Cycloaddition reactions between the chiral 2-pyrone derivatives 9 and various dienophiles, catalysed by lanthanide shift reagents, afforded diastereomerically pure bicyclic lactones 3. A catalytic asymmetric version of these reactions, using optically active lanthanide catalysts, was successfully investigated.
[Posner, Gary H.; Cho, Cheon-Gyu; Anjeh, Tizah E. N.; Johnson, Neil; Horst, Ronald L.; et al. Journal of Organic Chemistry, 1995 , vol. 60, # 14 p. 4617 - 4628]
