12. QUINONES: STRUCTURE AND DISTRIBUTION
709
(LI) in Eleutherine bulbosa. Naphthoquinones of the plumbagin type
(XLVIII), the benzoquinones with long alkyl side chains, like embelin
(LXIII) (the side chains all contain an odd number of carbon atoms),
can also be fitted into the acetate scheme without difficulty.
In the animal kingdom evidence concerning the biosynthesis of
quinones is badly needed. Structural comparisons, and the existence of
polyhydroxylated compounds, suggest that an acetate origin is plausible
in many cases, but it must be emphasized once again, that other biogenetic schemes must exist. The polyhydroxynapthoquinones in echinoids
are conceivably derived from acetate units, assuming that oxygen can
be introduced at several points. A common origin for flaviolin (XCIV)
and spinochrome N (CL) seems likely. It has already been noted
that the anthraquinones evolved by crinoids (p. 690) are similar in
structure to some of the lichen quinones. In the insect world, the
anthraquinones can be fitted into an acetate scheme (the formation of
erythrolaccin would require rearrangement of one ring) and so can
the aphin pigments (345) (p. 685). The latter view is supported by the
fact that the xanthoaphins and chrysoaphins, the hydrated forms, and
precursors, of the fully aromatic erythroaphins, are similar to the type
of structure elaborated by bacteria and fungi. Moreover, pigments of the
erythroaphin type are actually produced by fungi (234).
C. ISOPRENOID QUINONES
It has already been noted that thymoquinone (p. 648), perezone
(LXI), and the tanshinones (p. 643) are products of terpene metabolism.
A number of other quinones are partly terpenoid in character, and
belong to a fairly extensive group of natural products in which an isoprene or polyisoprene unit is attached to an aromatic or heterocyclic
ring system. The most obvious examples are the ubiquinones, vitamins
K and plastoquinone; others are lapachol, lomatiol, and dunnione (XLIXLIII), and the quinols auroglaucin, flavoglaucin, and pyrolatin
(LXXXII, LVII). Birch (318) suggested that these hydrocarbon side
chains were introduced biosynthetically by a C-isopentenylation reaction
subsequent to ring formation, the process being analogous to C-methylation and closely related to that involved in the junction of isopentene
units in the formation of terpenes. Evidence supporting this view has
been obtained and it is very striking that the quinones most widely dispersed in plants and animals, are of this type. When Birch et al. (347)
cultured Aspergillus novus in a medium containing 2-C
14 -mevalonic
lactone, the auroglaucin which they isolated was radioactive, the activity
being confined almost entirely to a terminal carbon atom as shown be-
709
(LI) in Eleutherine bulbosa. Naphthoquinones of the plumbagin type
(XLVIII), the benzoquinones with long alkyl side chains, like embelin
(LXIII) (the side chains all contain an odd number of carbon atoms),
can also be fitted into the acetate scheme without difficulty.
In the animal kingdom evidence concerning the biosynthesis of
quinones is badly needed. Structural comparisons, and the existence of
polyhydroxylated compounds, suggest that an acetate origin is plausible
in many cases, but it must be emphasized once again, that other biogenetic schemes must exist. The polyhydroxynapthoquinones in echinoids
are conceivably derived from acetate units, assuming that oxygen can
be introduced at several points. A common origin for flaviolin (XCIV)
and spinochrome N (CL) seems likely. It has already been noted
that the anthraquinones evolved by crinoids (p. 690) are similar in
structure to some of the lichen quinones. In the insect world, the
anthraquinones can be fitted into an acetate scheme (the formation of
erythrolaccin would require rearrangement of one ring) and so can
the aphin pigments (345) (p. 685). The latter view is supported by the
fact that the xanthoaphins and chrysoaphins, the hydrated forms, and
precursors, of the fully aromatic erythroaphins, are similar to the type
of structure elaborated by bacteria and fungi. Moreover, pigments of the
erythroaphin type are actually produced by fungi (234).
C. ISOPRENOID QUINONES
It has already been noted that thymoquinone (p. 648), perezone
(LXI), and the tanshinones (p. 643) are products of terpene metabolism.
A number of other quinones are partly terpenoid in character, and
belong to a fairly extensive group of natural products in which an isoprene or polyisoprene unit is attached to an aromatic or heterocyclic
ring system. The most obvious examples are the ubiquinones, vitamins
K and plastoquinone; others are lapachol, lomatiol, and dunnione (XLIXLIII), and the quinols auroglaucin, flavoglaucin, and pyrolatin
(LXXXII, LVII). Birch (318) suggested that these hydrocarbon side
chains were introduced biosynthetically by a C-isopentenylation reaction
subsequent to ring formation, the process being analogous to C-methylation and closely related to that involved in the junction of isopentene
units in the formation of terpenes. Evidence supporting this view has
been obtained and it is very striking that the quinones most widely dispersed in plants and animals, are of this type. When Birch et al. (347)
cultured Aspergillus novus in a medium containing 2-C
14 -mevalonic
lactone, the auroglaucin which they isolated was radioactive, the activity
being confined almost entirely to a terminal carbon atom as shown be-