Current Organic Chemistry - Volume 17, Issue 14, 2013

Gating in host-guest chemistry refers to conformational changes that occur in response to some stimulus, resulting in opening or closing of a physical barrier that controls access to the host. Gating due to conformational changes of protein loops is common in enzyme catalysts: the binding of a substrate in an enzyme often involves the closing of a protein loop to bind the substrate, a type of gating [1]. Cram’s pioneering work in host-guest chemistry led to the discovery of one molecule inside another and the concept of container molecules. Cram’s nanometer-scaled container molecules were found by our group to involve gating to allow passage of guest molecules into or out of the container host [2]. The gating process in container molecules provides a novel way that binding and release of guest molecules can be controlled, and this has potential applications in controlled drug delivery.

The transport of ions across molecular cages that mimic the behavior of bio-membranes has been of wide interest to researchers, but the synthesis of cage or capsular entities with ion transport properties is often challenging. A variety of molecular building blocks, such as calixarenes or cavitands, has been employed to study encapsulation and ion-transport properties. Recently, we have studied the transport of ions into the interior of gallium-containing pyrogallol[4]arene nanoassemblies, and the results of those studies are the focus of this article. Specifically, the transport of silver(I), cesium(I) and thallium(I) ions into the interior of C-butylpyrogallol[4]arene nanocapsules has been investigated. The transformation of solid-state spherical gallium hexamers to solution-phase toroidal frameworks suggests a structural rearrangement and concomitant inclusion or exclusion of ions in solution. Cation…π interactions and the anion position in the assembly play important roles in the stability of the inclusion complexes.

This review provides a detailed overview on the research carried out in self-assembled supramolecular capsules enabling encapsulation of transition metals within their inner space. The supramolecular assemblies discussed are based on cationic/anionic interactions between cone-shaped building blocks, derived from quaternary ammonium and sulfonated salts, which upon assembly give rise to closed cavitands. Furthermore, one or both of the ionic building blocks contain diphosphine groups located at the cavity core. Thus, upon chelation of a transition-metal ion to the phosphorus(III) donor atoms, the metal fragment lies positioned in the molecular container. In addition, metal coordination does not affect the overall structure of the supramolecular capsule.

The reaction mechanism for the hydrogenation of norbornadiene (nbd) catalysed by bis-norbornadiene rhodium(I) [Rh(nbd)2]+ complex has been studied by means of DFT based methods, in the absence and in the presence of a self-folding octaamide cavitand. Formation of three products, norbornene, nortricyclene and a dimer is discussed. When the metal complex is encapsulated inside the cavitand, the steric crowding prevents formation of the dimer, while in the absence of the cavity, the dimer is the major product. Although energy differences between the studied reaction pathways are small, the present study explains the changes in regioselectivity when the hydrogenation reaction is carried out in a confined space.

In recent years, asymmetric reactions catalyzed by the carbohydrate-based catalysts are the most intensively studied as they have been recognized as versatile starting materials for chiral auxiliaries, reagents, ligands and organocatalysts. The design and finetuning of carbohydrate-based catalysts is facilitated by the multiple functional groups present within this class of compounds. This strategy has provided unique ligand moieties which combine the performance of “privileged catalysts” with increased flexibility and accessibility. With the development of sugar-based ligands in selective catalysis, D-glucosamine has been selected as a cheap and readily available chiral scaffold for the synthesis of a series of novel ligands and organocatalysts. D-glucosamine-derived chiral catalysts has gained continuous increasing interest in asymmetric reactions. In this review, key development of asymmetric reactions catalyzed by Dglucosamine- derived chiral catalysts is summarized.

Phosphoesters are abundant in carbohydrate structures, yet their chemical reactivity is less well known than that of nucleoside phosphoesters. Both classes of compounds contain sugar bound phosphoesters, but structural versatility of carbohydrates means that the reactivity range is wider, and reaction mechanisms not feasible in nucleic acid chemistry, are possible. Sugar phosphates, as well as their phosphodiester and phosphoanhydride derivatives with a phosphate group in glycosylic position, react like acetals under acidic conditions. Substrates with a phosphate group attached to an alcoholic OH react by intramolecular transesterification similar to that of RNA provided that there is a suitably positioned HO-group and a suitable leaving group. If there is a free carbonyl group allowing anomeric equilibria, base-catalyzed phosphate elimination through enediolate intermediates may compete with the cleavage, particularly under alkaline conditions. The few reports on phosphate migration show that the reaction is conceivable, but the competition between cleavage and phosphate migration possibly is different from reactions of nucleic acids and nucleotides.

Diverse natural product-like N-benzyl cinnamamides were prepared by condensation of trans-cinnamic acid with substituted benzylamines in the presence of boric acid as catalyst. The further evaluation of this amides as a substrates of the bromo-arylation reaction, in the presence of N-bromosuccinimide (NBS) as the halogen source and under the catalysis of Yb(OTf)3, generate a different types of N-substituted amides: N-aryl 2,3-dibromopropanamides, N-(2-bromobenzyl) cinnamamides and tetrahydro-2-benzazepin-3-one as a new molecules. Here we discuss the formation of these main products on the basis of the electronic nature of the substituents present in the N-benzyl aromatic ring of the prepared cinnamamides.

Chiral chromans and flavanones derivatives are widely exist in plants, asymmetric synthesis of biological active chiral flavanone and chromanone derivatives is a valuable research field. A chroman derivatived (+)-(S,R,R,R)-Nebivolol (β1-adrenergic receptor blocker) has been synthesized by natural chiron and sulfoxide-directed method recently. Enantioselective method for the synthesis of flavanones and chromanones by the oxa-Michael addition and other cyclization reactions are described, asymmetric Michael-Michael cascade reaction for the construction of chiral chromans provideas a novel method in the synthesis of chiral flavanones and chromanones. Trost’s AAA reaction (Pd-catalyzed asymmetric allylic alkylation) provides a novel strategies in the synthesis of flavanones and chromanones.

A new proline derivative of (S)-5-prolylamide-triazole was synthesized and evaluated as an organocatalyst for the direct asymmetric aldol reaction of acetone with aromatic aldehydes under no extra solvent. At room temperature and in the absence of extra solvent, 15 mol% catalyst efficiently catalyzed the direct asymmetric aldol reactions to give aldol adducts, with modest enantiomeric excess (ee) of up to 74%. The titled reactions can be carried out in an environmentally friendly manner of no extra solvent and less catalyst loading.