Chelation strategies

Figure 2.22. Strategies for the transition metal-templated synthesis of catenanes. The metal (in) predisposes two fragments as open chelates (A) (strategy I) or as a macrocyclic chelate (E) and an open chelate (strategy II) in intermediates (B) and (F), respectively. Cyclization of these intermediate complexes with the chain fragments (C) provides the [2]-catenate complex (D).

Using chelation and non-chelation strategies in the context of double asymmetric reactions, Panek and Jain demonstrated that by judicious choice of protecting groups alt four of the dipropionate stereotriad fragments can be prepared [151]. Reactions of the TBDPS-protected )S-alkoxy aldehyde 97c with (R)-crotylsi-... 

Blok D, Feitsman H I J, Kooy Y M C, et al. (2004). New chelation strategy allows for quick and clean Tc-labeling of synthetic peptides. Nucl. Med. Biol. 31 815-820. 

An unsolved and hardly to be solved problem appears to be the development of chelators that immobilize a-emitters. The recoil energy will destroy all currently used chelators. Strategies to use fullerenes to encapsulate a-emitters are under way. These and other nanocontainers may finally serve as the ultimate chelators. 

Silylformylation, in which the H2 component of the H2/CO mixture of hydroformylation is replaced by a silane to give net addition of RsSi— and —CHO across an unsaturated bond, is normally effective for alkynes but not alkenes. Leighton and co-workers have used a chelation strategy (Eq. 14.73) to produce products that can be further functionalized by oxidation of the C—Si bond. 

A synthetic strategy which ensures retention of the monomeric form of SnR2 even in the crystalline state is to use functionalized R groups which contain a chelating substituent, e.g. by replacing the H atom in -CH(SiMe3)2 with a 2-pyridyl group. 

DOX, as EPI seems to form fewer amounts of ROS and secondary alcohol metabolite, (ii) encapsulation of anthracyclines in uncoated or pegylated liposomes that ensure a good drug delivery to the tumor but not to the heart, (iii) conjugation of anthracyclines with chemical moieties that are selectively recognized by the tumor cells, (iv) coadministration of dexrazoxane, an iron chelator that diminishes the disturbances of iron metabolism and free radical formation in the heart, and (v) administration of anthracyclines by slow infusion rather than 5-10 min bolus (Table 1). Pharmacological interventions with antioxidants have also been considered, but the available clinical studies do not attest to an efficacy of this strategy. 

Most of the anionic compounds that have been reported contain six oxygen atoms at the periphery of the pseudo-octahedral phosphorus. Two different strategies have been used for the preparation of the moieties depending upon the homogeneous (three times the same chelate) or heterogeneous (two different types) distribution of the bidentate ligands. 

Figure 7 Mixld for iron (Fe) deficiency induced changes in root physiology and rhizo-sphere chemistry associated with Fc acquisition in strategy I plants. (Modified froin Ref. 1.) A. Stimulation of proton extru.sion by enhanced activity of the plasnialemma ATPase —> Felll solubilization in the rhizospherc. B. Enhanced exudation of reductanls and chela-tors (carhoxylates. phenolics) mediated by diffusion or anion channels Pe solubilization by Fein complexation and Felll reduction. C. Enhanced activity of plasma membrane (PM)-bound Felll reductase further stimulated by rhizosphere acidificalion (A). Reduction of FolII chelates, liberation of Fell. D. Uptake of Fell by a PM-bound Fell transporter.

In contrast to strategy 1 plants, grasses are characterized by a diffeient mechani.sm for Fe acquisition, with Fe-mobilizing root exudates as main feature. In response to Fe deficiency, graminaceous plants (strategy II plants) (39) are able to release considerable amounts of non-proteinaceous amino acids (Fig. 8B), so called phytosiderophores (PS), which are highly effective chelators for Felll (Fig. 8)... 

From the fundamental knowledge concerning the interfacial complexation mechanism obtained from the kinetic studies on chelate extraction, ion-association extraction, and synergistic extraction, one can design the interfacial catalysis. The main strategy is to raise the concentration of the reactant or intermediate at the interface. 

Arylation of alkynes via addition of arylboronic acids to alkynes represents an attractive strategy in organic synthesis. The first addition of arylboronic acids to alkynes in aqueous media catalyzed by rhodium was reported by Hayashi et al.89 They found that rhodium catalysts associated with chelating bisphosphine ligands, such as 1,4-Ws(diphenyl-phosphino)butane (dppb) and 1,1 -/ E(diphenylphospliino)fcrroccnc... 

In iron-deficient conditions Strategy I plants (Figure 4.1) acidify the soil, through the activation of a specific H+-ATPase (Guerinot and Yi, 1994), increase their reducing capacity at the root plasma membrane, and possibly also release reduc-tants into the rhizosphere/I 11 They also release iron chelators, such as caffeic acid... 

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