Highly stereoselective photocyclodimerization of alpha-cyclodextrin-appended anthracene mediated by gamma-cyclodextrin and cucurbit[8]uril: a dramatic steric effect operating outside the binding site.

Photocyclodimerization of alpha-cyclodextrin (CD)-appended anthracene was studied in the presence of gamma-CD and cucurbit[8]uril (CB[8]) hosts to manipulate the stereodifferentiating photoreaction occurring inside the cavity by the bulky attachment located outside. The gamma-CD-mediated photodimerization afforded the head-to-tail photodimers in 98% combined yield, in particular, the syn-head-to-tail photodimer of 91% ee in 68% yield, which are much greater than 32% ee and 44% yield obtained with unmodified anthracene carboxylate. The use of CB[8] also led to a striking inversion of the head-to-tail/head-to-head selectivity, affording exclusively the head-to-head photodimers in 99% combined yield.