Indolenine oxides, IX. Novel polycyclic linearly conjugated cyclohexadiene imines from rearrangement of unstable tetrahydroisoxazolo[2,3‐a]‐indoles

From 1,3‐dipolar cycloaddition of the symmetrically substituted olefins 2a–e, to the sterically overcrowded nitrone 6, the primary cycloadducts, i.e. the 2,3,3a,4‐tetrahydroisoxazolo[2,3‐a]‐indoles 7a–e, are obtained only by direct crystallization from the chilled reaction mixtures in the absence of acid. Upon storage of the solutions at room temperature, especially in the presence of catalytic quantities of acid, or during chromatography on silica gel, 7a, c–e undergo rearrangement to the cyclohexadiene imines 8a, c–e. The latter in turn of limited stability in solution and are transformed into indolines or derivatives of 3H‐indole under partial removal of the epoxyethano bridge. The structure 8a has unambiguously been proven by X‐ray crystallography. — The analogous primary cycloadducts 3a–d of the sterically non crowded nitrone 1 are stable and — with one exception in very low yield — do not undergo the rearrangement reported for 7a, c–e.