166 Heterocycles in Nature
hydroxyl groups markedly affects the UV–Vis absorption. The petals of hydrangeas, which can exhibit any colour from
blue to red, contain delphinidin 3-O-glucoside, shown below in protonated and O-deprotonated forms.
The naturally occurring fl avones (page 77) are yellow and are very widely distributed in plants. They accumulate in
almost any part of a plant, from the roots to the fl ower petals. Flavones are stable and have, from time immemorial, been
used as dyes, because they impart various shades of yellow to wool, for example the commercial ‘quercitron bark’ from a
North American oak, Quercus velutina, contains quercetrin. The corresponding aglycone, quercetin, is one of the most
widely occurring fl avones, found, for example, in Chrysanthemum and Rhododendron species, horse chestnuts, lemons,
onions and hops. Coumarin has the sweet scent of newly-mown hay, and is used in perfumes (see pages 76, 191, 192).
In recent years explorations of sea animals have thrown up a staggering variety of heterocyclic substances, and we
present just a few structures here, as illustrations. Many contain halogen(s), as one might anticipate from creatures living
in the marine environment – tetrabromopyrrole and hexabromo-2,2'-bipyrrole are amongst the structurally simplest.
Ascididemine (tunicate, Didemnum sp.), variolin B (sponge, Kirkpatrickia varialosa), lamellarin B (mollusc, Lamellaria
sp.), diazonamide A (ascidian, Diazona chinensis) and telomestatin (Streptomyces anulatus) all have multi-heterocycle
structures, which challenge the ingenuity of synthetic chemists who might seek to achieve their total synthesis.