Intramolecular Hydrogen Abstraction by the C—O Double Bond

The process of intramolecular abstraction of an hydrogen atom from the y carbon atom by the excited carbonyl group is commonly referred to as Norrish type II reaction. The resulting diradical can close to a cyclo- 

This efficient cyclobutanol forming reaction has been utilized in the synthesis of terpenes404) (4.4), medium ring compounds40s) (4.5) and strained molecules as in (4,6)406) and (4.7) 407). In the last example the tricycloP. O.O jhexane skeleton is formed. 

The cleavage of the intermediate 1,4-diradical can also become the major path as in the synthesis of methylenecyclopentane (4.8) 408). 

If no y-H atom is available, or if for sterie reasons abstraction of a 6-H atom is facilitated, this latter reaction occurs with formation of a cyclopentanol. A series of such sequences has been used in the synthesis of dodecahedrane 409a,b . 

An interesting application of this Norrish type II reaction consists in the photochemical oxydation of alcohols to carbonyl compounds by irradiation of their pyruvic esters (4.11) 412). 

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