Stereoselective synthesis of (1R,4R)-N-acyl-2-oxa-5-aza-bicyclo[2.2.1]heptan-3-ones via mesoionic compounds.: An improved synthesis of cis-4-hydroxy-D-proline

We report an asymmetric synthesis of (1R,4R)-N-acyl-2-oxa-5-aza-bicyclo[2.2.1]heptan-3-ones, starting from the inexpensive and commercially available trans-4-hydroxy-L-proline and achieved by treating N-acyl-trans-4-hydroxy-L-prolines with acetic anhydride. The formation of intermediate mesoionic compounds may explain the formation of N-acyl-2-oxa-5-azabicyclo[2.2.1]heptan-3-ones with (R)-absolute configuration at C(4). Acidic cleavage of these lactones readily affords N-acyl-cis-4-hydroxy-D-prolines or cis-4-hydroxy-D-proline in good yields. (C) 2002 Elsevier Science Ltd. All rights reserved.