Dichotomy in the ring opening reaction of 5-[(2-furyl)methylidene]-2,2-dimethyl-1,3-dioxane-4,6-dione with cyclic secondary amines

5-[(2-Furyl)methylidene]-2,2-dimethyl-1,3-dioxane-4,6-dione (1a) treated with equimolar amount of pyrrolidine or hexahydroazepine afforded 5-(pyrrolidine)- (2a) or 5-[(hexahydroazepine-1-yl)-2-hydroxypenta-2, 4-dien-1-ylidene]-2,2-dimethyl-1, 3-dioxane-4,6-dione (2d). Their treatment with hydrobromic acid led to cyclization and formation of stable 5-cyclopentenyl-4H-1,3-dioxine hydrobromides (3a, 3d). Under the same conditions 1a treated with morpholine or piperidine yielded a mixture of 2b, 3b and 2c, 3c, respectively The corresponding 3-substituted furans 1b-1e gave only substituted 5-cyclopentenyl-4H-1,3-dioxines (3e-3i). The use of an excess amine in reaction with 1a yielded unexpectedly 5-(3,5-dihetaryl-cyclopent-2-en-1-yliden)-2,2-dimethyl-1,3-dioxane-4,6-diones (9a-9c) and 5-[5-hexahydroazepin-1-ium-1-ylidene-2-(hexahydroazepin-1-yl)cyclopent-1-en-1-yl]-2,2-dimethyl-4-oxo-4H-1,3-dioxin-6-olate (10).