Controlling the Substitution Pattern of Hexasubstituted Naphthalenes by Aryne/Siloxyfuran Diels-Alder Additions: Regio- and Stereocontrolled Synthesis of Arizonin C1 Analogs

3,4-Dimethoxybenz-1-yne and 2-siloxylated furans without or with a bromine atom at C-3 undergo Diels-Alder reactions with orientational selectivity. Hydrolysis furnished a bromine-free or a bromine-containing naphthalene, respectively. Bromination of the former provided a regioisomer of the latter. Either of the two compounds was processed to give a variety of unnatural naphthoquinonopyrano-gamma-lactones. This occurred by a succession of (1) Heck coupling, (2) asymmetric dihydroxylation, (3) oxa-Pictet-Spengler cyclization, and (4) oxidation. The fifteen monomeric naphthoquinonopyrano-gamma-lactone structures that we prepared resemble the natural product (-)-arizonin C1 or its C-5 epimer. Accordingly, they represent hexasubstituted naphthalenes likewise. The sixteenth naphthoquinonopyrano-gamma-lactone that we synthesized is a kind of dimer. Its moieties are bridged differently than those in naturally occurring naphthoquinonopyrano-gamma-lactone dimers.