A practical stereoselective 5-exo-trig radical cyclization procedure was developed in order to prepare enantiomerically pure GABA derivative precursors (4-alkyl-pyrrolidin-2-ones). This procedure allows much more rapid access to optically pure GABA derivatives, such as the powerful antiepileptic agent (S)-(+)-3-aminomethyl-5-methylhexanoic acid (Pregabaline).
A novel stereoselective tin-free radical protocol for the enantioselective synthesis of pyrrolidinones and its application to the synthesis of biologically active GABA-derivatives
fashion, via a sequential 5-exo-trig radical cyclization-hydrogen or bromine atom-transfer process, under non-tin conditions. Interestingly, when N-allyl-α-bromoacetamides were treated with triethylborane/MeOH(excess)/BF3·OEt2 in toluene at −78 °C, a tandem 5-exo-trig radical cyclization-hydrogen atom-transfer reaction operated, on the other hand, a tandem 5-exo-trig radical cyclization-bromine atom-transfer
Free radical precursors 4a and 4b were synthesized and treated under reductive or oxidative conditions to obtain the corresponding optically pure pyrrolidinones 5-8, which were subsequently transformed into corresponding optically pure GABA derivatives 9-11. When reductive radical conditions (4a-->7 and 8) were used, a Ph1-5 migration product 14 was obtained; presumably depending upon the specific conformation of the rotamer precursor and also 14 was found to be a kinetic product. (C) 2004 Elsevier Ltd. All rights reserved.