Establishing Consensus Stereostructures for the Naphthoquinonopyrano-γ-lactone Natural Products (-)-Arizonin B1 and (-)-Arizonin C1 by Total Syntheses. Diastereocontrol of Oxa-Pictet-Spengler Cyclizations by Protective-Group Optimization

Previous total syntheses of arizonin C1 (4) led to opposite assignments of its absolute configuration. Here, we report the fourth total synthesis thereof. In addition, we disclose the first total synthesis of arizonin B1 (3) proceeding differently than via arizonin C1. The stereocenters of the two targets stemmed from an asymmetric dihydroxylation and an ensuing oxa-Pictet-Spengler cyclization. Their configurations were in line with Fernandes' assignments. Protective-group variation in the substrate modulated the diastereoselectivity of the Pictet-Spengler cyclization between 77:23 in favor of a trans disubstitution at C-3a vs. C-5 - used for preparing the natural (-)-arizonins C1 and B1 - and 100:0 in favor of a cis disubstitution - exploited for synthesizing the unnatural (+)-5-epi-arizonins C1 and B1. All naphthalenes of the present study were derived from the (benzyloxy)methoxynaphthalenediol 19. It resulted from a Diels-Alder reaction of the aryne 17a with the siloxyfuran 18a.