Organic synthesis

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This article is about artificial synthesis of organic compounds. For the journal Organic Syntheses, see Organic Syntheses. For synthesis in organisms, see Biosynthesis.

Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules often contain a higher level of complexity than purely inorganic compounds, so that the synthesis of organic compounds has developed into one of the most important branches of organic chemistry. There are several main areas of research within the general area of organic synthesis: total synthesis, semisynthesis, and methodology.

Total synthesis[edit]

Main article: Total synthesis

A total synthesisis the complete chemical synthesis of complex organic molecules from simple, commercially available (petrochemical) or natural precursors. [1][page needed] Total synthesis may be accomplished either via a linear or convergent approach.[citation needed] In a linear synthesis—often adequate for simple structures—several steps are performed one after another until the molecule is complete; the chemical compounds made in each step are called synthetic intermediates.[citation needed] For more complex molecules, a convergent synthetic approach may be preferable, one that involves individual preparation of several "pieces" (key intermediates), which are then combined to form the desired product.[citation needed]

Robert Burns Woodward, who received the 1965 Nobel Prize for Chemistry for several total syntheses[citation needed] (e.g., his 1954 synthesis of strychnine[2][non-primary source needed]), is regarded as the father of modern organic synthesis. Some latter-day examples include Wender's,[citation needed] Holton's,[citation needed] Nicolaou's,[citation needed] and Danishefsky's[citation needed] total syntheses of the anti-cancer therapeutic, paclitaxel (trade name, Taxol).[citation needed]

Methodology and applications[edit]

Each step of a synthesis involves a chemical reaction, and reagents and conditions for each of these reactions must be designed to give an adequate yield of pure product, with as little work as possible.[3][page needed] A method may already exist in the literature for making one of the early synthetic intermediates, and this method will usually be used rather than an effort to "reinvent the wheel". However, most intermediates are compounds that have never been made before, and these will normally be made using general methods developed by methodology researchers. To be useful, these methods need to give high yields, and to be reliable for a broad range of substrates. For practical applications, additional hurdles include industrial standards of safety and purity.[4][non-primary source needed]

Methodology research usually involves three main stages: discovery, optimisation, and studies of scope and limitations.[citation needed] The discovery requires extensive knowledge of and experience with chemical reactivities of appropriate reagents.[citation needed] Optimisation is a process in which one or two starting compounds are tested in the reaction under a wide variety of conditions of temperature, solvent, reaction time, etc., until the optimum conditions for product yield and purity are found.[citation needed] Finally, the researcher tries to extend the method to a broad range of different starting materials, to find the scope and limitations.[citation needed] Total syntheses (see above) are sometimes used to showcase the new methodology and demonstrate its value in a real-world application.[citation needed] Such applications involve major industries focused especially on polymers (and plastics) and pharmaceuticals.[citation needed]

Stereoselective synthesis[edit]

Main article: Chiral synthesis

Most complex natural products are chiral, and the bioactivity of chiral molecules varies with the enantiomer.[citation needed] Historically, total syntheses targeted racemic mixtures, mixtures of both possible enantiomers, after which the racemic mixture might then be separated via chiral resolution.[citation needed]

In the later half of the twentieth century, chemists began to develop methods of stereoselective catalysis and kinetic resolution whereby reactions could be directed to produce only one enantiomer rather than a racemic mixture.[citation needed] Early examples include stereoselective hydrogenations (e.g., as reported by William Knowles[citation needed] and Ryōji Noyori[citation needed]), and functional group modifications such as the asymmetric epoxidation of Barry Sharpless;[citation needed] for these specific achievements, these workers were awarded the Nobel Prize in Chemistry in 2001.[5] Such reactions gave chemists a much wider choice of enantiomerically pure molecules to start from, where previously only natural starting materials could be used.[citation needed] Using techniques pioneered by Robert B. Woodward and new developments in synthetic methodology,[citation needed] chemists became more able to take simple molecules through to more complex molecules without unwanted racemisation, by understanding stereocontrol,[citation needed] allowing final target molecules to be synthesised pure enantiomers (i.e., without need for resolution). Such techniques are referred to as stereoselective synthesis.[citation needed]

Synthesis design[edit]

Elias James Corey brought a more formal approach to synthesis design, based on retrosynthetic analysis, for which he won the Nobel Prize for Chemistry in 1990. In this approach, the synthesis is planned backwards from the product, using standard rules.[6][page needed] The steps "breaking down" the parent structure into achievable component parts are shown in a graphical scheme that uses retrosynthetic arrows (drawn as ⇒, which in effect, mean "is made from").

More recently,[when?] and less widely accepted,[citation needed] computer programs have been written for designing a synthesis based on sequences of generic "half-reactions".[7]

See also[edit]

Publications[edit]

Specialized methods[edit]

  • Electrosynthesis, a synthetic method that uses electricity to initiate a desired chemical reaction.

References[edit]

  1. ^ Nicolaou, K. C.; Sorensen, E. J. (1996). Classics in Total Synthesis. New York: VCH. [page needed]
  2. ^ Woodward, R. B.; Cava, M. P.; Ollis, W. D.; Hunger, A.; Daeniker, H. U.; Schenker, K. (1954). "The Total Synthesis of Strychnine". Journal of the American Chemical Society 76 (18): 4749–4751. doi:10.1021/ja01647a088. [non-primary source needed]
  3. ^ March, J.; Smith, D. (2001). Advanced Organic Chemistry, 5th ed. New York: Wiley. [page needed]
  4. ^ Carey, J.S.; Laffan, D.; Thomson, C. & Williams, M.T. (2006). "Analysis of the reactions used for the preparation of drug candidate molecules". Org. Biomol. Chem. (print, online research report) 4: 2337–2347. doi:10.1039/B602413K. [non-primary source needed]
  5. ^ Service. R.F. (2001). "Science Awards Pack a Full House of Winners" (print, online science news). Science 294 (5542; October 19): 503–505. doi:10.1126/science.294.5542.503b. PMID 11641480. Retrieved 2 March 2016. 
  6. ^ Corey, E. J.; Cheng, X-M. (1995). The Logic of Chemical Synthesis. New York: Wiley. [page needed]
  7. ^ Todd, Matthew H. (2005). "Computer-aided Organic Synthesis". Chemical Society Reviews 34 (3): 247–266. doi:10.1039/b104620a. PMID 15726161. 

External links[edit]