Core units

Proteins often have the same high-affinity isotherms as do synthetic polymers and are also slow to equilibrate, due to many contacts with the surface. Proteins, however, have the additional complication that they can partially or completely unfold at the solid-liquid interface to expose their hydrophobic core units to a hydrophobic surface... 

The dichloride of molybdenum(II) [13478-17-6] M0CI2, contains Mo CF g core units (Fig. 6c) having chloride bridges in its soHd-state stmcture. Similar or identical hexanuclear units are known in soluble species such as Mo3Ch 24 other derivatives containing the Mo CF g core. These compounds have been under investigation because of their photochemical and photoluminescent activity (see Photochemical technology) (36,37). The hexanuclear... 

The core unit of the chromatin, the nucleosome, consists of histones arranged as an octamer consisting of a (H3/ H4)2-tetramer complexed with two histone H2A/H2B dimers. Accessibility to DNA-binding proteins (for replication, repair, or transcription) is achieved by posttranslational modifications of the amino-termini of the histones, the histone tails phosphorylation, acetylation, methylation, ubiquitination, and sumoyla-tion. Especially acetylation of histone tails has been linked to transcriptional activation, leading to weakened interaction of the core complexes with DNA and subsequently to decondensation of chromatin. In contrast, deacetylation leads to transcriptional repression. As mentioned above, transcriptional coactivators either possess HAT activity or recruit HATs. HDACs in turn act as corepressors. 

The degree of branching can be regarded as the ratio of branched units in the polymer to those in a perfect dendrimer. Thus, the hmiting values are DB=0 for hnear polymers and DB=1 for a perfect dendrimer. Various definitions of DB have been given. If we do not take into account the vinyl group or initiator unit (the core unit ), the DB is defined as... 

FuUerodendrimers also allow an evaluation of the accessibility of the Cgo core unit by studying bimolecular deactivation of its excited states by external quenchers. Recently Ito, Komatsu, and co-workers have used this approach to investigate a series of fuUerodendrimers (9-11) in which Frechet-type dendrons have been connected to a fullerene moiety via an acetylene linker (Fig. 5) [35]. 

The complex [Au2(PPh3)2] has been prepared by reduction of [AuI(PPh3)] with sodium napht-halide. Its core unit is isoelectronic with mercurous halides Hg2X2 but, curiously enough, it has a dinuclear trans bent structure with d(Au—An) = 2.76 A and a Au—Au—P angle of 129°.3325 There is no confirmation as yet of the original preliminary report. 

More recently Frechet and Gitsov [130] used a similar approach as above and synthesized a novel series of dendritic copolymers derived from a central penta-erythritol core unit. These hybrid star molecules behaved as unimolecular micelles with different core-shell conformational-structures as a response to the polarity of the solvent used. 

Host-guest systems made from dendritic materials have potential in the areas of membrane transport and drug delivery [68, 84, 85]. In a recent report [136] Tomalia and coworkers investigated structural aspects of a series of PAM AM bolaamphiphiles (e.g., 50) with a hydrophobic diamino do decane core unit. Fluorescence emission of added dye (nile red) was significantly enhanced in an aqueous medium in the presence of 50 unlike the cases when 51 and 52 were added (Fig. 23). Addition of anion surfactants to this mixture generated supramolecular assemblies which enhanced their ability (ca.by 10-fold) to accommodate nile red (53). Further increase in emission was noted by decreasing the pH from the normal value of 11 for PAMAM dendrimers to 7. At lower pH values the... 

In our group, several dendrimers based on ( R)-3-hydroxybutanoic acid (HB) have been prepared [56-58]. The dendrimers were synthesized by the convergent strategy. Trimesic acid has been used as core unit and the benzyl esters of the dimer and the tetramer of HB as elongation units. In such a way dendrimers of 1st and 2nd generation (27-30) have been constructed (Fig. 13). Since poly(.R)-3-hydroxybutanoic acid (PHB) is known to be biodegradable [59, 60] the stability of the dendrimers 27-30 was tested in the presence of PHB-de-... 

At the beginning of investigations on chiral dendrimers in our own group was the question of how to synthesize chiral, non-racemic derivatives of tris(hydroxymethyl)-methane [82], which we wanted to use as dendrimer center pieces. We have developed efficient diastereoselective syntheses of such triols [83-85] from ( R)-3-hydroxybutanoic acid, readily available from the biopolymer PHB [59,60] (cf. Sect. 2.4). To this end, the acid is converted to the dioxanone 52 [86, 87], from which various alkylation products and different aldol adducts of type 53 were obtained selectively, via the enolate (Fig. 20). These compounds have been reduced to give a variety of enantiopure chiral building blocks for dendrimers, such as the core unit 54, triply branching units 55a and 55b or doubly branching unit 56 [1,88]. 

In the last few years McCleverty, Ward, and co-workers have reported the NIR electrochromic behavior of a series of mononuclear and dinuclear complexes containing the oxo-Mo(iv) v core unit [Mo(Tp )(0)Cl(OAr)], where Ar denotes a phenyl or naphthyl ring system [Tp = hydro-hydrotris(3,5-dimethylpyrazolyl)borate].184-189 Mononuclear complexes of this type undergo reversible MoIV/Mov and Mov/MoVI redox processes with all three oxidation states accessible at modest potentials. Whilst reduction to the MoIV state results in unremarkable changes in the electronic spectrum, oxidation to MoVI results in the appearance of a low-energy phenolate- (or naphtholate)-to-MoVI LMCT process.184,185... 

Lithiation of the complex [(cp)(CO)2W]2(HC=CPh) with its tetrahedral W2C2 core unit followed by treatment with (Ph3P)AuCl gives the corresponding cluster with the hydrogen atom replaced by the Au(PPh3) group. A diaurated complex is obtained with (dppm)(AuCl)2.94 95... 

Cud has been shown to form a 1 1 adduct with (Ph3P)AuC=CFc (Fc = ferrocenyl), which has a dimeric structure with a square (CuCl)2 core unit of which each Gu atom is coordinated side-on to a triple bond of the substrate. A similar 2 2 aggregate is formed with CuBr and (Ph3P)AuC=CAu(PPh3).66... 

The investigations have been extended to include calixarenes as core units bearing both (L)AuC=C- and crown... 

Substrates with two labile substituents such as (tht)AuCl can be converted into ylide complexes in two steps. An example of this type is the product [(Ph3PGH2)2Au]+C104 obtained upon the reaction with two ylides, followed by anion exchange. The cation with its linear CH2-Au-CH2 core unit crystallizes with the tetracyanochinodimethane anion as a green crystalline product.1... 

In the crystal, most trinuclear carbeniate complexes are known to be stacked with close aurophilic interactions, and organic 7r-acids can be intercalated into these stacks.252 Similar intercalation takes place with the cations of simple silver and thallium(i) salts. The cations become attached to form polymetallic core units through metallophilic bonding. With the trinuclear silver pyrazolates, both metal and ligand exchange have been observed to give new stacks of trinuclear units.254... 

The same research group extended their studies, proposing a systematic investigation of the core activities for different carbohydrate densities during biosensing processes.279 Three new mannosylated dendrimers with a Ru(bipy)3 core unit were synthesized, and the influence of the number and size of dendritic branches on the rate of electron and energy transfer, as well as the lectin-biosensing abilities (Scheme 41),... 

Moody and coworkers have employed a biomimetic hetero-Diels-Alder-aroma-tization sequence for the construction of the 2,3-dithiazolepyridine core unit in amythiamicin D and related thiopeptide antibiotics (Scheme 6.243 a) [426]. The key cycloaddition reaction between the azadiene and enamine components was carried out by microwave irradiation at 120 °C for 12 h and gave the required 2,3,6-tris(thi-azolyl)pyridine intermediate in a moderate 33% yield. Coupling of the remaining building blocks then completed the first total synthesis of the thiopeptide antibiotic... 

Stereoselective synthesis of pseudo C2-symmetrical 1,3-dibenzyldiamino alcohol (S,S) (323) a core unit of HIV protease inhibitors, and the two meso-stereoisomers (323a) and (323b) was achieved by stereocontrolled addition of benzylmagnesium chloride to nitrones (63a) and (63b) (Scheme 2.137). The yield of (S,S)-(323), based on N-Boc-L-phenylalaninal, accounts for 23% (Scheme 2.137) (211). 

A direct method for the respective preparation of the core units of statin analogues (3R,AR)-2, (3S,AS)-2, (3R,AS)-3, and (3S,AR)-3 in enantiomerically pure form is described. These analogues are prepared from the same molecule 1 in a one-pot, sequential asymmetric hydrogenation process utilizing Rh(I)-and Ru(II)-chiral phosphine complexes. Some other examples are depicted in Table 13.1. 

Figure 1. General subdivision of dendrimers into core unit, branching units, and end groups.

The potential influence of the dendrylation on the functional core unit includes sometimes a drastically increased molecule size as well as a steric shielding (encapsulation) and a micro-environment different and isolated from its external surroundings, eg., unimolecular micellar structures, electron-rich shells, solubilization. It is even possible to activate the core unit by both energy and electron transfer processes. In the following subsections, these design possibilities will be dealt with in more detail. 

Figure 11. The implications for the total yield, assuming an idealized dend-rimer functionalization for the example of a dendrimer with AB2-branching and bifunctional core unit (G = generation).

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