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

Abstract

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.