Enantioselective supramolecular devices in the gas phase

Resorcin[4]arenes represents a class of widely studied macrocycles with remarkable complexing properties towards inorganic and organic compounds. In 2007 Zhang et al. indicated them as the third best host molecules, right after cyclodextrin and crown ethers. Their success in the host-guest chemistry is principally due to the tunability of their synthesis. In principle the lower rim’s chains of resorcin[4]arenes can be functionalized with a large number of chemical groups, which strictly influences the conformational space available to the structure. Furthermore the conformation of the macrocycle defines its coordinating capabilities as well as its selectivity. Such a versatility can include the possibility to employ even a supramolecular chirality which can be obtained in two ways: 1) the presence of stereogenic centres in the lateral chains; 2) the hindered spatial arrangement of achiral subunits forming a chiral macrocyclic scaffold. In the last decade the application of resorcin[4]arenes in different analytical and biochemical fields has been continuously growing: i) chromatography, as stationary phase as well as component of liquid phases;(1,2) ii) NMR, as solvating agent;(3) iii) pharmacology, as biocompatible drug carrier.(4) These applications are usually carried out in liquid media, where the host-guest interactions result from the superimposition of the intrinsic effect of the non-covalent supramolecular interactions and the solvation effect.Here we present a comprehensive gas phase study of the molecular recognition of several chiral resorcin[4]arenes towards different enantiopure compounds (i.e. amino acids and their derivatives, nucleosides, alkaloids).(5-9) The isolated state allows us to exclude the solvation effects and to purely investigate the intimate nature of the host-guest interactions. The gaseous adducts have been studied by means of different mass spectrometry approaches which provided us multifaceted information on the structure (IRMPD), thermodynamics (ESI-CID), and reactivity (ESI-FT-ICR) of resorcin[4]arenes based adducts, in order to shed light on the effect which supramolecular chirality exerts on chemico-physical properties of these host-guest complexes. Information of this kind provide the organic chemists a positive feedback to stimulate or even inspire the synthesis of new macrocycles applicable to different chemical fields.