Preparation isotopes

Preparing isotopically enriched carbonyls and related species is not always a trivial problem. We have recently developed (20) a method which looks particularly promising in some cases. CW CO2 laser irradiation of a gas phase mixture containing SF5 as an energy transfer agent can promote thermal chemistry without complications due to wall reactions, e.g. 

We have seen that isotope effects on the properties of atoms and molecules are usually small, and this is true for all except the lightest elements. Consequently separation of single isotopes from mixtures of isotopes or isotopomers is tedious and difficult. The difficulty is compounded by the fact that the desired isotope is often present at low or very low concentration in the starting material (normally a naturally occurring fluid, ore, or mineral). Even so, the nuclear properties of certain separated isotopes are enough different from their sisters to justify the (usually enormous) expense of preparing isotopically pure or nearly pure materials. Three important examples follow ... 

Vitamin B12s reacts rapidly with alkyl iodides (e.g., methyl iodide or a 5 -chloro derivative of adenosine) via nucleophilic displacement to form the alkyl cobalt forms of vitamin B12 (Eq. 16-31). These reactions provide a convenient way of preparing isotopically labeled alkyl cobalamins, including those selectively... 

Since frequency shifts are expected, it is also helpful if crystals that yield isotopic variants of the probe molecule are easy to prepare. Isotopic molecules help in assigning bands definitively and in explaining band shifts. 

The A-nitroimidazolcs are stable for a time even in the presence of water, but treatment with concentrated sulfuric acid cleaves the N-nitro group, and strong base opens the ring. Much of the interest in such compounds is related to their multistep complex substitution reactions in which sequential nucleophilic addition of arylamines, ring opening, ring closure, nitroamide elimination, and rearomatization gives l-aryl-4-nitroimidazoles, e.g., 840 839. This method can also be used to prepare isotopically labeled imidazoles when labeled amino acids are used as the amine. 

W.R. SMYTHE, L.H. RUMBAUGH and S.S. WEST succeed in the first preparative isotope separation [22],... 

The method has been used to prepare isotopically labelled amino acids. While Boc-BMI enolate adds to aldehydes with only moderate diastereoselectivity, reduction of the acylation products (5) gives allothreonine derivatives (6). Michael additions to o(,p-unsaturated esters, ketones, and nitro compounds lead to products of type (7) and (8) (for a general discussion see Suzuki and Seebach ). 

Carbitol, bp 231°) may be added, and the desired alcohol may be distilled directly from the reaction in pure, anhydrous form. This property is particularly valuable in preparing isotopically labeled compounds. 

To obtain C o we prepare isotopically pure carbon-13 rods using a straightforward method that can be carried out with... 

The first condition is usually met in deuterium compoimds but the heavier isotopes such as C, N, and 0 are more difficult to obtain pure and are consequently very expensive. It is also impractical to prepare isotopically pure compounds containing tritium or " C as apart from their expense and possible hazard these compounds would decompose from their own radiation. 

Various isotopes have been used for this purpose, but at the present time most of the work is done with an artificially-prepared isotope of technetium, Tc, The letter m indicates that this isotope is an isomeric form of technetium 99 another form,... 

Several laboratories have prepared isotopically enriched organotin species to use as internal standards in the technique of isotope dilution analysis. This MS technique can provide higher levels of accuracy than other calibration techniques and is currently used in many reference laboratories. 

Labeled compound Preparation Isotope recovered in (or as) Remarks Ref. ... 

Most curium isotopes heavier than Cm have longer half-lives but cannot be prepared isotopically pure by neutron capture, and, except for Cm, isotope enrichment is accomplished in a mass separator. Approximately 1 kg of high-mass curium has received substantial neutron exposure at the Savannah River plant. The major isotopes formed are (54 %), Cm (34 %), and Cm... 

A review of the methods for preparing isotopically enriched glycosyl phosphates and nucleoside diphosphates also covers enzymic preparations and solution conformations. A method for synthesizing aldosyl phosphates involving oxidation of the... 

Interestingly, complex 21 underwent essentially instantaneous deutera-tion, which was already complete during the NMR sample preparation. Isotope exchange involved the heterocyclic proton at C4 exclusively. Complex 23 was less reactive and H/D exchange occurred first at the C2-bound CHj group (one hour). Subsequent deuteration of the heterocyclic carbon C4 position was significantly slower and required two days to reach completion. These observations point to a more electrophilic character of the triazolylidene heterocycle (fast reactivity towards OH"), while the imidazolylidene is rather nucleophilic (fast reaction with H ), hence highlighting a remarkable difference between the two types of abnormal carbene. 

Biotransfonnations are selective chemical modifications of molecules by biological entities ranging from isolated enzymes (ceU-free systems) to intact organisms. In particular cases they are exploited by the chemical and pharmaceutical industries for production of chemicals this approach can be especially advantageous for structurally complex compounds. Preparative biotransformations have been used on a laboratory scale by radiochemists to prepare isotopically labeled compounds necessary for drug development and other applications in the life sciences. Their synthetic use for isotope labeling includes ... 

Growing plants or plant tissue cultures have been used to prepare isotopically labeled secondary metabolites an example is that of three pyrrolizidine alkaloids from Senicio... 

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