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I recently encountered a problem on a test. It was a standard synth problem, "Provide reagents to produce y from x". The starting material was DHA and the ending molecule was a branched pentose. (I included an image of the solution I came up with). In my solution I use acetaldehyde under basic conditions to form the pentose.
My question is would pentose form under basic conditions or do I need to acidifying at the end? Would acidifying just encourage the formation of side products? Is there some other way to get this pentose as a primary product that I am not thinking of?

DHA to branched pentose

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If your reaction was carried out in basic conditions the product is an hemicetal. If your reaction was carried out in acid conditions the product is an acetal.

Why? This is catalysis question.

When you have the hemicetal to form an acetal you need the protonation of the -OH group, the oxonium ion formation and a new atack. But this can't occur in basic conditions.

If you need more explation you can read this:

Jonathan Clayden: Organic Chemistry Chapter 14: Nucleophilic substitution at C=O with loss of carbonyl oxygen

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Using base means that the reaction will stop at the hemiacetal and acetal formation would not take place. Acidifying at the end might encourage acetal formation.

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  • $\begingroup$ Using base means that the reaction will stop at the hemiacetal and acetal formation would take place. Aren't these two statements contradictory? Also please can you explain your answer in more detail. At the moment this might be deleted for being too short. $\endgroup$ – bon Nov 6 '15 at 14:31
  • $\begingroup$ I was hoping for some kind of explanation. I have a very fuzzy understanding of that hemi-acetal mid step. Are you saying that neither acidic or basic conditions would work in this reaction? $\endgroup$ – sharkpanic Nov 6 '15 at 15:32
  • $\begingroup$ Oops that's a typo; I'll try to write up an explanation when I get home in an hour $\endgroup$ – Laksh Nov 6 '15 at 16:53

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