Contrast reagents

The reactions with (25,35,45,55)-5-(tm-butyldimethylsilyloxy)-3-(4-methoxyphenylmethoxy)-2,4-dimethylheptanal (15) are particularly informative reagent (5)-3 is incapable of overriding the intrinsic diastereofacial preference of 15, and the normal Felkin product 17 is obtained with >95% selectivity. In contrast, reagent-controlled mismatched double diastereoselectivity is evident in the reaction with (5)-4 that provides 16 as the major component of a 73 22 5 mixture. The minor product 18 apparently derives from a reaction with the contaminating (/ )-4, since (5)-4 that was used is not enantiomerically pure. 

The structures of some gadolinium chelates used as contrast reagents are shown in Fig. 12.37. 

The chemical factors which contribute to the stability of contrast reagents are (i) multidentate ligands, (ii) high thermodynamic stability, (iii) kinetic inertness in some cases. In future, efforts need to be directed towards the search and synthesis of new CRs with balanced higher relaxivity and acceptable stability. 

Applications of blocking principle based contrast reagents are many and wide-ranging. The application in gene expression and as a method of study of embryonic development will be briefly described. The contrast reagent, 1,4,7,10-tetraazacyclododecane-... 

The formation of a number of lanthanide texaphyrin complexes has been reported [95]. In all cases, metal insertion and oxidation proceeds smoothly (Scheme 16) [95]. The complexes demonstrate fair water solubility and good stability towards hydrolysis. Detailed kinetic studies of complex 147, for instance indicated that the half-life for decomplexation and/or decomposition of this complex is 37 days in a 1 1 mixture of MeOH H20 (pH7). Thus, it appears that gadolinium (III) complexes of texaphyrins could provide the basis for a new approach to paramagnetic MRI contrast reagent development [95]. 

CD probes for chirality sensing of biological substrates, Enantiomer 5, 13-22, 2000). Smart contrast reagents have also been developed for magnetic resonance stndies (Lowe, M.R, Activated MR contrast reagents, Curr. Pharm. Biotechnol. 5, 519-528, 2004). 

Lemieux GA, Yarema KJ, Jacobs CL, Bertozzi CR. Exploiting differences in sialoside expression for selective targeting of MRI contrast reagents. J. Am Chem. Soc. 1999 121 4278-4279. Gross HJ. Huorescent CMP-siaUc acids as a tool to study the specificity of the CMP-siaUc acid carrier and the glyco-conjugate sialylation in permeabilized cells. Eur. J. Biochem. 1992 203 269-275. 

Botta, M. Seeond eoordination sphere water molecules and relaxivity of gadolinium (III) eomplexes Implications for MRI contrast reagents. Eur. J. Inorg. Chem. 2000. 3. 399-407. 

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