[Pace2014]

Treatment of acyl-protected sugars with certain reagents such as hydrazine acetate or benzylamine in THF gives selective deacylation at the anomeric oxygen.

What is the feature of the hemiacetal oxygenl that results in this chemoselectivity? I can easily see why the presence of the ring oxygen enhances the reactivity at the anomeric carbon, but there doesn't seem to be anything blatantly different about the steric or electronic environment of the oxygen that’s attached to C1.

I’m not even sure if the deprotection reaction is reversible or not. If it’s the former, then maybe the deprotected product is favoured due to the formation of the low-energy beta anomer, stabilised by the anomeric effect. But this is a weak guess and I’m still at a loss.

[salteen]

You are correct that the anomeric oxygen is more reactive than the others - this is key.  What's the reason for it's enhanced reactivity? 

If you haven't already, try drawing out the mechanism of the reaction.  This will help show why attack at C1 is more productive than at the other carbons.

[Pace2014]

C1 has the neighbouring ring oxygen which can donate a lone pair to stabilise a carbocation, allowing substitution reactions to occur which could not take place at the other positions. So, for example, you can selectively protect the anomeric hydroxyl by using acid and a PG like Bz or Bn.

However, this deprotection doesn't look like an attack at C1. If there were water involved in the reaction it might make sense; -OAc is an alright leaving group and there could be stabilisation from the ring oxygen as well as neighbouring group participation from the acetyl group followed by attack by water to give the hydroxyl. But the reactions given involve amines in polar aprotic solvent, eg. BnNH2 in THF. The only reaction I can see happening here is attack on the carbonyl centre by the amine to give an amide and the deprotected hydroxyl.

If this is what happens then i guess I phrased my initial post incorrectly, as in this case it's about the reactivity of the carbonyl centre of the acetyl bonded to the anomeric centre and not the oxygen itself. I still can't see how the ring oxygen would alter the reactivity of this position to any great extent as there's no conjugation going on. I feel like I'm missing something blitheringly obvious.

[Dan]

The anomeric O is a better leaving group than the others. The geminal endocyclic O has an electron withdrawing effect (cf anomeric effect and induction).

This is also evident from the pka of an anomeric OH (~12).

[salteen]

Your hypothesis of the mechanism is correct - Attack of the hydrazine at the acetate carbonyl to form the acylhydrazine.  Dan's explanation of leaving group ability is the reason why only the C1 acetate is deprotected.

[Pace2014]

Thanks guys. It really was something pretty obvious that I was overlooking.