Esterification of Monosaccharides

[SaintJude]

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Since MCAT is moving more heavily towards biochem, I just wanted to quickly discuss the "reactions of monosaccharides" topic on the AAMC list. Kaplan focuses on 3 types of rx, first being:

1. Ester formation: Conversion of Monosaccharides to Esters Using Acid Anhydride & Base


View attachment Esterification in Base.jpg

I thought it was odd that ALL the hydroxyl groups are esterified. Why/how is that? What would happen under acidic conditions. Sorry if that's a stupid question, but orgo is seriously just slowly coming back...

 

[MedPR]

That's interesting. I figured you would need an acid to cleave the acid anhydride (protonate an oxygen, etc) but it appears not.

Edit: Wait! I remember. Carboxylic acid + heat ==> acid anhydride. SO at 0deg C, the acid anhydrides decompose to their corresponding carboxylic acids. I assume the pyridine is in aqueous solution, so there is water to help stabilize the carboxylate ions. The pyridine can (I think) deprotonate the OH groups on the monosaccharide, thus making them primetime nucleophiles that will attack the ever so vulnerable carbonyl carbon. Bond forms, water comes along and protonates the OH from the carboxylic acid, water falls off, and now you have your acetylated sugar.

Does that seem plausible?

 

[chiddler]

That's interesting. I figured you would need an acid to cleave the acid anhydride (protonate an oxygen, etc) but it appears not.

Edit: Wait! I remember. Carboxylic acid + heat ==> acid anhydride. SO at 0deg C, the acid anhydrides decompose to their corresponding carboxylic acids. I assume the pyridine is in aqueous solution, so there is water to help stabilize the carboxylate ions. The pyridine can (I think) deprotonate the OH groups on the monosaccharide, thus making them primetime nucleophiles that will attack the ever so vulnerable carbonyl carbon. Bond forms, water comes along and protonates the OH from the carboxylic acid, water falls off, and now you have your acetylated sugar.

Does that seem plausible?

It seems plausible. Alternatively, to simplify it a bit, you can just consider it a nucleophilic attack onto the anhydride by the alcohol groups of the sugar.

 

[MedPR]

It seems plausible. Alternatively, to simplify it a bit, you can just consider it a nucleophilic attack onto the anhydride by the alcohol groups of the sugar.

Yea. The only thing I'm concerned about is how huge the nucleophile is in this case..

 

[chiddler]

Yea. The only thing I'm concerned about is how huge the nucleophile is in this case..

maybe. maybe it's a slow reaction.

well then lets look at other reactions with like tbutyl lithium. it's super reactive but it's huge. or LDA.

anhydrides are only less reactive than the "oyl chloride"'s RCOCl. perhaps this is why it is a reasonable reaction even at zero degrees.