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. [ABSTRACT FROM AUTHOR]