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Despite the large variety of modified nucleosides that have been reported, the preparation of constrained 4′‐spirocyclic adenosine analogues has received very little attention. We discovered that the [2+2]‐cycloaddition of dichloroketene on readily available 4′‐exo‐methylene furanose sugars efficiently results in the diastereoselective formation of novel 4′‐spirocyclobutanones. The reaction mechanism was investigated via density functional theory (DFT) and found to proceed either via a non‐synchronous or stepwise reaction sequence, controlled by the stereochemistry at the 3′‐position of the sugar substrate. The obtained dichlorocyclobutanones were converted into nucleoside analogues, providing access to a novel class of chiral 4′‐spirocyclobutyl adenosine mimetics in eight steps from commercially available sugars. Assessment of the biological activity of designed 4′‐spirocyclic adenosine analogues identified potent inhibitors for protein methyltransferase target PRMT5. [ABSTRACT FROM AUTHOR]

In the last decade, halogen bonds, noncovalent interactions formed between positive regions in the electrostatic potential on halogen atoms, often referred to as σ-holes, and electron-rich sites, have gained a lot of interest. Recently, this interest has been expanded towards interactions with Group V and Group VI elements, giving rise to pnicogen and chalcogen bonds. Although chalcogen bonds have already shown some promising results for applications in crystallography and catalysis, experimental results characterising these noncovalent interactions remain scarce. In this combined experimental and theoretical study, original data allowing the characterization of S⋅⋅⋅O chalcogen bonds is obtained by studying the 1:1 molecular complexes between 2,2,4,4-tetrafluoro-1,3-dithiethane (C2F4S2) and dimethyl ether (DME). Ab initio calculations of the C2F4S2⋅DME dimer yield two stable chalcogen-bonded isomers, the difference being the presence or absence of secondary F⋅⋅⋅H interactions. Liquid-krypton solutions containing C2F4S2 and DME were studied using FTIR and Raman spectroscopy. Upon subtraction of rescaled monomer spectra, clear complex bands are observed. The observed complexation shifts agree favourably with the ab initio calculated shifts of the chalcogen-bonded complexes. The 1:1 stoichiometry of the complex is confirmed and a complexation enthalpy of −13.5(1) kJ mol−1 is found, which is in good agreement with the calculated values. A Ziegler-Rauk energy decomposition analysis revealed that electrostatic interactions prominently dominate over orbital interactions. Nevertheless, significant charge transfer occurs from the oxygen in DME to one of the sulfur atoms in C2F4S2 and the carbon along the extension of the chalcogen bond. [ABSTRACT FROM AUTHOR]

The contributions of covalent and noncovalent interactions to the formation of classical adducts of bulky Lewis acids and bases and frustrated Lewis pairs (FLPs) were scrutinized by using various conceptual quantum chemical techniques. Significantly negative complexation energies were calculated for fourteen investigated Lewis pairs containing bases and acids with substituents of various sizes. A Ziegler-Rauk-type energy decomposition analysis confirmed that two types of Lewis pairs can be distinguished on the basis of the nature of the primary interactions between reactants; dative-bond formation and concomitant charge transfer from the Lewis base to the acid is the dominant and most stabilizing factor in the formation of Lewis acid-base adducts, whereas weak interactions are the main thermodynamic driving force (>50 %) for FLPs. Moreover, the ease and extent of structural deformation of the monomers appears to be a key component in the formation of the former type of Lewis pairs. A Natural Orbital for Chemical Valence (NOCV) analysis, which was used to visualize and quantify the charge transfer between the base and the acid, clearly showed the importance and lack of this type of interaction for adducts and FLPs, respectively. The Noncovalent Interaction (NCI) method revealed several kinds of weak interactions between the acid and base components, such as dispersion, π-π stacking, CH ⋅⋅⋅π interaction, weak hydrogen bonding, halogen bonding, and weak acid-base interactions, whereas the Quantum Theory of Atoms in Molecules (QTAIM) provided further conceptual insight into strong acid-base interactions. [ABSTRACT FROM AUTHOR]