Preparation of uracil by cycloreversion. Structure of cycloalkane/ene‐ and norbornane/ene‐fused dihydrouracils

The reactions of the 2‐amino‐1‐cycloalkane‐, cycloalkene‐, norbornane‐ and norbornenecarboxylates 1–9 with potassium cyanate gave urea esters, which were cyclized to cycloalkane‐, cycloalkene‐, norbornane‐ and norbornene‐fused 5,6‐dihydrouracils 10–17 . On cyclization, the urea ester formed from trans ‐4‐cyclohexene‐1‐carboxylate, furnished the cis ‐fused 5,6‐dihydropyrimidine‐2,4(1 H ,3 H )‐dione. On heating, the norbornene‐ diexo ‐fused dihydrouracil 16 yielded 2,4‐pyrimidinedione through the splitting‐off of cyclopentadiene. The structures of the compounds were proved by 1 H and 13 C nmr spectroscopy.