Compounds labeling, preferential

Positron emission tomography (PET) makes use of a short-lived positron emitter such as fluorine-18 to image human tissue with a degree of detail not possible with x-rays. It has been used extensively to study brain function (see illustration) and in medical diagnosis. For example, when the hormone estrogen is labelled with fluorine-18 and injected into a cancer patient, the fluorine-bearing compound is preferentially absorbed by the tumor. The positrons given off by the fluorine atoms are quickly annihilated when they meet... 

Alkanes are only poorly directly iodinated with iodine radicals. Aliphatic compounds with a double bond will be preferentially subject to an addition rather than to a substitution. Also, for aromatic compounds, labeling occurs mainly by the substitution of a halogen atom, and rarely by substitution of a hydrogen atom, with ortho and para positions favored, the ortho position being even more reactive if there is no steric hindrance (Argentini, 1982). 

Therefore, directed evolution was applied to solve these problems. To screen for enantioselectivity, the Miilheim MS-based high-throughput ee assay (92,93) (Section III.C) was applied (46). In this case, the necessary isotope labeling focused on the use of in the pseudo-meso compound N-(J )-17 (see Section III.C for a detailed discussion). An (5)-selective nitrilase leads preferentially to N-(5)-18, whereas an 7 -selective variant results in the picw o-enantiomer (J )-18. They differ by one mass unit and can therefore be distinguished by MS, both qualitatively and quantitatively (by integration of the relevant peaks). 

Scheme 18. If the repetitive Diels-Alder reaction adducts reported were being formed under thermodynamic control, heating the hexadecadeutero 2 1 adduct 68 under reflux in toluene in the presence of a 10 molar equivalent of the bisdiene 37 should give rise to a mixture of compounds comprised of 68, 69, and 41. A retro Diels-Alder reaction of 68, followed by scrambling of the labeled bisdiene 66 with the excess of the bisdiene 37 should favor preferential cycloaddition with 37 to afford 69. The excess of 37 should ultimately drive the equilibration process toward the unlabeled 2 1 adduct 41. No evidence for such an equilibration process has been found... 

Oxidation of ADP using periodate, followed by partial reduction with boro-hydride, affords a mixture of the fully reduced bis(hydroxymethyl) product and the semialdehydic reduction intermediate. By dint of comparison with model compounds using n.m.r. spectrometry, it has been shown that the first reduction step takes place at C-3, to give (107) (which will prefer to cyclize to the hemiacetal). It is not clear why preferential reduction at C-3 occurs either the aldehydic group at this position is more reactive per se, or it is more accessible to borohydride. It seems likely that (107) and its analogues could prove to be useful new affinity labels. 

For the first time, Locher (1936) noted that if the reaction of B (n, a) Li is conducted in tissues, the energy of the particles produced would destroy the immediate cell but not its neighbors. The effect of such treatment depends on the total number of captured neutrons and concentration of B isotopes in the tissue. The minimum B concentration needed in a tumor to achieve a therapeutic effect was estimated to be 20-30 ag/g (Javid et al. 1952). BNCT uses B labeled compounds that are accumulated preferentially in selected targets such as tumor cells. 

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