Dearomatization by Photocycloaddition

SCHEME 15.11 Dearomatization pathways in arene-alkene photocycloadditions. 

Related dearomatizations by [2+2] and [4+2] photocycloadditions are less common. It has been observed that cycloadditions featuring high degrees of charge separation in the reactive exciplex favor the [2+2] pathway [27a]. Consequently, examples of this reaction often involve arenes and/or alkenes substituted with electron-withdrawing groups. Benzocyclobutane products formed in [2+2] 

SCHEME 15.12 Synthesis of a-cedrene by intramolecular arene [3+2] photocycloaddition. 

Aromatic k systems are well-known participants in various pericyclic reactions, and aromaticity of substrates is often transiently disrupted in these transformations (e.g., Claisen rearrangement of O-allyl phenol). In most cases, a pathway for rearomatization exists (such as tautomerization) that ultimately leads to substituted aromatic products. In other instances, dearomatization upon pericyclic rearrangement results in formation of stable ahcyclic products, and this tactic has been successfully employed in several synthetic studies. 

Certain functionalized allyl phenyl ethers undergo Claisen rearrangement to afford stable cyclohexadienones. Steric congestion in the substrates and products is believed to facilitate these transformations [35]. For example, dimethylallyl ether 38 undergoes regioselective thermal 

---