Catechol units

As with the crowns, the situation becomes more complicated when there are other heteroatoms or substituents in one of the cryptand bridges. The symbol B is used to designate a benzo- or catechol unit in the bridge and subscripts are used to designate heteroatoms when non-oxygen heteroatoms are present. Examples of this are shown in structures 12 and 13 above. 

The ortho-xylyl unit may be considered as the synthetic complement of the catechol unit so often used in crown synthesis. This is especially so for the 1,2-bromomethyl compounds which may be used as electrophiles. These units have been incorporated in numerous crowns over the years and the syntheses are all quite similar. 

Fig. 6. (a) (b) Replacement of a peptide-bound l-hydroxy-2-pyridinone unit by a catechol unit on a tris(ammoethyl)aniine (tren) cap. (c) A tren-capped hexadentate ligand with two 3-hydroxy-2-pyridinone (3,2-hopo) and one terephthalamide (tarn) chelating units. 

A-(dihydroxybenzoyl)serine linear trimer (257) and its dimer analogue are 10" and 10 , respectively. Enthalpies of formation for the Fe complexes of the trimer and of enterobactin are reported. Linear hexapeptides and decapeptides bearing catechol units derived from dopa (dihydrox ghenylalanine) form stable complexes with Fe +, with stability constants in the range... 

Several macrocyclic polycatechols, with up to six catechol units incorporated into the ring, have been designed for possible treatment for iron overload and were prepared using high dilution techniques. They form stable iron(III) complexes the complex with the three-catechol ring as ligand has log K= ii.l... 

Tris(bpy) complexes of ruthenium(II) with pendant catechol units are represented by [Ru(bpy)2(114)], isolated as the BF4 salt. Interest in this fluorescent complex stems from its use as a potential skin sensitizer. " In the complex [Ru(bpy)2(115)] + (R = H), the deprotonated catechol unit can act as a binding site for other metal fragments, thereby forming homo- and... 

Octadentate ligands with four catechol units like 140 exhibiting eight coordination sites are capable of effective complexation of not only iron ions, but also much larger Pu4+ [49]. 

Tire early-late transition metal complex of Raymond et al. is interesting in that it requires both metal atoms to form the basic C3 structure (ideally D i, may form). Titanium complexation to three catechol units leads to a tridentate ligand, and, only when palladium bromide is added, trans coordination to palladium gives the agglomerate 31 [78]. 

A smaller turn structure is induced in an N,N -bis(2,3-dihydroxybenzoate)-substituted lysine derivative on coordination to the Mo(VI)C>2 fragment E. In E the conformation of the lysine residue is highly constrained, because of intramolecular hydrogen bonding in combination with coordination of the catechol units to the dioxomolybdenum(VI) moiety [13, 14]. 

Fig. 1.3.6. Schematic representations of all seven isomers of the complexes [l2(OCH3)2Ti2]2. Catecholate units at the N-termini are indicated as gray bars, those at the C-termini as black bars.

This can be achieved by using the cis molybdenum(VI)dioxo ion instead of titanium(IV). In this ion two comers of the octahedron at the molybdenum are already blocked by oxygen atoms and only two catechol units can be bound to the metal. Thus, reaction of ligand 3-H4 with MoC>2(acac)2 in the presence of potassium carbonate leads to a mononuclear macrocydic complex [3Mo0212 in which a loop-type conformation is stabilized at the peptide (Scheme 1.3.3) [23]. (Similar... 

The effective molarity measured for [Phen2]PAAcPEI may be compared with that for enterobactin, the strongest microbial siderophore, containing three catechol units connected by a spacer. The effective molarity of a catechol unit towards a Fe(m) ion bound to another catechol unit contained in enterobactin is estimated to be 3 x 104 m [37]. Enterobactin contains three catechol units, whereas the Cu(ii) binding site of [Phen2]-paAcPEI consists of only two phenanthrolines. Nevertheless, the effective molarity observed for [Phen2]PAAcPEI is extraordinary for a synthetic system. 

Within the cage M8[Ti4(L21)4] (M = Li+, Na+ or K+) four counterions are bound to the internal oxygen atoms of the titanium tris(catecholate) units together with three DMF molecules per cation. Those counterions could be exchanged successively with primary ammonium ions as shown in Fig. 18. This exchange could be monitored by hi NMR spectroscopy, e.g. for the Lig[Ti4(L21)4] complex [139]. 

Native, as well as technical lignins, whether present in pulp (wood) or in the form of isolated samples, usually contain trace amounts of catechol structures These can be identified and quantified if the alkylation step in the oxidative degradation procedure is performed with diethyl sulfate since, in this case, various types of catechol units in the form of 1,2-diethoxybenzene derivatives are distinguished from guaiacyl and synngyl structures (Gellerstedt and Lindfors... 

The determination of catechol groups in various lignins is of practical and fundamental importance (Chap. 5.2.3.3). That such compounds have not been detected in acidolysis studies (Gellerstedt et al. 1984) may be attributed to their unstability under the acidolysis conditions. The indicated detection of catechol groups implies that structures such as p-hydroxycinnamyl alcohol end groups and catechol units, which are unstable in acidic aqueous media, may be determined by thioacidolysis. 

Reaction of indoline with 66 yields a more stable condensation product (76), which was isolated as its crystalline perchlorate salt (97) (Scheme 5). Saponification of 76 with concentrated HCl afforded the dicarboxylic acid (77), which was cleaved with aqueous ammonia to ( )-betalamic acid (3) and then characterized as its dimethyl ester (58). Interestingly, it was not possible to cleave ester 76 directly because of smooth oxidative aromatization to the neo derivative 78. This proves that the catechol unit is of no importance for the formation of neo compounds from betalains. 

The catecholamines all contain a 3,4-dihydroxyphenyl group (catechol unit) as common strucmre element, but differ with respect to the 1-substituent (Figure 7.1). The enzyme phenylamine-p-hydroxylase or dopamine-p-hydroxylase introduces the hydroxyl group stereospecificafly and converts the achiral DA into chiral NE in the R stereoisomeric... 

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