Photolytic cleavage Norrish Type

Norrish Type I reaction The photolytic cleavage of an aldehyde or ketone to give RCO and Rv Norrish Type II reaction The photolytic cleavage of R2CH-CR2-CR2COR to give R2C=CR2 and... 

A mechanism involving Norrish type I cleavage followed by radical cyclization has been proposed to explain this photolytically induced rearrangement.An alternative possibility might involve bonding from the carbonyl carbon to the 6-carbon followed by alkoxy radical expulsion and ring closure. 

The photochemical cycloaddition of azirine 35 with cyclopentanone has been found to depend on the experimental conditions. When 35 is irradiated and cyclopentanone is slowly added, the expected s/ /ro-S-oxazoline 53 is the main productHowever, when the cyclopentanone is irradiated first and the irradiation is continued in the presence of azirine 35, the sole product is 3-oxazoline 54. Under the latter conditions, cyclopentanone reacts first by a Norrish type I cleavage and hydrogen transfer to yield 4-pentenal. This aldehyde reacts faster with the nitrile ylide than does the cyclic ketone still present, so that only 54 is formed. Norcamphor and camphor also react with azirine 35 under photolytic conditions via the Norrish type I reaction route to give 3-oxazolines 55 and 56. 

---