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I have spotted a protecting acetal group and thought this would undergo hydrolysis … My working is such:

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But I cannot get to the desired isomer. Could someone please help? (Diagrams would be very much apprecited thanks!)

  • $\begingroup$ The two acetal oxygen's are different. Try protonating the "other" acetal oxygen. $\endgroup$
    – ron
    Dec 21, 2015 at 19:35
  • $\begingroup$ Use the 1- and 3- hydroxys (where the carbonyl would be on the carbon at position 6). The original molecule has the acetal formed from the hydroxys at positions 1 and 2. $\endgroup$
    – Beerhunter
    Dec 21, 2015 at 21:08

1 Answer 1


You have managed to draw the correct acyclic hydrolized compound with a decent mechanism.

This is a problem that can be solved by numbering the carbon atoms. Number the carbon atoms sequentially along the chain, starting with the acetal carbon (which has to be the carbonyl group). Likewise number the unpacked acyclic molecule. Use what you get to figure out which of the three hydroxy groups are used to make the second acetal.

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In general, numbering your atoms using common structural features as anchors can help unravel complicated mechanism problems.

  • 4
    $\begingroup$ Upvote for numbering atoms. $\endgroup$
    – jerepierre
    Dec 21, 2015 at 22:04
  • 3
    $\begingroup$ I read a book called The Art of Writing Reasonable Organic Reaction Mechanisms and the first chapter was all about numbering atoms and mapping bonds broken and bonds formed. $\endgroup$
    – Ben Norris
    Dec 21, 2015 at 22:45
  • 1
    $\begingroup$ That book is on my reading list too! Here's an Amazon link for anyone interested. $\endgroup$ Dec 22, 2015 at 5:39

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