Chemical structures, lanthanide-induced shifts

The chemical shifts and lanthanide-induced shifts of the methyl resonances of a series of stictane and related flavicane triterpenoids have been examinedand provide supporting evidence for previous structural assignments (see Vol. 4, p. 219). 

Solution structure determination essentially relies on NMR data, via either a classical analysis of the number of signals and of their chemical shifts with the help of two-dimensional COSY, NOESY, and NOEDIF measurements, or the more sophisticated investigation of both lanthanide-induced shifts (LIS) and relaxation times (LIR). When the major species in solution is the heterobimetallic helicate, analysis of the... 

Detailed solution structures have been obtained thanks to the analysis of the paramagnetic NMR lanthanide-induced relaxation and chemical shifts (Piguet and Geraldes, 2003, see Section 3.4) combined with high-resolution emission spectra recorded for the Eu helicates (Bunzli and Piguet, 2005 see Section 3.5). In all cases, the crystal structures represent... 

Solution structures deduced from LIS data. - Andre et a/." have prepared new ditopic trihelicate lanthanide (La-La -L3) complexes, which they consider of potential utility as biomedical probes. The solution structures of these species were determined by analyses of lanthanide-induced chemical shifts and proton spin relaxation data of mixed complexes containing one diamagnetic Lu + ion and one paramagnetic lanthanide ion. 

Hyperflorin, an antibiotic isolated from St. John s wort Hypericum perforatum L.), has been found on the basis of chemical and spectroscopic evidence to possess structure (639). ° A review on the conformational analysis of bicyclo[3,3,l]nonanes has appeared,and the conformations of l,5-disubstituted-3,7-dimethyIenebicyclo-[3,3,l]nonane derivatives have been shown, on the basis of lanthanide-induced n.m.r. shifts, to be predominantly in the double-chair form. In a notable paper that contains a wealth of information pertinent to adamantane synthesis, it has been reported that bicyclo[3,3,l]nonane-3,7-dicarboxylic acid, its dimethyl ester, and various derivatives exist mainly in the chair-boat conformation. The synthetic entry to these systems is illustrated for the above diester (643) in which the starting point is adamantanone (640) thus, ring expansion with diazomethane followed by SeOj oxidation gave the a-diketone (641) which was cleaved with periodate to give the diacid (642). Esterification using diazomethane gave (643) which exists, as does the diacid, as... 

These comparisons should be taken into account when substituting lanthanide ions for first-row transition-metal ions in order to induce chemical shifts or change the relaxation behavior of a n.m.r. spectrum to elicit ion sequestering and structural information (see later). 

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