The nitrones derived from β-allyloxyaldehydes with a β-quaternary centre cyclized to give six- instead of seven-membered O-heterocycles exclusively whereas a nitrile oxide derived from a γ-allyloxyaldehyde with a γ-quaternary center cyclized to yield an oxepane ring as the sole product. Using the latter intramolecular nitrile oxide cycloaddition as the key step, optically active oxepane 5, which is a suitable intermediate for a synthesis of zoapatanol 1, has ...
