Maremycins C and D, new diketopiperazines, and maremycins E and F, novel polycyclic spiro-indole metabolites isolated from Streptomyces sp.

New diketopiperazines, named maremycins C-1/C-2 (1a/1b) and D-1/D-2 (2a/2b), as well as the novel spiro-indoles, maremycins E (3) and F (4), have been detected alongside the known maremycin B (6) in the culture broth of Streptomyces sp. (strain GT 051237) by chemical screening. The structures have been determined by detailed NMR spectroscopic investigations of the isolated metabolites. Maremycins C-1/C-2 (1a/1b) as well as D-1/D-2 (2a/2b) are diastereomers and were identified as mixtures. Structurally, maremycins C-1/C-2 (1a/1b) are the diastereomers of the sulfur oxidation products of maremycin B (6), while D-1/D-2 (2a/2b) are the demethylmercapto analogues of maremycins A (5) and B (6), respectively. Maremycins E and F possess a novel structural skeleton, where a spiro moiety is formed between the B-position of the cyclopenta[f]quinoxaline moiety and the 3'-position of the in-dol-2-one moiety of the initial diketopiperazine product.