In retrosynthetic analysis, a synthon is a destructural[clarification needed] unit within a molecule which is related to a possible synthetic operation. The term was coined in 1968 by E.J. Corey. He noted in 1988 that the "word synthon has now come to be used to mean synthetic building block rather than retrosynthetic fragmentation structures". It was noted in 1998  that the phrase did not feature very prominently in Corey's 1981 book, The Logic of Chemical Synthesis, as it was not included in the index.
In planning the synthesis of phenylacetic acid, two synthons are identified: a nucleophilic "-COOH" group, and an electrophilic "PhCH2+" group. Of course, both synthons do not exist per se; synthetic equivalents corresponding to the synthons are reacted to produce the desired reactant. In this case, the cyanide anion is the synthetic equivalent for the -COOH synthon, while benzyl bromide is the synthetic equivalent for the benzyl synthon.
The synthesis of phenylacetic acid determined by retrosynthetic analysis is thus:
- PhCH2Br + NaCN → PhCH2CN + NaBr
- PhCH2CN + 2 H2O → PhCH2COOH + NH3
- C2 synthons - acetylene, acetaldehyde
- -C2H4OH synthon - ethylene oxide
- carbocation synthons - alkyl halides
- carbanion synthons - Grignard reagents, organolithiums, substituted acetylides
Alternative use in Synthetic Oligonucleotides
This term is also used in the field of gene synthesis—for example "40-base synthetic oligonucleotides are built into 500- to 800-bp synthons".
- E.J. Corey (1967). Pure App. Chem. 14: 30–37 http://www.iupac.org/publications/pac/pdf/1967/pdf/1401x0019.pdf
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- E. J. Corey (1988). "Robert Robinson Lecture. Retrosynthetic thinking—essentials and examples". Chem. Soc. Rev. 17: 111–133. doi:10.1039/CS9881700111.
- W.A. Smit, A.F. Buchkov, R.Cople (1998). Organic Synthesis, the science behind the art. Royal Society of Chemistry. ISBN 0-85404-544-9.
- Elias James Corey, Xue-Min Cheng (1995). The logic of chemical synthesis. Wiley-Interscience. ISBN 0-471-11594-0.
- Sarah J. Kodumal, Kedar G. Patel, Ralph Reid, Hugo G. Menzella, Mark Welch, and Daniel V. Santi (November 2, 2004). "Total synthesis of long DNA sequences: Synthesis of a contiguous 32-kb polyketide synthase gene cluster". PNAS 101 (44): 15573–15578. Bibcode:2004PNAS..10115573K. doi:10.1073/pnas.0406911101.