Gaseous- versus solution-phase recognition of some aromatic amino esters by 2,8,14,20-tetrakis(L-valinamido)[4]resorcinarene

The effects of the physical environment on the molecular recognition of some aromatic amino esters (A = 3-(3,4)dihydroxyphenyl)alanine methyl ester (DOPAOMe), 3-(3,4)dihydroxyphenyl)alanine ethyl ester (DOPAOEt) and tryptophan ethyl ester (trpOEt)) by the flattened-cone 2,8,14,20- tetrakis(L-valinamido)[4]resorcinarene (1L) have been investigated in both the gas-phase by ESI-MS spectrometry and in CDCl3 solutions by 1H and 13C NMR spectroscopy. It is found that the non-covalent [1L·H·DOPAOMe]+ and [1L·H·DOPAOEt]+ complexes are stable in the gas-phase. The last one is stable in CDCl3 solutions as well, while [1L·H·trpOEt]+ is not. The formation of the stable [1L·H·DOPAOEt]+ complex in CDCl3 is not affected by the presence of traces of additives, like D2O, DCl or ethyl acetate, or by absorption on silica. APT-13C NMR analysis of [1L·H·DOPAOEt]+ suggests that the amino ester is mainly located inside the resorcinarene cavity, in conformity with the most stable structure arising from MC/MD simulations

Gas-Phase Enantioselective Reactions in Noncovalent Ion-Molecule Complexes

Noncovalent diastereomeric ion-molecule complexes are produced in the gas phase and are ideal for the study of chiral recognition in the absence of complicating solvent and counterion effects. This review article describes the state-of-art in this field with special emphasis on the most recent mass spectrometric studies of the structure, dynamics, and reactivity of diastereomeric ion/molecule aggregates. Chirality 21:69-86, 2009. (C) 2008 Wiley-Liss, Inc

Synthesis of amino and ammonium resorcin[4]arenes as potential receptors

Starting from the tetrabromide la (cone conformation) a number of resorcin[4]arene derivatives (3a, 4a, 5a, 6a) containing an amino function in the side chains have been prepared. Preliminary investigations, including UV, NMR, and NIS analyses, of mixtures of the resorcinarene and its potential guests, such as metal cations or amino acids, evidenced promising new properties, depending on the nature of the substituents. Notably, treatment of 1a with pyridine gave readily the corresponding pyridinium salt 7a, capable of interacting with Ga(III) salts in aqueous solution. The same results have been obtained starting from the tetrabromide 1b (1,2alternate conformation) for two of the above reactions, giving derivatives 3b and 7b

Gas-phase enantioselectivity of chiral amido[4]resorcinarene receptors

Diastereomeric proton-bound [1LHA]+ complexes between selected amino acids (A=phenylglycine (Phg), tryptophan (Trp), tyrosine methyl ester (TyrOMe), threonine (Thr), and allothreonine (AThr)) and a chiral amido[4]resorcinarene receptor (1L) display a significant enantioselectivity when undergoing loss of the amino acid guest A by way of the enantiomers of 2-aminobutanes (B) in the gas phase. The enantioselectivity of the B-to-A displacement is ascribed to a combination of thermodynamic and kinetic factors related to the structure and the stability of the diastereomeric [1LHA]+ complexes and of the reaction transition states. The results of the present and previous studies allow classification of the [1LHA]+ complexes in three main categories wherein: i) guest A does not present any additional functionalities besides the amino acid one (alanine (Ala), Phg, and phenylalanine (Phe)); ii) guest A presents an additional alcohol function (serine (Ser), Thr, and AThr); and iii) guest A contains several additional functionalities on its aromatic ring (tyrosine (Tyr), TyrOMe, Trp, and 3,4-dihydroxyphenylalanine (DOPA)). Each category exhibits a specific enantioselectivity depending upon the predominant [1LHA]+ structures and the orientation of the 2- aminobutane reactant in the relevant adducts observed. The results may contribute to the understanding of the exceptional selectivity and catalytic properties of enzyme mimics towards unsolvated biomolecules