Enantiomerically Pure Highly Functionalized α-Amino Ketones from the Reaction of Chiral Cyclic N-(9-Phenylfluoren-9-yl) α-Amido Esters with Organolithium Reagents

The reaction of methyl N-(9-phenylfluoren-9-yl)pyroglutamate with several organolithium reagents afforded the corresponding ketones in excellent yields and with complete retention of enantiomeric purity. The success of this transformation is due to the unusual stability of the tetrahedral intermediates 5, which stems from two factors: the electron-withdrawing effect of the amide nitrogen and the lithium complexing ability of the fluorenyl ring of the 9-phenylfluoren-9-yl group. This ester-to-ketone transformation was also successfully applied to oxazolidinone 9 and imidazolidinone 20 and provided a ketone (21b) that was ultimately transformed into the urea-lactam 26 which incorporates the bicyclic core of streptolidine lactam, a component of the streptrothricin antibiotics.