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So I have a question concerning a reaction mechanism for the reaction below.

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I know for certain that I get the alfa anomer, but I can't explain it. When I draw out the mechanism it doesn't make sense to me.

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I think what throws me off is the NGP from the Ac- group since it would 'force' the beta configuration. I was then considering anomerization of the beta anomer to the alpha anomer, but when I look at mechanisms for this I can't see how it would be possible when I have a bromide on C-1.

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I was then wondering if the endocyclic oxygen gets protonated in the first step rather than the exocyclic oxygen in the mechanism I drew, but that would lead to NGP again because we get an oxocarbenium ion.

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