The retro Diels-Alder reaction as a valuable tool for the synthesis of heterocycles

Applications of the retro Diels-Alder reaction (cycloreversion) under mild conditions for the laboratory preparation of heteromonocycles or condensed-ring heterocycles are reviewed. A new method utilizes diendo- or diexo-3-aminonorbornene-2-carboxylic acids and their derivatives for the formation of heterocyclic ring systems which are difficult to access by other routes or which can be prepared only in complicated multi-step procedures. The synthesis of substituted 1,3-dienes (e.g. furan derivatives) and O,N- or N,N-heterocycles (oxazoles, isoxazoles, pyrazoles, imidazoles, triazoles, pyrimidinones, oxazinones, pyridazinones etc) are discussed.