The (apparently disrotatory) thermal ring opening of 4‐nitro‐1‐cyclobuten‐1 ‐amines

A series of 4‐nitro‐1‐cyclobuten‐l‐amines 3 was isolated from the thermal (2 + 2) cycloaddition of nitro(cyclo)alkenes 1 and ynamines 2. Heating of 3a‐d yielded products corresponding with disrotatory ring opening viz. the 1,3‐dienes 6a‐d. Heating of 3e,f at 60°C gave the dienes 7a,b, as the result of a conrotatory ring opening reaction, together with the trans‐2‐nitrocyclobutenes 8a,b, as the result of a hydrogen shift in the cyclobutene ring. Upon prolonged heating at 80°C 8b was converted into the diene 7b. Treatment of 3e with acid yielded the 2‐nitrocyclobutenes 9a and 9b. The structures of 3a, 3f, 6a and 7b were elucidated by X‐ray analysis. The formation of 6a‐d is attributed to E/Z isomerization of the product, initially formed by a conrotatory ring opening.