Intramolekulare Cycloaddition von Nitronen

Abstract

Intramolecular Cycloaddition of NitronesNitrones 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.