The Ajudazols A and B, Novel Isochromanones fromChondromyces crocatus (Myxobacteria): Isolation and Structure Elucidation

European Journal of Organic Chemistry
2002.0

Abstract

Two novel metabolites, ajudazols A (1) and B (2), were isolated in a screening of the myxobacterial genus Chondromyces from several strains of C. crocatus. Both 1 and 2 are unique isochromanone derivatives with an extended side chain containing an oxazole, a Z,Z-diene, and a 3-methoxybutenoic acid amide as characteristic structural features. In this paper we report on the isolation and structural elucidation of the ajudazols A (1) and B (2), a further group of novel metabolites produced by C. crocatus. Even though the ajudazols failed to show any significant biological activity in our primary screening, their ease of access and their promising UV and NMR spectra prompted their isolation as promising candidates for secondary screenings. The ajudazols were isolated from the acetone extract of the wet cell mass of C. crocatus by partition between methanol and heptane which initially removed most of the more lipophilic by-products. Separation and purification of 1 and 2 was accomplished by consecutive chromatographic separations on RP-18 silica gel and Sephadex LH-20. Due to a severe decomposition of ajudazols during normal-phase silica-gel chromatography, this step was avoided by the modified separation procedure. The structural elucidation of the ajudazols was based mainly on data obtained from ajudazol A (1). High resolution EI MS of the molecular ion at m/z = 590 furnished the elemental composition C34H42N2O7, which implies 15 double bond equivalents. The UV spectrum showed two intense, broad bands at 213 and 235 nm and a weak band at 320 nm while the IR spectrum indicated lactone, amide and hydroxy groups by bands at 1710, 1674, 1650 and 3360 cm−1. The structure elucidation by NMR spectroscopy was hampered by a broadening and doubling of the signals, although this could be overcome by measuring the spectra at 80 °C in [D6]DMSO. Taking the direct and stronger long-range correlations in the 1H,1H-COSY NMR spectra into account three basic structural elements, designated as A, B and C, were recognized. These were complemented with the remaining quaternary carbons from the correlations derived from the 1H,13C-HMBC spectra. Structural element A is an isochromanone substituted with a methyl group, a chelated phenol and a secondary benzylic alcohol. The extension of unit A with an oxazole residue was indicated by 1H,13C-HMBC correlations between methylene C-11 and the aromatic methine C-13. The linking of the oxazole and structural element B by a vinylidene group was indicated by HMBC correlations. The remaining carbon atoms were connected to structural part C to give the terminal methoxybutenoic acid methyl amide residue. Vicinal coupling constants and NOE correlations confirmed the E configuration of the Δ23 double bond, Z configuration of both Δ17 and Δ19 double bonds, and Z configuration of 27-H and 28-methoxy group.

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