Novel asymmetric photodimerization reaction of coumarin derivatives bearing a chiral 2-oxazolidinone auxiliary.

A novel asymmetric photodimerization reaction of coumarin derivatives bearing the ( S )-4-benzyl-2-oxazolidinone auxiliary provides only the syn -head-to-tail ( syn -HT) dimer with moderate diastereoselectivity (up to 75 : 25). The mechanism of complete syn -HT selectivity and moderate diastereoselectivity is proposed based on the result of density functional theory (DFT) calculation. The benzyl group of the ( S )-4-benzyl-2-oxazolidinone auxiliary in combination with a Lewis acid exerts effective diastereofacial shielding of the reaction site.