Circular Dichroism and Conformational Analysis of Diastereomeric Bicamphors

Stereospecific syntheses afforded endo,endo- (1) and exo,exo- (2) bicamphors, while the third possible diastereomeric exo,endo-bicamphor (3) originated from nonselective camphor radical dimerization. The stereochemistry of bicamphor linkage was confirmed by 1H NMR analysis. Chiroptical and ultraviolet spectral data are presented for the three diastereomers 1-3 to show interchromophoric interaction. Conformational analysis to evaluate the relative orientation of each pair of carbonyl chromophores was accomplished by 1H NMR spectroscopy and molecular mechanics calculations.

Directed biosynthesis of novel derivatives of echinomycin by Streptomyces echinatus. I. Effect of exogenous analogues of quinoxaline-2-carboxylic acid on the fermentation

Streptomyces echinatus A8331 cultured on a maltose minimal salts medium normally produces a single antibiotic, echinomycin (quinomycin A), containing two quinoxaline-2-carbonyl chromophores. Echinomycin is powerfully active against experimental tumours and can be assayed by its activity against Gram-positive bacteria. Grown in the presence of aromatic carboxylic acids related to quinoxaline, S. echinatus responds in favourable circumstances by incorporating the added material into analogues of the natural antibiotic having replacement chromophores. Both mono- and bis-substituted derivatives are formed. With quinoline-2-carboxylic acid as precursor, large quantities of analogues are produced, and the time course of synthesis, extraction, purification, assay, and characterization of the derivatives are described. Twenty-two other aromatic acids have been tested as potential substrates for antibiotic analogue biosynthesis. Half of them did not significantly affect growth and echinomycin production. Five appeared to stimulate antibiotic synthesis, while the remainder proved inhibitory. New biologically active antibiotics were detected in cultures supplemented with 7-chloroquinoxaline-2-carboxylic acid; 1,2,4-benzo-as-triazine-3-carboxylic acid; thieno[3,2-b]pyridine-5-carboxylic acid; and 6-methylquinoline-2-carboxylic acid.