Crystal structures of proline-derived enamines

The isolation and structural characterization of both aldehyde-and ketone-derived proline enaminones are reported and discussed. Crystal structures of 10 proline enamines provide information on stereochemical aspects, i.e., double bond configuration and syn-vs. anti-positioning of the carboxylate relative to the enamine double bond. Furthermore, the obtained crystal structures are compared with the density functional theory-calculated structures of the ground and transition state and the postulated Seebach-Eschenmoser transition state.