source: Journal of Chemical Education year: 2006 volume: 83 first page: 1225 last page: 1228 doi: 10.1021/ed083p1225
Abstract: The Robinson annulation is a three-step process involving a Michael addition followed by an internal aldol condensation and a dehydration. Under appropriate experimental conditions, it is possible to stop the reaction after every step and to isolate the three products separately. This feature is particularly attractive in the frame of an organic chemistry course. It allows students to confirm experimentally the validity of the stepwise mechanism and to get a more thorough understanding of the whole process. It also permits them to synthesize a rich set of related molecules that can be compared and characterized through various analytical techniques. Thus, a stoichiometric mixture of chalcone and ethyl acetoacetate was reacted in ethanol. Depending on the quantity of barium hydroxide monohydrate used as catalyst, the reaction time, and the temperature, three different products were obtained. Their full IR, 1H, 13C, COSY, NOESY, and HETCOR NMR spectra are supplied. Examination of the spectroscopic data helps uncover many challenging structural analysis problems. Among them, the diastereoselective formation of chiral centers during the annulation process, the distinction between axial and equatorial substituents on a cyclohexane ring, and the possibility of a keto-enol tautomerism are extensively discussed.
Keywords: Catalysis, Chirality / Optical Activity, Conformational Analysis, Diastereomers, Hands-On Learning, Manipulatives, IR Spectroscopy, Laboratory Instruction, NMR Spectroscopy, Organic Chemistry, Second-Year Undergraduate, Stereochemistry, Synthesis, Upper-Division Undergraduate