Intramolekulare Cycloaddition von Nitronen

Intramolecular Cycloaddition of Nitrones Nitrones 9 and 10 , formed by treatment of the corresponding aldehydes with N ‐substituted hydroxylamines, are converted into the cis fused bicyclic compounds 11 and 12 , respectively. While this intramolecular cycloaddition as a rule occurs at room temperature, the formation of compounds 17/18 and/or 20/21 from the corresponding nitrones needs refluxing in toluene with exception of 17Ca . The N ‐aryl‐substituted nitrones 14C/15C form dimers 22C/23C spontanously at room temperature. However, dimerization is reversible; thus 22/23C afford the bicyclic compounds 17/18 and/or 20/21 on heating in toluene via the corresponding nitrone intermediates. — Hydrogenation cleaves isoxazolidine ring of the bicyclic compounds. Thus, 17Ca and 18Ca yield the hexahydrothiepine 27C and oxepane derivatives 28C , respectively, 21Ca gives the tetrahydropyran 30C . Diastereomerically pure 29C arises by prolonged hydrogenation of 17Ca under reinforced conditions via 27C .