"The Step-by-Step Robinson Annulation of Chalcone and Ethyl Acetoacetate: An Advanced Undergraduate Project in Organic Synthesis and Structural Analysis"

Lionel Delaude, Jean Grandjean, and Alfred F. Noels

journal cover
          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

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