Illustrating the Utility of X-ray Crystallography for Structure Elucidation through a Tandem Aldol Condensation/Diels-Alder Reaction Sequence

Two introductory organic chemistry laboratory experiments are described based on the Diels-Alder reaction of 2,3,4,5-tetraphenylcyclopentadienone, which is synthesized prior to or in a one-pot reaction, with styrene. Students are presented with three possible products, the endo and exo diastereomers and the decarbonylated product, for the beautiful crystalline compound isolated. Analysis by IR and ¹H NMR spectroscopy aid investigators in proposing an initial structure for the product, but the solved X-ray crystal structure provides unequivocal evidence of its stereochemistry. Both versions of the synthesis introduce two important and atom economical reactions, employ the Karplus relationship for ¹H NMR interpretation, and illustrate the power of X-ray crystallography for molecular structure determination.