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A search for precursors of lasalocid A has led to the isolation and X-ray crystallographic analysis of an isomer of the antibiotic. The isomer differs from lasalocid A in both the size and absolute configuration of the terminal cyclic ether. These differences lead to speculation on the nature of cyclization mechanisms involved in the biosynthesis of lasalocid A and the other polyether antibiotics.

SumnaryThe structure and absolute configuration of antibiotic X-537A have been determined by physical and chemical methods.~~ THE isolation of three crystalline antibiotics, X-206, X-464, and X-53'iA from three different Streptomyces species was reported from this laboratoryt in 1951. The three antibiotics had similar biological activity as well as the unusual property that their alkali salts were soluble in such nonpolar solvents as benzene and ether, but virtually insoluble in water. Antibiotic X-464 has recently been shown2 to be identical to nigericin, (polyetherin A4) using a combination of i.r., mass spectral, and polarimetric methods. Nigericin belongs to the same class of polyether antibiotics as the m~nensins.~ These compounds have the common property of being monocarboxylic acids containing a number of cyclic ether moieties.Antibiotic X-537A (C34H~408)~ m.p. 110-1 14", [a]:-7.55" (c 1, MeOH) was unique in this class of antibiotics in possessing an aromatic chromophore with U.V. maxima at 248 ( E 6750) and 318nm (E 4200) in 50% aqueous isopropyl alcohol. The sodium salt (C34H5308Na) m.p. l68-l7l0, [a]L5 -30" (G 1, MeOH) had an inflection in the U.V. a t 245 and a maximum at 308nm (E 4100). The presence of a phenolic group was indicated by a positive iron( 111) chloride reaction' in chloroform suggesting that the chromophore was a hydroxybenzoic acid. Both m-and p-hydroxybenzoic acids gave negative iron ( I I I) chloride reactions whereas salicylic acid gave an identical reaction to the antibiotic. Potentiometric titration in 66% dimethylforrnamide gave a single pka = 5.13 (salicylic acid = 4-55, $72-hydroxybenzoic acid = 6.8, 12.9). Further proof of a salicylic chromophore for the antibiotic was the hypsochromic shift in the U.V. on transformation of the free acid to its salt form. Salicylic acid behaves similarly, but wz-hydroxybenzoic acid undergoes a bathochromic shift under these conditions. N.m.r. (CDCl,) of the barium salt showed the presence of an aromatic methyl singlet a t 8 2.14, and two aromatic protons at 8 6.38 and 6.91 (Jortho 8 Hz).

L-2-Amino-4-methoxy-£nz?zs-3-butenoic acid was isolated from a fermentation broth of Pseudomonas aeruginosa ATCC-7700. This substance inhibited the growth of Bacillus species 1283B in a chemically defined medium. The growth inhibition was reversed by a variety of amino acids. An amino acid antimetabolite which inhibited the growth of Bacillus sp. 1283B was discovered in a fermentation broth of Pseudomonas aeruginosa ATCC-7700. The compoundwas isolated from the broth by adsorption onto an anion-exchange column followed by elution with trimethylammonium bicarbonate2), a volatile buffer. The structure was determined to be L-2-amino-4-methoxy-/r£ms-3-butenoic acid, I*, by physical chemical characterization and by reduction to L-2-amino-4-methoxybutanoic acid3), Ilia, identical with a sample prepared by enzymatic resolution of the synthetic racemate4), Illb. An unexpected product, L-2-amino-butanoic acid, II, was isolated on reduction of I employing a platinum-charcoal catalyst. The title compound is the first amino acid reported to contain an enol ether group.

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