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Synthesis of a Bicyclic Proline Analogue from l-Ascorbic Acid

The efficient synthesis of a bicyclic α-amino acid from L-ascorbic acid is presented. The synthetic procedure is a three-step process involving an S N 2 reaction of an amino acetal with an L-ascorbic acid derivative, followed by protection of the amine as a Fmoc urethane, and acid-promoted trans-acetalization to give the title compound. Inversion of the configuration at the stereocenter of the precursor derived from L-ascorbic acid allowed the formation of the corresponding bicyclic α-amino acid bearing the carboxylic group in the 2-exo configuration. Such Fmoc-protected α-amino acids can be considered as bicyclic mimetics of proline, and are particularly suited for solid-phase peptidomimetic chemistry.