A diastereoselective oxa-Pictet-Spengler-based strategy for (+)-frenolicin B and epi-(+)-frenolicin B synthesis.

An efficient diastereoselective oxa-Pictet-Spengler reaction strategy was developed to construct benzoisochroman diastereomers. The utility of the reaction was demonstrated in the context of both the total synthesis of naturally occurring pyranonaphthoquinones (+)-frenolicin B and epi-(+)-frenolicin B as well as a range of frenolicin precursor analogs. The method is versatile and offers exquisite stereocontrol and, as such, offers a synthetic advance for the synthesis of pyranonaphthoquinone analogs.