Practical synthesis of Boc-protected cis-4-trifluoromethyl and cis-4-difluoromethyl-L-prolines

A short, efficient, and diastereomerically pure synthesis of N-Boc-cis-4-trifluoromethyl-L-proline (7) and N-Boc-cis-4-difluoromethyl-L-proline (9) from N-Boc-4-oxo-L-proline (4) is described. The reaction of 4 with Me3SiCF3 and the conversion of the carbonyl group of 4 into the difluoromethylene group are the key steps for the synthesis of 7 and 9, respectively.