Unexpected synthesis of conformationally restricted analogues of gamma-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.70111011

Efficiency of zeolite MCM-22 with different SiO2/Al2O3 molar ratios in gas phase glycerol dehydration to acrolein

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)The gas-phase dehydration of glycerol was conducted over MCM-22 zeolite with different SiO2/Al2O3 molar ratios. MCM-22 zeolites were synthesized and characterized by XRD, FTIR, TG, SEM, EDX, textural analysis by N-2 physisorption and NH3-TPD. The MCM-22 zeolites have presented good crystallinity, high surface areas and a decreased amount of acid sites and strength with increasing SiO2/Al2O3 ratio was observed. The glycerol conversion and acrolein selectivity increased with increasing reaction temperature from 280 degrees C to 320 degrees C on MCM-22 (30) catalyst. As long as the concentration of glycerol solution was in the range from 10% to 36.6%, there was no noticeable difference in the glycerol conversion but the acrolein yield increased with water amount in the feed. The glycerol conversion and selectivity to acrolein at 320 degrees C using glycerol feed of 36.6% during 10 h of reaction decreased in the following order: MCM-22 (30) > MCM-22 (50) > MCM-22 (80). This result shows that glycerol dehydration to acrolein is strongly dependent on the acid sites density and external surface area of the zeolite MCM-22. (c) 2013 Elsevier Inc. All rights reserved.1817482Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)CNPq [476657/2010-5