Macrocyclic functionalization

Pyridyl donors are common and feature in later sections as part of many multidentate ligands, such as macrocycles. Functionalized pyridyl donors will also be discussed and Section 6.8.4.7 provides examples of the bidentate bipyridyl and phenanthroline complexes. 

Zinc complexes have been characterized with hexadentate macrocycles formed from a tetraaza macrocycle functionalized with pyridylmethyl or pyrazolylemthyl groups.700... 

Macrocycles Functionalized with the Dansyl Chromophore as Zinc (II)... 

Much of the work in this area concentrates on metal-free macrocycles, often with the nitrogen atoms of the macrocycle functionalized as amides [82], Reduction of these amides to facilitate complexation to a metal ion destroys the mesomorphism. 

Obviously, many structural variations are possible in the design of ferrocene oxa-aza cryptands. Some of the oxa-azaferrocene cryptands form alkali metal and lanthanide complexes [90]. FAB mass spectrometry experiments have shown that the cryptands have strong selectivity for the potassium cation compared with Li+, Na, or Cs" " [94], In these complexes the macrocycle functions as a host, but in Mg + complexes the cation is coordinated by the amide carbonyl groups [95]. In the lanthanide complexes the metallocene moiety acts as an efficient center for radiationless deactivation of the lanthanide excited state [96]. 

SAMs of thiol terminated organic molecules on metal surfaces such as gold, silver, and mercury are well known. However, the surface that has been investigated in more detail is gold [151] as it provides the more reproducible results. A series of authors [152-160] have reported on the synthesis of macrocyclics functionalized with moieties containing—SH or disulfide-S-S- groups. Using these approaches, it... 

Conceptually the most simple syntheses of complex molecules involve the joining of structural units in which all functional groups and all asymmetric centres are preformed. This technique can usually only be applied to compounds in which these units are connected by —C—X— bonds rather than C—C. It is illustrated here by the standard syntheses of oligonucleotides, peptides, and polydentate macrocyclic ligands. 

Another group of macrocyclic ligands that have been extensively studied are the cycHc polyethers, such as dibenzo-[18]-crown-6 (5), in which the donor atoms are ether oxygen functions separated by two or three carbon atoms. The name crown ethers has been proposed (2) for this class of compounds because of the resemblance of their molecular models to a crown. Sandwich stmctures are also known in which the metal atom is coordinated with the oxygen atoms of two crown molecules. 

The structural variations reported by Cram and coworkers relate to an appreciable extent to the various ancillary functions which have been appended to the binaphthyl units or elsewhere in the macrocyclic system. Enhancements of the chiral barrier or functionalization through arms has generally been effected at the 3-or 6-positions. These positions are adjacent to the hydroxyl group or directly across the second ring from it, respectively. 

The enantioselectivity of the macrocyclic CSPs are different in each of the operating modes, probably because of different separation mechanisms functioning in the different solvent modes. The possible chiral recognition mechanisms for three mobile phase compositions on glycopeptide phases are listed in Table 2-3 in descending order of strength. 

This relatively new class of CSPs incorporates glycopeptides attached covalently to silica gel. These CSPs can be used in the normal phase, reversed phase, and polar organic modes in LC [62]. Various functional groups on the macrocyclic antibiotic molecule provide opportunities for tt-tt complexation, hydrogen bonding, and steric interactions between the analyte and the chiral selector. Association of the analyte... 

Macrocyclic polyamines may be viewed as an extended form of linear polyamines 13-161 with one less degree of saturation. Polyamines with lower degrees of saturation, have important biological functions. Compounds such as macrocyclic polyimines 171 and porphyrines 18), function as 02 carriers and activators, promote photosynthesis, form the basic structure of vitamine B12, etc., and for these reasons have been subjects of intense investigation. 

The introduction of another organic cation function, guanidinium group, into macrocyclic structures such as (IV)-(VI) produces ligands which also display affinity for phosphate anions58). 

Poly (macrocyclic) compounds. The analytical application of compounds such as crown polyethers and cryptands is based on their ability to function as ligands and form stable stoichiometric complexes with certain cations. Special importance is due to their preference for alkali metal ions which do not form complexes with many other ligands. A number of these compounds are commercially available and their properties and analytical applications have been described by Cheng et a/.11... 

A number of oxepin derivatives with alkano bridges across the 3- and 6-positions and across the central C-C double bond have been oxidized with ruthenium(VIII) oxide. Usually, all of the double bonds of the heterocycle are cleaved and a macrocycle 5 is formed that contains two 1,2-diketone functions.142,199... 

An obvious drawback in RCM-based synthesis of unsaturated macrocyclic natural compounds is the lack of control over the newly formed double bond. The products formed are usually obtained as mixture of ( /Z)-isomers with the (E)-isomer dominating in most cases. The best solution for this problem might be a sequence of RCAM followed by (E)- or (Z)-selective partial reduction. Until now, alkyne metathesis has remained in the shadow of alkene-based metathesis reactions. One of the reasons maybe the lack of commercially available catalysts for this type of reaction. When alkyne metathesis as a new synthetic tool was reviewed in early 1999 [184], there existed only a single report disclosed by Fiirstner s laboratory [185] on the RCAM-based conversion of functionalized diynes to triple-bonded 12- to 28-membered macrocycles with the concomitant expulsion of 2-butyne (cf Fig. 3a). These reactions were catalyzed by Schrock s tungsten-carbyne complex G. Since then, Furstner and coworkers have achieved a series of natural product syntheses, which seem to establish RCAM followed by partial reduction to (Z)- or (E)-cycloalkenes as a useful macrocyclization alternative to RCM. As work up to early 2000, including the development of alternative alkyne metathesis catalysts, is competently covered in Fiirstner s excellent review [2a], we will concentrate here only on the most recent natural product syntheses, which were all achieved by Fiirstner s team. 

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