Unreacted precursors

Eor virtually all radiopharmaceuticals, the primary safety consideration is that of radiation dosimetry. Chemical toxicity, although it must be considered, generally is a function of the nonradio active components of the injectate. These are often unreacted precursors of the intended radioactive product, present in excess to faciUtate the final labeling reaction, or intended product labeled with the daughter of the original radioactive label. 

The high diastereoselectivity is attributed to the directing influence of the COOCH, group. Incomplete hydrogenation of 5 with the chiral catalyst 4 results in formation of (R,R)-6 further hydrogenation of the unreacted precursor with 1 gives (S,S)-6. Thus the two sy -2,3-dialkylsuccinates can be prepared in >96% ee. ... 

Another characteristic concentration in Fig. 17 is point , which gives the solubility limit of the solvent in the unreacted precursor mixture. If the solvent concentration exceeds this solubility hmit, demixing occurs in the initial state. 

Chlorine dioxide is readily soluble in water, forming a greenish-yellow solution. It is not unusual to simultaneously have multiple chlorine species present in chlorine dioxide solutions originating from byproducts or unreacted precursors. Table 6-3 lists the various chlorine species that might be present in solutions of chlorine dioxide (Gordon 2001). 

Because no tin hydride is present, intermediate radicals are only slowly intercepted by hydrogen atom abstraction. Thus, the fragmentation method is a clever alternative that avoids this course, and low concentrations are not required. As a result of this, even relatively unreactive precursors, such as glycosyl chlorides and phenylsulfides, can be used. Therefore, the method is compatible with the same molecular complexity and an extended spectrum of functionality as found in the tin hydride method. 

The mechanism of this transformation is outlined in Scheme 38 and each step has important features. In step 1, the tributyltin radical abstracts the radical precursor X. A possible side reaction, the addition of the tributyltin radical to the allylstannane, is much slower than comparable additions to activated alkenes. Even if this addition occurs, the stannyl radical is simply eliminated to regenerate the starting materials. Thus, for symmetric allylstannanes, this reaction is of no consequence. As a result, the range of precursors X that can be used in allylation is more extensive than in the tin hydride method. Even relatively unreactive precursors like chlorides and phenyl sulfides can be used if they are activated by adjacent radical-stabilizing groups. 

The LC-purification of 299 has been simplified by addition of w-heptyl iodide, CH3(CH2)6I, to the post-reaction mixture in order to decrease the amount of the unreacted precursor. 

Fig. 8 Evolution of the molar mass (T) and polymer content (O) of hybrid silicones prepared from precursor H (Table 2 full symbol) and precursor M (Table 3 empty symbol). Remaining content represents the fraction of unreacted precursor...

FIGURE 15. High-temperature photoelectron spectrum of the products of the thermolysis at 600 °C of 2-(chloromethyl)-3-methylthiophene (upper spectrum) compared with calculated (MNDO-PERTCI) vertical IEs of 39 and 40. The arrow indicates the presence of unreacted precursor. Reproduced by permission of VCH Verlagsgesellschaft from Reference 38... 

Hence FAIMS can analyze product ions if precursors convert early in the separation. This mode resembles the annealing regime in DT IMS, where ions are rapidly isomerized or dissociated by heating upon energetic injection into the drift tube from a lower-pressure region and the unreacted precursor(s) and/or product(s) are separated or characterized by mobility.Unlike in DT IMS, the remaining precursor(s) and resolved product(s) cannot pass FAIMS under same conditions if their Eq values differ by more than the instmmental resolution, one must be eliminated. However, the precursor(s) or product(s) may be selected by FAIMS and appear as separate peaks in the Eq spectrum. This distinction reflects that DT IMS is a dispersive and FAIMS is a filtering method, and has important implications for FAIMS analyses. 

Chemical vapor deposition is a technique that involves the flow of a carbon precursor gas into a chamber containing one or more heated objects to be coated. Chemical reactions occur on and near the hot surfaces, resulting in the deposition of a thin film on the surface. Ihis is accompanied by the production of chemical byproducts that are exhausted out of the chamber along with unreacted precursor gases. A large variety of materials can be deposited therefore there are many variants of the CVD process [24]. 

J.R. Molek, A.L. Zydney, Separation of PEGylated a-lactalbumin from unreacted precursors and byproducts using ultrafiltration. Biotechnol. Prog., 23 (6) pp.1417-1424,2007. 

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