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Would it be somehow possible to conduct asymmetric synthesis if all chiral molecules would exist only as racemates in nature?

EJC
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    Why, chirality pretty well may emerge spontaneously, upon crystallization or otherwise. – Ivan Neretin Jul 06 '16 at 10:20
  • I guess that if a random "odd" number of molecules is taken from a sample, the collection will have an enantiomeric excess, but I don't see how decent enantiomeric excess can be obtained. – EJC Jul 06 '16 at 10:24
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    Suppose a compound racemizes easily in the solution, but not in the solid state. Suppose it crystallizes in an enantiopure form rather than as a racemate. Prepare a (super)saturated solution and wait until it starts to crystallize. Given the right conditions, the entire bulk may be converted to one enantiomer (which one in particular is defined by the first seed crystal, i.e. by blind luck). – Ivan Neretin Jul 06 '16 at 10:31
  • Why would it crystallize so easier in the enantiopure form? – EJC Jul 06 '16 at 10:36
  • Why not? Crystal structure depends on many things and is not easy to predict; there is no universal law that a racemic crystal should be more energetically favorable than the enantiopure one. Sometimes it would, and sometimes it would not. – Ivan Neretin Jul 06 '16 at 10:41
  • @Marko Any single crystal usually contains only one enantiomer. There may possibly be substances which form a special unit cell with pairs of enantiomers, but I am not aware of any specific example. – Karl Jul 06 '16 at 10:43
  • So precipitation is one way. Are there any others? – EJC Jul 06 '16 at 11:27
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    You could use polarized light to selectively/preferentially destroy one enantiomer in the the racemate. – ron Jul 06 '16 at 12:53
  • How does that work? Please someone explain both procedures in the answer and I may accept it. – EJC Jul 06 '16 at 13:11
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    The interaction between one enantiomer in the racemate and any chiral probe (e.g. your hand, polarized light, etc.) will be diastereomeric (e.g. different energy) to the interaction of the other enantiomer and the same chiral probe. – ron Jul 06 '16 at 13:24
  • There wouldn't be any chiral probe in the universe. – EJC Jul 06 '16 at 14:12
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    As sunlight passes through the atmosphere it becomes partially polarized; it is a chiral probe. – ron Jul 06 '16 at 14:28
  • Looking at it from a more philosophical point of view... if there was no chirality in the world, there would be no life and therefore no chemists to worry about the racemates ;) – NotEvans. Jul 06 '16 at 19:45
  • Haha, true, but this was more a question for contemplation. – EJC Jul 06 '16 at 20:13
  • see here for example, or google something like "preferential destruction of one enantiomer by polarized light" – ron Jul 06 '16 at 20:19
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    @NotNicolaou perhaps it would be better to say, if there was no chirality in the world, there would be no life as we know it. – ron Jul 06 '16 at 21:34
  • @ron You are confusing linear and circular polarization. Scattered light from the sky is partially polarized all right, but still not chiral. – Ivan Neretin Jul 07 '16 at 05:51
  • @IvanNeretin Thanks for pointing that out. I'll fall back to the chirality found in some mineral and clay surfaces. – ron Jul 07 '16 at 14:46

1 Answers1

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Yes! There is a simple possibility. Take one racemate and do what Pasteur did:

  1. be lucky, in that both enantiomers crystallise separately;
  2. be lucky again in that they form macroscopically distinguishable crystals (in this case probably: mirror images);
  3. collect the different enantiomers. Voilà, you have enantiopure substances and can use them in synthesis.

(Thankfully, using tartaric acid immediately gives access to the Sharpless asymmetric epoxidation.)

Once you have any single enantiopure compound, you are able to generate further ones. For example, reacting a racemate with an enantiopure compound in a way that preserves the respective stereocentres will result in diastereomeric products. Diastereomers can theoretically always be separated (it may be harder for some and some may interconvert under ambient conditions but the theory stands). You could also generate diastereomeric salts, by taking an enantiopure anion (say (R)), and a racemic cation. You will receive the diastereomers (R,S) and (R,R) which can then be separated and so on and so forth.

Jan
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    Great combination of Pasteur experiment and Sharpless epoxidation. It could also be used for resolution either by crystallization as salts or ester derivatizaion or for kinetic resolution through Sharpless epoxidation. – EJC Jul 06 '16 at 20:17