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 (<b>2</b> and <b>3</b>, respectively) have been isolated from the marine-derived <i>Streptomyces spinoverrucosus</i> strain SNB-048 and, by another group, from the Solar Saltern-derived <i>Streptomyces</i> sp. KMF-004. Two distinct pathways have been proposed for the conversion of the co-occurring pyrrolizidine alkaloid bohemamine D (<b>1</b>) into compound <b>3</b>. Here, we report that the readily prepared compound <b>10</b>, which embodies the 2-hydroxy-1,2-dihydro-3<i>H</i>-pyrrol-3-one core of bohemamine D (<b>1</b>) and is the bis-<i>O</i>-methyl ether of the alkaloid discoipyrrole C, is converted into 1,3-oxazin-6-one <b>11</b> 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 <b>10</b> in the presence of oxygen is also detailed and, again, the possible biosynthetic implications of the resulting conversion are considered.