Chiral auxiliaries, diastereoselectivity, asymmetric facial selectivity

Control of absolute asymmetry is a relatively untouched area for [2 + 2] photochemical cycloaddition reactions despite the recent advances in the field of asymmetric synthesis. The first example of the use of a removable chiral auxiliary was reported by Tolbert, who obtained impressive enantioselectivity in the photocycloaddition of bomyl fumarate to stilbenes (equation 37). More recently, Lange has shown that menthyl cyclohexenonecarboxylates are useful in control of absolute stereochemistry (equation 38). Baldwin and Meyers have also obtained excellent facial selectivity in systems where the stereogenic center which controls the diastereoselectivity can be excised to afford products of high enantiomeric purity (equations 39,40). 

---