Studies Related to the Proposed Biotransformation of Bohemamine D into the Co-occurring Marine Natural Product Spinoxazine B

The 1,3-oxazin-6-one-containing spinoxazines A and B (2 and 3, respectively) have been isolated from the marine-derived Streptomyces spinoverrucosus strain SNB-048 and, by another group, from the Solar Saltern-derived Streptomyces sp. KMF-004. Two distinct pathways have been proposed for the conversion of the co-occurring pyrrolizidine alkaloid bohemamine D (1) into compound 3. Here, we report that the readily prepared compound 10, which embodies the 2-hydroxy-1,2-dihydro-3H-pyrrol-3-one core of bohemamine D (1) and is the bis-O-methyl ether of the alkaloid discoipyrrole C, is converted into 1,3-oxazin-6-one 11 on heating at elevated temperatures in air. The mechanism of this conversion was studied using density functional theory and the biosynthetic implications of it are discussed. The photochemical reaction of compound 10 in the presence of oxygen is also detailed and, again, the possible biosynthetic implications of the resulting conversion are considered.