Short and efficient enantioselective synthesis of cis and trans pyrrolidine-2,5 dicarboxylic acids

N-BOC-ethyl pyroglutamate 1 undergoes nucleophilic ring opening with lithium methyl p-tolyl sulfinyl anion delivering the p-tolylketosulfoxide 2 . Treatment of 2 with TFA gave rise to a mixture of thioesters 3a,3b . The hydrolysis of 3b afforded (2S, 5S) pirrolidine-2,5 dicarboxylic acid 5 , a constituent of the red Alga Schizymenia dubyi . Under Pummerer reaction conditions (TFAA/Pyr), 2 yielded the 5-oxo-L-pipecolic acid derivative 6 . Cis and trans pyrrolidine 2,5-dicarboxylic acids 4 and 5 were obtained by ring opening of 1 with lithium methyl p-tolyl sulfinyl anion followed by treatment with TFA and basic hydrolysis. Under Pummerer reaction conditions(TFAA/Py) 2 delivered the 5-oxo-L-pipecolic acid derivative 6 .