Target compound

Baccalaureate chemists who never took part in research familiarize themselves with its rudiments, particularly the elements of traditional solution-phase 

Is the starting material for the first reaction of the sequence an item of commerce If so, do we possess it now if not, how soon can I obtain it and how much do I need  

Does the appropriate functional group of the available reactant form the desired atomic grouping in the product sought and do so safely  

What is the stoichiometiy of the planned reaction, and what byproducts does it form, if any  

What reagent(s), catalyst(s), solvent(s), atmosphere, and conditions effect the change, and, if a choice exists, which maximizes the purity and yield of the product  

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Developing a suitable synthesis strategy for a target compound by searching for synthesis precursors, starting materials and synthesis reactions... 

This is the domain of synthesis design (Figure 10.3-Ic). The product of the reaction is known and one has to work back from the reaction product to synthesis precursors that provide, on reacting, the desired target compound. This procc.ss has to be repeated until one arrives at available starting materials, A , Synthesis design is the theme of Section 10.3-2. 

How do chemists find a pathway to the synthesis of a new organic compound They try to find suitable starting materials and powerful reactions for the synthesis of the target compound. Thus, synthesis design and chemical reactions are deeply linked, since a chemical reaction is the instrument by which chemists synthesize their compounds synthesis design is a chemist s major strategy to find the most suitable procedure for a synthesis problem. 

The target compound is searched for a rctron. A retron is the structural subunit required to be present in the target in order to apply a transform. In Figure 10,3-30 the rctron of a Michael addition is a sequence of five carbon atoms with two carbonyl functions in the 1,5-position. For a Michael addition transform to be applied, it has to be present,... 

A siynthestis tree is the graphical representation of the result of a retrosynthetic analysis. The target compound (T) is set to the top of a tree that is turned upside down (Figure 10..3-3.3). 

Tran orm-based or long-range strategies The retrosynthetic analysis is directed toward the application of powerful synthesis transforms. Functional groups are introduced into the target compound in order to establish the retion of a certain goal transform (e.g., the transform for the Diels-Alder reaction, Robinson annulation, Birch reduction, halolactonization, etc.). 

Substructure searches provide another method of searching for available starting materials. They arc used primarily for planning the synthesis of combinatorial libraries. After the target compound has been dissected into a set of suitable precursors, substructure searches can provide for each of them a series of representatives of a certain class of compounds, Siibsti ucturc searches enable the user to specify attributes such as open sites or atom lists at certain positions of the structure. Figure 10.3-38 shows the possible specification elements for the query in a substructure search. 

In order to draw a desired target compound WODCA provides a molecule editor which has to be launched. This can be performed by pressing the "New Compound" button in WODCA s button bar, The molecule editor is then started and aftei"wards the target compound can be drawn directly with the mouse. 

Figure 10.3-56. Entire synthesis tree for target compound 1.

The design of the synthesis foi 1 is now finished. The target eompound (1) can be simplified to eommeicially available starting materials which can easily be converted to the target compound in the synthesis direction, Figure 10.3-56 shows the entire synthesis tree interactively developed with the WODCA program. 

The retrosynthetic analysis of a target compound is a systematic approach in developing a synthesis plan starting with the target structure and working backward to available starting materials. 

Most programs take a retrosynthetic approach. This is a means for systematically working backward from the target compound to available precursors... 

Complex environmental samples originate from diverse matrices (the predominant material of which the sample to be analyzed is composed). These matrices, usually either water or soil/sediment, can contain as many as 50 to 100 organic components at widely varying concentrations. The EPA approach to the analysis of these samples involves the analysis of specific (or target) compounds and the use of authentic standards for quality control. The current number of standards in the EPA repository is about 1500, and their analysis is covered by various approved methods. 

Identification of some target compounds in an environmental sample. The report lists the closeness of match and the estimated quantities for the listed compounds. 

The next step is to show that the response for the analysis of any target compound is linear. This step is known as the initial calibration and is achieved by the analysis of standards for a series of specified concentrations to produce a five-point calibration curve (Figure 41.2a, b). On subsequent days, a continuing calibration must be performed on calibration check compounds to evaluate the calibration precision of the GC/MS system. 

Comparison of the mass spectrum from a target compound (top), with the three best fits from the library of standard spectra (lower three traces). The closeness of fit of the mass spectra and the chromatographic retention time lead to a positive identification of 2, 6-dimethylheptane. 

Chromatographic peak areas are calculated automatically by the data system by reference to the response obtained from certain specified, compound-dependent ions. From the peak areas of the target compounds, quantification is achieved by comparison with the internal standards, which are present in known concentration. The laboratory responsible for the analysis must report the target compounds and all tentatively identified (nontarget) compounds. Standard EPA forms must be completed and submitted. A laboratory is said to be in compliance when it has satisfied all aspects of its CLP contract. 

Target compounds are specified for each Series Method. Volatile compounds that need to be analyzed can be extracted from the matrix by a purge-and-trap device. 

To produce a quantitative result, chromatographic peak areas of identified target compounds are compared with peak areas of the internal standards, which are of known concentration. 

Quantitative mass spectrometry, also used for pharmaceutical appHcations, involves the use of isotopicaHy labeled internal standards for method calibration and the calculation of percent recoveries (9). Maximum sensitivity is obtained when the mass spectrometer is set to monitor only a few ions, which are characteristic of the target compounds to be quantified, a procedure known as the selected ion monitoring mode (sim). When chlorinated species are to be detected, then two ions from the isotopic envelope can be monitored, and confirmation of the target compound can be based not only on the gc retention time and the mass, but on the ratio of the two ion abundances being close to the theoretically expected value. The spectrometer cycles through the ions in the shortest possible time. This avoids compromising the chromatographic resolution of the gc, because even after extraction the sample contains many compounds in addition to the analyte. To increase sensitivity, some methods use sample concentration techniques. 

Important to environmental analysis is the ability to automate the injection, as weU as the identification and quantitation of large numbers of samples. Gc/ms systems having automatic injectors and computerized controllers have this capabiUty, even producing a final report in an unattended manner. Confirmation and quantitation are accompHshed by extracting a specific ion for each of the target compounds. Further confirmation can be obtained by examining the full scan mass spectmm. 

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