Diastereofacial selectivity photocycloadditions

The intermolecular photocycloaddition of alkenes to cyclic enones was found to afford cis- and trans-fused bicyclic systems. This stereoselectivity and the diastereofacial selectivity of chiral alkenes and/or enones is discussed below. 

High stereoselectivity was found in the first example of intramolecular [2 + 2] photocycloaddition discovered by Ciamician and Silber111 in the irradiation of camphor 233 to carvon camphor 234. Generally, if the tethered alkenyl side chain is connected to the enone via an asymmetric center, the configuration of this center plays an important role on the diastereofacial selectivity. For example, high stereoselectivity was found in the irradiation of the diketone 235 or its corresponding enol acetates112, when substituents on the alkenyl side chain affect the selectivity of the enolization of 235 but not the diastereofacial selectivity (Scheme 50). 

Systematic study on the diastereofacial selectivity in the intramolecular photocycloaddition of alkenes to chiral dioxinones was recently reported by Haddad and coworkers129 on compounds of type 298. Preferred pyramidalization in the direction of the less exposed side (the axial methyl at the acetal center) described in structure 298b, and first bond formation at this position (found to be the case in dioxinones 143 and 146, Scheme 31), are essential features for obtaining selective photocycloadditions of alkenes to chiral dioxinones from this side, leading to the kinetically favored products. In such cases the preferred approach is not necessarily from the more exposed side (Figure 6). 

---