Study 6.17 Asymmetric synthesis photocycloaddition

Recently, many investigations have been carried out on the [4 + 4] photocycloaddition of 2-pyridones and its application to organic synthesis. Although the studied reactions involved only racemic materials, high diastereoselectivities have been obtained with chiral substrates. This makes this type of cycloaddition very promising for asymmetric synthesis [93]. 

Asymmetric [2 + 2] Photocycloadditions. Intramolecular copper-catalyzed [2 + 2] photocycloaddition is a useful methodology for the preparation of bicyclic cyclobutanes and recent studies deal with its asymmetric version albeit with variable success. Diastereoselective reactions are achieved under the control of stereogenic centers incorporated in the dienic precursors. Both CuOTf and the more stable and easy to handle Cu(OTf)2 are suitable catalysts in this context. In the latter case, it is assumed that the copper(I) species is generated from Cu(OTf)2 under the photochemical conditions. A noteworthy example is the application of the CuOTf-catalyzed [2 + 2] photocycloaddition in the stereoselective total synthesis of the tricyclic sesquiterpene kel-soene (eq 128). ... 

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