Lead tetraacetate is highly selective for oxidation of α-hydroxy-hemiacetal groups and hence most readily attacks cyclic forms of the sugars. The reaction proceeds stepwise; the hemiacetal α-glycol being cleaved and the monoester of a correspondingly shorter-chained sugar formed. After cyclization the new sugar in turn is oxidized at the hemiacetal α-glycol group to yield a diester of a still-lower-order member of the series. In this manner D-glucose first yields mono-O-formyl-D-arabinose and then di-O-formyl-D-erythrose. Similarly, D-fructose is degraded to a glycolate ester of D-erythrose and finally to a formate–glycolate diester of D-glyceraldehyde. Some relatively rare sugars thus may conveniently be prepared directly from abundant higher-order members of the series. The reactions appear to involve preferential attack of the furanose form of a sugar rather than of the normally-predominant pyranose form, or possibly migration of ester groups towards the reducing end of the sugars.