Alkylpyrazines

Alkylpyrazines are not usually considered as alkaloids, although they are basic nitrogen compounds; they are better known as flavour components, present in very small quantities, and apparently detectable by humans and insects in parts per billion. They are very common among ants, as trail pheromones in their venoms and of unknown function in the mandibular glands of many ponerine ants. Their biosynthesis can be reasonably predicted from the known chemistry of their formation and the knowledge that the amino-acid threonine decomposes by way of oxidation to a (3-keto-acid and decarboxylation to amino-acetone (Figure 9.15). The intermediate dihydropyrazine can react with an aldehyde in a way that has been demonstrated in the laboratory, to give the final alkylpyra-zine. Trisubstituted pyrazines with C3 to C5 alkyl groups and tetrasubsti-tuted pyrazines with an additional isopentyl group (hydroxylated or unsaturated) are found in the mandibular glands of a number of ponerine ants. The trisubstituted pyrazines can be converted to the tetrasubstituted ones in vitro by free radical addition of isopentanal followed by reduction

H3C^OH

H00C^NH2 threonine

HaC^O

HOOC^NH2

NHo O^CHg

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