Current Organic Chemistry - Volume 15, Issue 9, 2011

Since the discovery of Pedersen that dibenzo[18]crown-6 complexed alkali metal ions in 1967, supramolecular chemistry has expanded quickly and evolved to an interdisciplinary science. Although early works focused on the development of various crown ethers for investigating their binding affinity towards metal ions and ammoniums, in early 1970s linear molecules as synthetic receptors had begun to attract the attention of supramolecular chemists. Currently an ocean of linear molecules have been reported, which either form designed assembled structures or exhibit required properties or functions. Compared with cyclic molecules, linear molecules are usually more readily available and their structural modifications are also relatively easier. Furthermore, discrete functional units or binding sites can be introduced in a tailored manner. Another driving force for the investigations of linear molecules is mimicking the secondary and higher-grade structures of biological molecules. The effort along this line has led to the field of foldamer chemistry, which, to some extent, may be considered as intramolecular supramolecular chemistry. This special issue focuses on the recent progress on the design of some new series of linear molecules and their applications in researches in supramolecular chemistry. Professors Yuan and Gong report the applications of hydrogen-bonded oligoamide duplexes as programmable molecular recognition units to realize directed self-assembly and controlled non-covalent synthesis. Professor Li summarizes the researches on molecular recognition by using linear molecules as receptors. Professor Jiang describes the extensive investigations of the structuresproperty relationships of aromatic oligoamide foldamers. Professor Chen summarizes the versatile applications of aromatic hydrazides to construct hydrogen bonding-stabilized supramolecular systems. Professor Li provides a review on perylene diimide-based molecular selfassemblies, folding motifs, and reaction-directing codes. Together these contributions provide a timely update on the supramolecular chemistry of linear molecules, with an emphasis on hydrogen bonding and stacking interaction as the driving forces.

Association units with high strength and specificity are getting increasing attention in the construction of well-defined selfassembling systems, which are making controlled non-covalent synthesis a reality. A class of hydrogen-bonded duplexes consisting of oligoamide strands carrying various combinations (sequences) of hydrogen-bond donors and acceptors has exhibited superb ability as intermolecular association units. These duplexes are featured by programmable sequence-specificity, predictable binding strength, ready synthetic availability and modifiability. With association constants ranging from ∼104 to 109M-1, these hydrogen-bonded duplexes offer a diverse set of association units capable of specifying intermolecular association in a controlled and directed manner. The efficacy of these molecules as molecular assembler has already been demonstrated by a number of examples such as the construction of supramolecular block copolymers, the templation of β-sheets, directed chemical reactions such as olefin metathesis and disulfide exchange. By integrating hydrogen bonding with dynamic covalent interactions, covalently linked duplexes are formed sequence-specifically in a selfassembling fashion in highly competitive media, which represents a major progress in creating association units combining the strength of covalent bonds and the specificity of multiple hydrogen bonds. Recently, hydrogen-bonded and/or covalent crosslinked duplexes have been found to serve as gelators for organic solvents, which may open a new avenue for developing novel materials.

This review highlights the advances in molecular recognition based on the linear molecular receptors in the past decade. Three kinds of linear receptors that bind discrete neutral, cationic and anionic organic molecules, metal ions, and halide anions are described. The first are foldamer receptors that are induced by intramolecular non-covalent forces to spontaneously fold into compact conformations. The second are linear molecules that themselves are structurally flexible but turn into folded conformations upon binding. The third are functionalized foldamers that consist of hydrogen bonding-driven preorganized arylamide-based linkers and attached binding sites fullerene units.

Aromatic oligoamide foldamers whose helical structures were stabilized by intramolecular hydrogen bindings between consecutive units not only can mimic the secondary or tertiary structures of its natural counterparts but also provide alternative approaches to study the properties of biopolymers in artificial systems. Aromatic oligoamide foldamers are sequence-specific oligomers as demonstrated in peptides and DNA, while variation of orientation and size of aromatic monomers and lengths of oligomers can generate many foldamers with desirable structures and in turn lead to different properties. In this article, we illustrate the structure-property relationships of aromatic oligoamide foldamers reported by us and collaborators.

Based on the discovery of hydrazide dimer supramolecular synthon, various supramolecular structures were successfully constructed. Attaching hydrazide unit into a quinolinone skeleton led to a new quadruply hydrogen bonding mode. With properly encoded hydrogen bonding sites the oligohydrazide strands self-assembled into duplex strands. Linear arrangement of two hydrazide units led to a new supramolecular zipper system. If the two hydrazide units were linked with a 120° spacer, the resulting molecules spontaneously selfassembled into shape-persistent cyclic hexamers with high dynamic and kinetic stability in solution.

Perylene diimides (PDIs) have been playing important roles in several active fields. Therefore, we will first review the current methods to modify the skeleton of perylene diimides. Next, PDI self-assemblies and PDI folded nanostructures and chemically restrained PDI nanostructures are presented. Unusual physical properties due to nanostructures formation are discussed including the absorption band intensity reversal and π-stack red-fluorescence emission. Because of interesting π-stacked structures, PDI derivatives are considered as potential candidates for photovoltaic applications and organic electronics. More importantly, π-stacking can be tuned when the dihedral angle of the two naphthalene planes varies, thus resulting in adjusted interacting forces. Such cooperative forces lead to the discovery of molecular code phenomena, which will have profound impact an future bi-molecular reaction designs.

Pyridinium ylids are versatile tools in organic synthesis. The stability of pyridinium ylids and their generation is commented. This review gives an overview of the possible modes through which pyridinium ylids can react. It is demonstrated that pyridinium ylids are suitable building blocks for the synthesis of indolizines, cyclopropanes, 2,3-dihydrofurans, 2(1H)-pyridin(ethio)ones, mono- & oligopyridines, pyranonaphthoquinones, nitrones and azepines.

The Meliaceae family is rich sources of limonoid (meliacin), which are well know for their antifeedant activity and bitterness. Although their existence also could be found in Rutaceae, Cneoraceae, Ptaeroxylaceae, and Simaroubaceae family, they are especially abundant and structurally diversified in the Meliaceae family. Structurally, they are highly oxygenated and modified nortriterpenoids either containing or derived from a precursor with a 4,4,8-trimethyl-17-furanylsteroid skeleton. Nearly three decades ago, Taylor divided so far found three hundreds of the meliacins from the Meliaceae family into protolimonoids and nine classes of limonoids based on which of the four triterpenoid skeleton rings have been oxidatively opened. From then on, much progress has been made on the field. Up to very recently, approximately 1300 limonoids, exhibiting more than 35 different carbon frameworks have been observed from the four families. Consider the complex structures and broad biological activity of limonoids, however, their chemical synthesis is rather rare. Recently, there were several reports on partly and total synthesis of limonoids, in which a currently successful total synthesis of azadirachtin marked a breakthrough. This review covers the chemistry of limonoids of Meliaceae before July 2009. It mainly focuses on the identification of novel skeletons and ring system, and provides a new expanded category and corresponding biosynthetic relationship map of limonoids aiming at expanding and deepening the understanding of this type complex compound. The progress in limonoid synthesis is also reviewed for the first time.

Salt-Type Organic Acids (STOA) catalysts consisting of organic components and acidic sites, possessing salt characteristics simultaneously, have been paid much attention from the whole world as reaction promoters to acid-catalyzed reactions. In recent years, the replacement of the conventional mineral acids with environmentally benign ones has become one of the prime concerns of the chemists for the sake of the development of green strategy for the industrial production. Of special interests to this review are acidic ionic liquids, sulfamic acid and polyaniline salts, the three important representatives for STOA catalysts. Using these catalysts for organic and catalytic reactions possesses the advantages like simple and convenient product isolation, reduced equipment corrosion, and minimized contamination of the waste streams combined with reusability of the catalysts. Importantly, some special features such as regio- or chemo-selectivity could be obtained using STOA catalysts. To some extent, it seems that they are the perfect combination of heterogeneous solid acid and homogeneous organic acid catalysts. Therefore, STOA catalysts may be in principle served as a new generation of green acidic catalysts, and very interesting applications in many organic reactions and scientific research fields are addressed in this review. The category, characteristics and applications of STOA catalysts were described in details.

Pyrazoles are of considerable interest due to the broad variety of their biological activities. This overview summarizes structures of pharmacologically interesting pyrazoles, natural products, technically applied pyrazoles as well as pyrazole syntheses published during the last decade. In addition, new insights into the properties such as tautomerisms, conformations, pyrolyses and decomposition reactions of pyrazoles are reviewed.