By E. P. Abraham, G. G. F. Newton (auth.), David Gottlieb, Paul D. Shaw (eds.)
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A study of the tautomerism of 2-hydroxypyridine-N-oxides as model compounds has also supported the formulation of aspergillic acid as an hydroxamic acid (SHAW, 1949). For these reasons, it is probable that the pyrazinone form given in Fig. 1 a is the betterrepresentation ofthe structure of aspergillic acid. Accordingly, in this review, compounds (Figs. 2-6) related to aspergillic acid that have ultraviolet spectra similar to that of aspergillic acid have been given pyrazinone structures rather than 2-hydroxypyrazine structures.
It is obvious that the aryl ring is not formed by a condensation of acetate units since neither acetate-1-14C or _2_14C resulted in the incorporation of 14C into the ring. The source of this group is also not a preformed aryl ring since neither uniformly labeled phenylalanine nor tyrosine contributed 14C to the phenyl portion of chloramphenicol. An alternative pathway for ring synthesis could be the shikimic pathway; this mechanism is common among the bacteria whereas acetate condensation into aryl rings is more conspicuous among the mycelial fungi.
Fig. 2. Hydroxyaspergillic acid Production In the various investigations discussed below, unless stated otherwise, a spore inoculum of A. flavus was used and the organism was grown aerobically in stationary culture. Aspergillic acid was first isolated by WHITE and HILL (1943) from culture filtrates of A. 5 % NaCl. The yields of aspergillic acid were variable (0-67 mg/liter) and were generally 5-7 mg/liter. It was suggested that the variations in yield were the result of a series of spontaneous mutations of the organism.