Isolation, structure elucidation and racemization of (+)- and (−)-pratensilins A–C: unprecedented spiro indolinone-naphthofuran alkaloids from a marine Streptomyces sp.

Three pairs of new enantiomeric alkaloids with an unprecedented spiro indolinone-naphthofuran skeleton were isolated from a marine Streptomyces sp. The pure enantiomers had a marked difference in the enantiomerization processes for the three compounds. DFT calculations in combination with chemical derivatization were performed to corroborate the racemization process via a keto–enol-type tautomerism.