Photooxygenation, of enamines

In the dye-sensitized photooxygenation of enamines, the carbon-carbon double bond is cleaved by singlet oxygen to give a ketone (or aldehyde) and an amide4,5 (Scheme 1). 

Photooxygenation of enamines by singlet oxygen has been proved to be particularly useful for the synthesis of natural products, for which mild reaction conditions must be used26. An example is given in Scheme 12. 

Dioxetanes are useful precursors to various oxidation products, such as carbonyl and carboxyl compounds. Photooxidative double bond cleavage, an alternative to ozonolysis, can successfully be adopted for the preparation of large heterocyclic rings or for ringopening procedures. For example, photooxygenation of enamine 526 leads to the 10-membered product 527 in 84% chemical yield via a dioxetane intermediate 528 (Scheme 6.255).1441... 

Photooxygenations of enamines. Photooxygenation of enamines of cyclic ketones at room temperature in methanol leads to a-diketones. If the reaction is conducted at -78 intermediate dioxetanes can be isolated. Thus photooxygenation of N-cyclohexenylmorpholine (1) at 25 gives cyclohexane-1,2-dione (3), isolated as the bisphenylhydrazone, in 75% yield. The intermediate... 

From the oxidation of enamines with aromatic nitro compounds a-keto-enamines were obtained in modest yields (70J). Photooxygenation led to cleavage of the enamine double bond (706,707). 

Protopine alkaloids. A notably efficient route to protopanes is the photooxygenation (Rose Bengal or Methylene Blue) of an enamine such as 1 to give an amido ketone (2), which can be converted into 3 by reduction with lithium aluminum hydride in THF followed by oxidation with activated manganese dioxide. The overall yield is about 65%. 

A slurry of 3-ketobisnor-4-cholen-22-al 22-morpholine enamine in dimethyl-formamide containing rose bengal as sensitizer photooxygenated 17.5 hrs. at 15° progesterone. Y ca. 100%. J. E. Huber, Tetrah. Let. 1968, 3271. 

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