Azulene‐Based Macrocyclic Receptors for Recognition and Sensing of Phosphate Anions

Herein we report the synthesis and detailed studies of the anion‐binding properties of two 20‐membered macrocyclic tetramide receptors: one symmetrical, containing two identical azulene‐based bisamide units, the other a hybrid, containing a dipicolinic bisamide unit and an azulene‐based bisamide unit. Analysis of the crystal structures of the macrocyclic receptors revealed their preference for adopting similar well‐preorganized bent‐sheet conformations, both as free receptors and in their complexes with anions. Studies of the optical properties of both receptors revealed abilities to selectively sense phosphate anions (H₂PO₄^?, HP₂O₇^3?), allowing for naked‐eye detection of the presence of these guests in DMSO. Binding studies in solution confirmed that the receptors bind strongly to a series of anions even in highly demanding media, such as mixtures of DMSO with water or with methanol. Comparison of the anion affinity of linear analogues with that of the macrocyclic receptors evidenced the importance of macrocyclic topology. Quantitative analysis revealed that the macrocyclic receptors are selective for H₂PO₄^? over other anions. The affinity to H₂PO₄^? seen for the symmetrical receptor, containing two azulene‐based subunits, is much higher than for the hybrid macrocycle containing both the azulene‐based and pyridine‐derived subunits. This highlights that the azulene‐based building block serves efficiently as both a binding site and a structure‐preorganizing motif.     

Macrocyclization of polymers via ring‐closing metathesis and azide/alkyne‐“click”‐reactions: An approach to cyclic polyisobutylenes

The end‐to‐end cyclization of telechelic polyisobutylenes (PIB's) toward cyclic polyisobutylenes is reported, using either ring‐closing metathesis (RCM) or the azide/alkyne‐“click”‐reaction. The first approach uses bisallyl‐telchelic PIB's (M_n = 1650, 3680, 9770 g mol^?1) and Grubbs 1st‐, 2nd‐, and 3rd‐generation catalyst leading to cyclic PIB's in 60–80% yield, with narrow polydispersities (M_w/M_n = 1.25). Azide/alkyne‐“click”‐reactions of bisalkyne‐telechelic PIB's (M_n = 3840 and 9820 g mol^?1) with excess of 1,11‐diazido‐undecane leads to the formation of mixtures of linear/cyclic PIB's under formation of oligomeric cycles. Subsequent reaction of the residual azide‐moieties in the linear PIB's with excess of alkyne‐telechelic PEO enables the chromatographic removal of the resulting linear PEO‐PIB‐block copolymers by column chromatography. Thus pure cyclic PIB's can be obtained using this double‐“click”‐method, devoid of linear contaminants. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 671–680, 2010     

On the redox reactions of radicals with AlIII(phthalocyaninate) pendants covalently bonded to poly(ethyleneamide). Mechanistic alterations when radicals are formed inside pockets of micelle‐like polymer aggregates

The mechanisms of the redox reactions between a polymer containing Al(III) sulfonated phthalocyanine pendants, (Al^III(^?NHS(O₂)trspc)^2?)₂, and radicals have been investigated in this work. Pulse radiolysis and photochemical methods were used for these studies. Oxidizing radicals, OH^?, HCO₃^?, (CH₃)₂COHCH₂^?, and N₃^?, as well as reducing radicals, e_aq^?, CO₂^??, and (CH₃)₂C^?OH, respectively accept or donate one electron forming pendent phthalocyanine radicals, Al^III(^?NHS(O₂)trspc ^?)^{? or 3?}. The kinetics of the redox processes is consistent with a mechanism where the pendants react with radicals formed inside aggregates of five to six polymer strands. Electron donating radicals, that is, CO₂^?? and (CH₃)₂C^?OH, produce one‐electron reduced phthalocyanine pendants that, even though they were stable under anaerobic conditions, donated charge to a Pt catalyst. While the polymer was regenerated in the Pt catalyzed processes, 2‐propanol and CO₂ were respectively reduced to propane and CO. The reaction of SO₃^?? radicals with the polymer stood in contrast with the reactions of the radicals mentioned above. A first step of the mechanism, the coordination of the SO₃^?? radical to the Al(III), was subsequently followed by the formation of a SO₃^?? ‐ phthalocyanine ligand adduct. The decay of the SO₃^?? ‐ phthalocyanine ligand adduct in a ～10² ms time domain regenerates the polymer, and it was attributed to the dimerization/disproportionation of SO₃^?? radicals escaping from the aggregates of polymer. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Solar Thermal Energy Storage in a Photochromic Macrocycle

The conversion and efficient storage of solar energy is recognized to hold significant potential with regard to future energy solutions. Molecular solar thermal batteries based on photochromic systems exemplify one possible technology able to harness and apply this potential. Herein is described the synthesis of a macrocycle based on a dimer of the dihydroazulene/vinylheptafulvene (DHA/VHF) photo/thermal couple. By taking advantage of conformational strain, this DHA–DHA macrocycle presents an improved ability to absorb and store incident light energy in chemical bonds (VHF–VHF). A stepwise energy release over two sequential ring‐closing reactions (VHF→DHA) combines the advantages of an initially fast discharge, hypothetically addressing immediate energy consumption needs, followed by a slow process for consistent, long‐term use. This exemplifies another step forward in the molecular engineering and design of functional organic materials towards solar thermal energy storage and release.     

Very Large Cyclic Compounds

Very large ring systems, containing more than 50 ring members, are becoming increasingly important in different topics in natural sciences. These so-called gigantocycles differ from smaller macrocycles in physical properties, special structural features and chemical behaviour. This article is meant to be the first summary of such ring systems and a synopsis of the most remarkable examples with their fascinating nano-scaled structures and ingenious synthesis. To restrict the scope of the article, only isolated and completely characterized, monodisperse compounds are presented. Furthermore, attention is mainly directed at organic gigantocycles. Some ultracycles with more than 100 ring members, mainly occurring in polymer chemistry and nature, will also be described.     

What can a chemist learn from nature’s macrocycles? – A brief, conceptual view

Macrocyclic natural products often display remarkable biological activities, and many of these compounds (or their derivatives) are used as drugs. The chemical diversity of these compounds is immense and may provide inspiration for innovative drug design. Therefore, a database of naturally occurring macrocycles was analyzed for ring size, molecular weight distribution, and the frequency of some common substructural motifs. The underlying principles of the chemical diversity are reviewed in terms of biosynthetic origin and natures strategies for diversity and complexity generation in relation to the structural diversity and similarities found in the macrocycle database. Finally, it is suggested that synthetic chemists should use not only natures molecules, but also natures strategies as a source of inspiration. To illustrate this, the biosynthesis of macrocycles by non-ribosomal peptide synthetases and terpene and polyketide cyclases, as well as recent advances of these strategies in an integrated synthesis/biotechnology approach are briefly reviewed.     

Macrocycles rapidly produced by multiple multicomponent reactions including bifunctional building blocks (MiBs)

Naturally occurring macrocycles often exhibit remarkable biological activities and, therefore, constitute an attractive starting point for diversity-oriented synthesis for lead discovery in drug development. Multicomponent reactions have been used for the introduction of chemical diversity in strategies towards macrocycle libraries, mostly by combinational synthesis of a linear precursor combined with a subsequent macrocyclization reaction. The Ugi reaction in particular may be used for the macrocyclization itself as well, and a library of natural product-like macrocycles can be constructed in a single step from simple precursors. The efficiency and versatility of both strategies is immense and is exemplarily illustrated by the construction of small libraries of cyclopeptide alkaloid derivatives and biaryl ether macrocycles. The syntheses of the latter compound group are examples of multiple multicomponent macrocyclizations including bifunctional building blocks (M³iB³ or MiB), of which the Ugi-MiBs and their variations are discussed in more detail.