respectively, which occur widely. Arbutin (LV) (hydroquinone monoglucoside) occurs in several families,
more especially in the Ericaceae, sometimes accompanied by its methyl
ether (methylarbutin). Usually it is present in the leaves, but in Geum
rivali Bourg. it occurs also in the roots (70), and in Rhododendron
pontica Linn., in the flowers, leaves, and stems (71). The genus Pyrola
is of particular interest; P. rotundifolia Linn, produces arbutin (72),
homoarbutin (LVI) (73), and the naphthoquinone chimaphilin
(XXXVIII), and pyrolatin (LVII) has been isolated from the leaves of
P. japonica Sieb. (74). The latter has a Ci 0 terpenoid side chain.
2,6-Dimethoxybenzoquinone has been found in several dicotyledonous
plants, but in wheat it is present in a reduced form. Both 2-methoxyand 2,6-dimethoxyquinol occur in wheat germ and wheat bran (75);
the former has been isolated from the germ as its monoglucoside (76).
The most interesting and most important benzoquinones produced
by higher plants are the ubiquinones (coenzymes Q) and plastoquinone (76a), which may occur generally in green leaves. Plastoquinone (2,3-dimethyl-5-solanesyl-l,4-benzoquinone, previously Q 25 4)
(LXVII) was first isolated by Kofler (78) from alfalfa and from horse
chestnut leaves and was also identified spectroscopically in the leaves of
oats, ivy, and stinging nettle. Recent investigations in the Merck (79)
and Hoffman-La Roche (80, 81) laboratories have established the structure of this quinone, which has a C 45 polyisoprenoid side chain and is
closely related to the ubiquinones. Kofler and co-workers (80) have
isolated plastoquinone from the leaves of some twenty plants (oak, ash,
beech, sycamore, willow, fig, sunflower, etc.), and Crane (82), using
spinach, has shown that it is concentrated in the chloroplasts. In general, it has been detected in the photosynthetic tissues of all higher
plants examined but is not limited exclusively to chlorophyll-containing
tissues. The work of Crane (82) revealed that plastoquinone is present
in the variegated leaves of the monocotyledon Pandanus vetchii
(Pandanacea), but the amount present in the green portions (0.0173 mg.
per gram fresh weight) is ten times that in the white portions
(0.0017 mg. per gram). Small amounts also exist in corn roots (Zea
mays) and cauliflower buds, which are capable of developing chloroplasts, and the quinone is possibly present in the proplastids of these