Is it possible, chemically, to change a dextrorotatory compound to its levorotatory counterpart? E.g., is it possible to change dextrodopa to levodopa?
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$\begingroup$ Related, but not a duplicate: chemistry.stackexchange.com/questions/105/… $\endgroup$– jonscaFeb 11, 2013 at 5:37
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2 Answers
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Yes,this process is called stereoconversion. Both SN1 and SN2 reactions can invert the chirality of a carbon center. In the case of the SN1 mechanism, departure of a leaving group generates a prochiral intermediate which is then open to nucleophilic attack on either face. If both faces are equally acessible to the nucleophile this will result in racemisation of the molecule. In the case of the SN2 reaction the nucleophile will enter a trigonal bipyramidal intermediate with the molecule. Assuming the nucleophile has the same stereochemical precedence as the leaving group, the stereochemistry will be inverted.
answered Jan 14, 2013 at 2:27
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1$\begingroup$ Doesn't "dextrorotatory compound to levorotatory counterpart" mean we have to preserve the groups on the chiral carbon, rather than substituting one of the groups with another. $\endgroup$ Jan 14, 2013 at 3:12
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1$\begingroup$ @AdityaSriram - Yes, but if the leaving group and nucleophile are the same species, you're in the clear. This can happen in the case of keto-enol tautomerism at a stereocenter. $\endgroup$ Jan 14, 2013 at 4:03
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1$\begingroup$ Adding an example to your answer will really help a lot. $\endgroup$ Jan 14, 2013 at 5:29
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1$\begingroup$ @RichardTerrett I think Aditya Sriram is right you need to add an example not just of how you would change the chirality from dex to levo but how you would preserve the groups on the chiral carbon. $\endgroup$ Jan 14, 2013 at 5:42
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$\begingroup$ I disagree with this. Under both SN1 or SN2 conditions, treatment of an alkyl bromide with bromide ion (for example) can at best only lead to racemisation of the compound and not complete inversion, which the question is asking for. $\endgroup$ Nov 17, 2016 at 18:08
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The following example(s) might help to illustrate the stereoconversion described by Richard Terrett:
In the case of alcohols, the "classical" method is the Mitsunobu reaction, using triphenylphosphine ($\ce{Ph3P})$ and diethyl azodicarboxylate (DEAD, $\ce{EtO2C-N=N-CO2Et})$ as reagents, and water as a nucleophile to quench the intermediate with inversion of configuration.
In the course of the reaction, $\ce{Ph3P}$ adds to DEAD in the presence of a proton donor to form an intermediate.
The latter adds to the alcohol, forming a stable $\ce{P-O}$ bond.
The addition of a nucleophile proceeds according to $S_N2$ with inversion of configuration, the formation of triphenylphosphine oxide renders the reaction irreversible.
answered Mar 17, 2014 at 16:06