Chemistry
Synthons
Synthons are hypothetical molecular fragments that are used as building blocks in retrosynthetic analysis, a method for planning the synthesis of complex molecules. They represent the disconnection of a molecule into simpler, readily available starting materials. By identifying synthons, chemists can work backwards from a target molecule to determine the most efficient route for its synthesis.
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eBook - PDFThe Chemistry of Natural Products
Main Lectures Presented at the Fourth International Symposium on the Chemistry of Natural Products Held in Stockholm, Sweden, 26 June—2 July, 1966
- Sam Stuart(Author)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
The synthetic chemist has learned by experience to recognize within a target molecule certain units which can be synthesized, modified, or joined by known or conceivable synthetic opera-tions. Thus, with the knowledge of the coupling reactions of phenoxy radicals, enolization reactions, Michael-type addition reactions, and 1,2-elimination reactions, two synthetic units can be recognized within the usnic acid structure which are related to the fragments (VI) and (VII) and, therefore, to the precursor (V). It is convenient to have a term for such units; the term synthon is suggested. These are defined as structural units within a molecule which are related to possible synthetic operations (and, therefore, to the reverse operations of degradation). A synthon may be almost as large as the molecule or as small as a single hydrogen ; the same atoms within a molecule may be constituents of several overlapping Synthons. Thus, for the molecule (VIII) many Synthons can be recognized, including a-h. The units (d) and (e) are valid Synthons, since they may be joined by a known synthetic operation, Michael addition (after minor modification to C 6 H 5 COCHCOOCH 3 and CH 2 =GH—COOCH3 + H+) In general, the greater the number and variety of Synthons which can be defined, the greater will be the complexity of the synthetic problem. Further, recognition of some Synthons (usually the larger or major Synthons) normally is more useful in analysis than that of others. The greater the degree of connectivity within a molecule (e.g., the larger the number of rings), the greater the number of possible Synthons. In principle there is no reason why the number of derivable Synthons cannot exceed the number of atoms in a molecule rich in internal connection. The recognition of Synthons within molecules is purely a utilitarian device ; the derivation of all the possible Synthons in a molecule may never be required.
eBook - ePubHybrid Retrosynthesis
Organic Synthesis using Reaxys and SciFinder
- Michael B. Smith, John D'Angelo(Authors)
- 2015(Publication Date)
- Elsevier(Publisher)
One target molecule may contain several groups that are identical, such as a molecule with six different alcohol units, or three different ketone units. Such diversity makes synthesis virtually impossible without a thorough understanding of many different reactions, and why certain ones are better choices under specific conditions. It is therefore important to understand several different ways to make or incorporate the same functional group. On the plus side, a working knowledge of synthesis and protocols used to synthesize molecules can help with remembering reactions and also understanding them. Students in their first organic chemistry course often struggle with correlating the correct reaction and reagent with the correct functional group and may not appreciate this observation. This book attempts to fix this failure to see the forest through the trees, at least for those who have completed the two-semester organic chemistry sequence. To that end, the discussions in this book will provide a working library of common reactions and protocols used to synthesize molecules. The chemical structure of medicines and other important molecules are characterized by the presence of many carbon atoms and often several functional groups. If such a molecule is not readily available from a commercial source, or a preparation is not found in the literature, a synthesis must be devised for that molecule. Normally, any synthesis requires choosing a commercially available molecule of fewer carbons as a starting material, and building the molecule, step-by-chemical step. Building a molecule in this manner is known as chemical synthesis, and it requires making carbon–carbon bonds to convert a smaller molecule into a larger and more complex one. Arguably, the fastest way to find out how well one understands reactions in organic chemistry is to attempt a synthesis that requires many different reactions
Available until 20 Nov |Learn more- Ann M. Fabirkiewicz, John C. Stowell(Authors)
- 2015(Publication Date)
- Wiley(Publisher)
8 PLANNING MULTISTEP SYNTHESES The challenge in synthesis is to devise a set of reactions that will convert inexpensive, readily available materials into complex, valuable products. Ordinarily this is not an obvious following of a roadmap, but rather a complex puzzle requiring much strategy. This chapter gives samples of the planning process with actual syntheses of relatively simple cases. Enough of the procedure is provided to enable you to analyze and devise syntheses for many molecules. A more extensive treatment is given in an excellent book by Warren [1]. 8.1 RETROSYNTHETIC ANALYSIS You’ll become familiar with the sorts of compounds that are available as you read and study the chemical literature, but the actual process of synthesis begins at the end; that is, you must study the desired structure and work backward. What penultimate intermediate would be readily converted to that product, and then what before that? This process is called retrosynthetic analysis, and each backward step is indicated by a double-shafted arrow (⇒). A backward scheme is drawn, and then a forward process is developed with actual reagents, indicated with ordinary arrows. In more complicated syntheses, you will need to look ahead toward steps in the middle of the process, but still a backward approach is most practical. The steps include functional group interconversions as given in Chapter 6 and carbon backbone construction as illustrated in Chapter 7. Viewed as the disassembly of the product, you should first disconnect the parts that are joined by functional groups; for example, esters should be separated to acid and alcohol parts. The carbon–carbon bonds should be disconnected at or near functional groups and at branch points in the backbone. There are often a great many choices of dividing points and starting materials. For example, jasmone and dihydrojasmone have been made by hundreds of routes [2]
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