The structure of manumycin. III. Absolute configuration and conformational studies

Chromic acid oxidation of manumycin (1), an antibiotic produced by Streptomyces parvulus (strain Tü 64), led to the isolation of 2-(2-methyl-4-oxo-2-pentenoylamino)-5,6-epoxy-1,4-benzoquinone (3) and (-)-(R)-2-methylhexanoic acid (4). From the absolute configuration of 4, determined by comparing its optical rotation with published data, follows the absolute configuration at the center of chirality in the diene side chain of manumycin (1) to be (6'R). Based on the direct comparison of the CD spectra of the two chromic acid oxidation products 2 and 3 with those of the antibiotic G7063-2 (5) and (-)-terreic acid (6) the stereochemistry at C-5 and C-6 of 1 was determined as (5R, 6S). From the negative CD-couplet of manumycin (1) its stereochemistry at C-4 was assigned as (4R).