The Photochemistry of Enamides

The preparation and properties of enamides and dienamides are described and their synthetically useful photochemical reactions illustrated. Simple enamides generally undergo a [1,3]-acyl radical shift to form vinylogous amides, although exceptions have been noted. The principal photochemical reaction of dienamides is 6π-electron photocyclization reaction to form polycyclic heterocycles. This photocyclization reaction has been extensively used in the preparation of otherwise difficultly accessible heterocycles and in the synthesis of natural products, particularly alkaloids. Examples are given, especially in the formation of phenanthrenes and analogs from stilbenes and stilbene enamides, where other photochemical reactions can occur in molecules containing the enamide chromophore. 1. Preparation and Stability of Enamides 2. Photochemical Reactions of Enamides 3. The Conversion of Stilbene Enamides into 4H-5,6-Dihydro-[de,g]quinolines (Phenanthrene Analogs) 4. Photocyclization Reactions of Dienamides 4.1. Mechanism 4.2. The Synthesis of Polycyclic Heterocycles 5. Dienamide Photocyclizations in Natural Product Synthesis 5.1. Benzylisoquinoline Alkaloids 5.2. Amaryllidaceae Alkaloids 5.3. Indole Alkaloids