Reactions, chelate

Doxorubicin and dannorabicin are antibiotics made from microorganisms of the family Streptomyces peucetius. The stmcture of these anthracyclines contains an aminosaccarhide residue daunozamine attached to a naphthacenequinone nucleus. Doxorubicin differs from daunorubicin in the presence of a hydroxyl gronp at C14. A nnmber of mechanisms have been suggested in which anthracyclines exhibit cytotoxicity. They canse DNA to denature, are involved in oxidation-rednction reactions, chelate bivalent cations and react with cell membranes, changing their fnnction. They are used for severe leukemia, lymphoma, breast and ovarian cancer, and other solid tumors. 

An approach to the regioselective synthesis of metal complexes was offered based on the joint use of chemical and electrochemical methods of synthesis of coordination compounds departing from the same ligands [11], for instance transformation (4.37). As a result of these reactions, chelates with the participation of different N atoms of the imidazole ring, of the ligand in coordination, are formed ... 

Besides allylic alkylation reactions chelated zinc ester enolates 231 also give good results in various types of standard enolate reactions, including alkylations, aldol reactions" and Michael additions" . 

The stereochemical outcome of the addition of carbanions to ketones yielding tertiary alcohols (or secondary alcohols in the case of aldehydes) is variable and depends on the substrate, the counterion and the solvent. Numerous applications of this strategy to natural product synthesis from carbohydrates can be found in the literature and this approach was fruitful in pioneering syntheses of polyketide-type products. Here again, the template effect of the sugar plays a tremendous role in the stereochemical outcome of the reaction. Chelation controlled nucleophilic addition can also be used to form chiral centers in a highly predictable way. 

In general, metal ion exchange by a chelating resin is intraparticle diffusion controlled (IPDC) accompanied by chemical reaction (chelation).Both Pick s law and Nernst-Planck have been used to describe... 

Reaction with a-ketoesters. In a regio- and diastereoselective addition reaction, chelation control provides optically active a-hydroxy esters. 

Rhodium(IV) complexes magnetic properties, 273 Ring-forming reactions chelate, 433... 

Copper complexes of the bisoxazoline ligands have been shown to be excellent asymmetric catalysts not only for the formation of carbocyclic systems, but also for the hetero-Diels-Alder reaction. Chelation of the two carbonyl groups of a 1,2-dicarbonyl compound to the metal atom of the catalyst sets up the substrate for cycloaddition with a diene. Thus, the activated diene 20 reacts with methyl pyruvate in the presence of only 0.05 mol% of the catalyst 66 to give the adduct 138 with very high enantiomeric excess (3.99). 

Most reactions among cyclometalation reactions are orthometalation reactions. These reactions are used for the synthesis of orthometalation products or derivatives of orthometalation products. Pfefifer et al. [19] named these synthetic reactions chelation-assisted reactions in 2002. 

The Kishi synthesis ot monensin teatures allylic conformational analysis to predict stereochemistry of hydroboratlon-oxidations in acyclic systems. The Still synthesis features acyclic diastereoselection in carbonyl addition reactions (chelation control and Felkin-Ahn control). 

The most popular promoters are BF3 OEt2, TiCU, and SnCU. They are followed, arguably, by AICI3, AlC Et, and TMSOTf. Protic acids are used more rarely since they tend to cause an unwanted protodesilylation. BF3, TiCU, and SnCL have somewhat different properties. BF3 does not react with allylsilanes directly, and has only one coordination site. TiCU is also inert towards allylsilanes, even at room temperature, and has two coordination sites. TiCU forms 1 1 or 1 2 adducts with aldehydes, depending on the stoichiometry of the reagents. It can also form chelates with a- and P-alkoxyaldehydes, which influences the stereochemistry of the reaction (chelation control). TiCU is best used at low temperatures (-78 °C) to prevent the formation of unwanted chlorination products that appear at room temperature. The major characteristic of SnCU is that, in the presence of an allylsilane, it undergoes a transmetallation within minutes at -80 °C. Hence,... 

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