Cyclenones. 2 + 2 photocycloaddition

The mechanism of the [2 + 2] photocycloaddition of alkenes with cyclenones is a multistep process that involves the addition of alkenes to the 3 7t,tt triplet state of a cyclic enone and the formation of 1,4-biradical intermediates [35]. Except for the reaction of 1-alkenes, up to four new stereogenic centers are formed during the cycloaddition (Scheme 10). 

Scheme 14. [2 + 2] Photocycloaddition of cyclenones with alkenes Asymmetric induction by chiral centers vicinal to the carbonyl group.

Scheme 20. Intramolecular [2 + 2] photocycloaddition Asymmetric induction when the cyclenone is linked to the alkenyl chain at an asymmetric carbon.

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