Preparation of 4- and 6-desphenyl analogues of (−)-clausenamide and alternative synthesis of (+)-epi-clausenamide

4- and 6-desphenyl analogues of (−)-clausenamide, 6 and 7, were prepared in optical active form from commercially available d-pyroglutamic acid and the known racemic pyrrolidinone 13, respectively. In order to confirm the absolute stereochemistry of (+)- and (−)-7, intermediate 19b was transferred into the (+)-epi-clausenamide 8.