Seven-membered rings

Progress in the synthesis of seven-membered heterocycles bearing up to and including three N, O, or S heteroatoms continues unabated. The application of classical ring expansion strategies continues to be investigated. However, many original ring expansion approaches were also developed to synthesize these frameworks. Cyclization methodology remains a cornerstone in the construction of seven-membered heterocycles, with Dieckmann cyclizations, Friedel–Crafts alkylations and acylations, Pictet–Spengler-type and Bischler–Napieralski-type reactions, and ring-closing metatheses successfully deployed. Enabling cascade reactions and powerful catalytic processes continue to enhance access to seven-membered heterocycles. Total syntheses of natural products containing oxepane, azepane, and 1,3,6-oxadiazepane cores were completed. There was only one report of the preparation of a seven-membered ring containing four N, O, or S heteroatoms–a tricyclic tetrazepine.