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Topic: Regioselective removal of O-acetyl group with benzylamine  (Read 8507 times)

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Offline g_orbital

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Regioselective removal of O-acetyl group with benzylamine
« on: March 09, 2007, 05:39:49 AM »
As shown in the following scheme (taken from J. Org. Chem. 1996, 61, 5208-5209), glucopyranose (15) was protected by acetic anhydride to form the triacetyl compound (16). Treatment of (16) with BnNH2 (Benzylamine), followed by hydrolysis with dilute aqueous acid furnished the pyranose (17).

As you can see, there was a selective hydrolysis which only removed the C-1 O-acetyl (at the anomeric position) while the two other O-acetyl groups were not affected. The author indicates that the reaction with benzylamine forms "amino glycoside", but I can't understand how and why. My question is: What is the mechanism of this deprotection - is it just a simple "addition-elimination" process, and if so - why does benzylamine react regioselectively with the C-1 O-acetyl group, and how does the "amino glycoside" look like? Also, what is the role of the hydrolysis with HCl(aq) in terms of mechanism?

Thank you very much in advance!

Offline movies

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Re: Regioselective removal of O-acetyl group with benzylamine
« Reply #1 on: March 09, 2007, 01:04:01 PM »
Sounds like an oxocarbenium mechanism to me.

Offline g_orbital

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Re: Regioselective removal of O-acetyl group with benzylamine
« Reply #2 on: March 10, 2007, 11:44:29 AM »
Movies, may you please elaborate on this oxocarbenium mechanism? At which step is this oxocarbenium ion created? And again, how do I get regioselectivity in this rxn? Thanks!

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Re: Regioselective removal of O-acetyl group with benzylamine
« Reply #3 on: March 10, 2007, 09:31:34 PM »
There is probably an oxocarbenium intermediate in both parts of step b.  The selectivity comes from the fact that in making the first oxocarbenium you kick out -OAc from the anomeric position.  The rest of the OAc groups are unaffacted.

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