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Monday, August 08, 2005

Benzoylation of a Polyol

Amino polyols are an important part of synthetically created amino acids. They are highly antibacterial and immunosuppressive and so are used in various antibiotics and antifungal products. This reaction shows one of the step necessary in creating the amino polyol.

Benzoyl chloride is added to the polyol to form a tribenzoate compound. In this reaction, the polyol has several R-O-H groups that act as weak nucleophiles. When the benzoyl-Cl bond breaks upon addition to the pyridine solvent, the benzoyl group acts as an electrophile. With the help of the DMAP (dimethylaminopyridine) catalyst in the reaction, the R-O-H group is deprotonated and the benzoyl group is added to the remaining R-O form the final tribenzoylated product. As seen above, the reaction has a yield of about 90%. The OTBS (t-buytldimethylsiloxy) groups do not participate in this reaction.

There is one hydroxyl group left on the molecule produced. All of the hydroxyl groups would be replaced by benzoyl groups if the reaction was not stopped after three groups had been added. To stop the reaction at this point, three equivalents of benzoyl chloride were used for every polyol.

For the full text of the article describing this process, see this report.


  • 1) Make sure you understand the difference between a benzyl and and a benzoyl group, and which is Bz vs. Bn
    1) Why is one of the alcohols left unreacted in this conversion?

    By Blogger Jean-Claude Bradley, at 4:27 PM  

  • 1) I do not see that the reaction was monitored by NMR from the article.
    2) There is no benzyl chloride used in this reaction - re-read my initial comments carefully.
    3) Any abbreviation you use in the scheme must be detailed in your text.

    By Blogger Jean-Claude Bradley, at 5:58 AM  

  • 1) you still have abbreviations you didn't explain
    2) you didn't fix all the problems with Bn vs. Bz
    3) you say that water was present during the reaction in pyridine - does that make sense?
    4) If they were trying to stop the reaction after only 3 benzoyl groups got added, how many equivalents of benzoyl chloride should they have used?

    By Blogger Jean-Claude Bradley, at 6:36 AM  

  • why do you say that 3 equivalents of benzoyl chloride were used?

    By Blogger Jean-Claude Bradley, at 11:18 AM  

  • 17/20. If you read the experimental section you'll see that they added 6 equivalents of benzoyl chloride. Always check the experimental section to find out exactly what was done. My guess is that last benzoyl group is prevented from adding because of steric hindrance from the adjacent TBSO group.

    By Blogger Jean-Claude Bradley, at 7:01 AM  

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