How do you explain the absence of aldehyde group in the pentaacetate of D-glucose?

D-glucose reacts with hydroxylamine (NH2OH) to form an oxime because of the presence of aldehydic (−CHO) group or carbonyl carbon. This happens as the cyclic structure of glucose forms an open chain structure in an aqueous medium, which then reacts with NH2OH to give an oxime.

But pentaacetate of D-glucose does not react with NH2OH. This is because pentaacetate does not form an open chain structure.