R

o

R = OH, laccaic acid HO R = H, xantholaccaic add

OH O CH3

OH O CH3

R = OH, laccaic acid HO R = H, xantholaccaic add

OH O carminic acid

O R R = OH, erythrolaccin

R = H, desoxyerythrolaccin

OH O OH emodin

OH O carminic acid

O R R = OH, erythrolaccin

R = H, desoxyerythrolaccin

OH O OH emodin

15-oxotetratriacontanyl 13-oxodotriacontanoate

Figure 8.16 Some anthroquinone pigments of scale insects, with a presumed parent polyketide. Kermesic acid is the artist's pigment Venetian red and carminic acid is the food colouring cochineal fabae) and protoaphin-s/ (first isolated from the brown willow aphid Tuberolachnus salignus and only differing from the first at one chiral centre) (Figure 8.17). There is restricted rotation about the bond joining the two parts of the molecule. The protoaphins are found in the haemo-lymph. The isolated material is brownish-yellow, but they are substantially ionized at physiological pH, and then give a deep purple colour. Different species of aphid vary in colour through shades of green, brown and red to almost black. Aphins are characteristic of the darker species. The green pigment is aphinin. Some species, e.g. greenfly, Macrosiphium rosae, contain only aphinin. The extraordinary situation exists that the structure of this pigment has been left incompletely solved for 35 years. It is probably as given in Figure 8.17. It was long presumed that these pigments were made by symbiont micro-organisms, but surprisingly, it has been shown that aphids treated with antibiotics and having no bacteria in them still produced aphins.

The aphid Aphis neri feeding on the toxic shrub Nerium oleander is bright orange, possibly as a warning colour (Plate 7). It contains gluco-side B (Figure 8.17) and a number of naphthalene derivatives related to it. Some are shown in Figure 8.18. The structure with the quinone methide a in Figure 8.18 is probably formed by condensation of neriaphin (before the glucoside is attached) with biacetyl and elimination of water, a rare example of biacetyl taking part in the formation of a natural product. The condensation can be repeated in vitro in weakly alkaline conditions.

o o ch3

qA^A/A^X^COOH presumed polyketide oh oh o o ch3

qA^A/A^X^COOH presumed polyketide oh oh

glucoside B OH

o oh oh differ here differ here ho

a perspective view oh a perspective view

Figure 8.17 The formation of the aphins, pigments of aphids. The naphthalene derivative first formed is converted into quinone A and glucoside B, which linked together give the aphins. Note that aphinin-fb and aphinin-รป differ by only one chiral centre. There is restricted rotation about the central bond. Structure a shows the twist in the molecule with the right side of the upper portion turned toward the reader and the right side of the lower portion turned away. The structural study of aphinin-fb has never been completed

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