Unexpected synthesis of conformationally restricted analogues of γ-amino butyric acid (GABA):: Mechanism elucidation by electrospray ionization mass spectrometry

From previous results with lower homologues, dehydroiodination of the three alkenyl-beta-enamino esters 3a-c was expected to provide six-membered N-heterocyclic products. The reactions of 3a-c with triethylamine are found to lead, however, to the unexpected stereoselective synthesis of the trisubstituted cyclopentane derivatives 4a-c, as confirmed by IR and NMR spectroscopy. Cyclopentanes 4a-c bear two chiral centers and a gamma-amino ester moiety, and are therefore conformationally restricted analogues of gamma-amino butyric acid (GABA), which is the major inhibitory neurotransmitter in the central nervous system. Use of electrospray ionization mass (ESI-MS) and tandem mass spectrometry (ESI-MS/MS) allowed the key iminium ion intermediates 5a-c(+), as well as the protonated molecules of both the reactant and final products, [3a-c + H](+) and [4a-c + H](+), to be intercepted and structurally characterized. From these findings a mechanism for this unexpected but synthetically attractive and efficient stereoselective reaction is proposed.