Singlet-Oxygen-Mediated Synthesis of Pandanusine A and Pandalizine C and Structural Revision of Pandanusine B

The first synthesis of racemic pandanusines A and B and pandalizine C, isolated from Pandanus amaryllifolius, is reported. The key synthetic step is an efficient tandem reaction sequence initiated by the photooxidation of an easily prepared furylalkylamine precursor. In this reaction sequence, methylene blue plays a dual catalytic role of photosensitizer and redox catalyst, first in generating singlet oxygen and second in facilitating a triplet oxygen reaction. Through the synthesis of pandanusine B, the proposed structure has been revised.