Intramolecular, addition Paterno-Buchi reaction

In addition to the intermolecular Paterno-Buchi reaction, the intramolecular variant has also been studied the latter allows for the construction of bicyclic structures in one step. For example the diketone 8 reacts quantitatively to the bicyclic ketone 9 ... 

Addition of an Unsaturated Carbon-Carbon Bond to a Carbonyl Function, intramolecular Paterno-Buchi reactions have been reported in the following systems. It should be pointed out that in most cases cls-trans isomerization, if possible, is a competitive process. 

Oxetans.—It has been known for many years that the electronically excited states of carbonyl compounds will add to double bonds to form oxetans. The majority of examples of the Paterno-Buchi reaction, as it is known, involve addition of a carbonyl triplet state to a tt-system, most commonly an isolated double bond. It is accepted that an intermediate in this reaction is a 1,4-biradical with a lifetime long enough to destroy the stereochemical integrity of an acyclic double bond. Intramolecular reactions are of synthetic value since in many cases they proceed regiospecifically. For example, 3-ent/o-acylbicyclo[3.3.1]non-6-enes give exclusively 2,4-oxo-bridged protoadamantanes upon excitation... 

Irradiation of the enone (80) at 282 nm brings about the formation of the oxetane (81), and this process has been used as a key step in the development of a synthesis of 2,7,9-trimethylenetricyclo[4.3.0.0 ]non-4-ene. Such intramolecular additions are popular methods for the formation of polycyclic compounds that can be used as starting materials in syntheses. An efficient example of this from earlier years is the formation of the oxetane (82,90%) from the intramolecular cyclization of the enone (83). The oxetane formed is a key intermediate in a new efficient route to e Jo-hirsutene. A review of the application of the Diels-Alder/Paterno-Buchi reaction as an approach to di- and tri-quinanes has been published. 

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