Enantiomerically Pure Building Blocks from Sugars, 9 An Expedient Approach to Pyranoid Ene and Enol Lactones by BF3‐Catalyzed Peroxidation of Glycal and Hydroxyglycal Esters

An effective and convenient one‐pot procedure, simply involving treatment with boron trifluoride/3‐chloroperbenzoic acid (MCPBA), is detailed for the high‐yield acquisition of enantiomerically pure dihydropyran‐2‐ones from mono‐ and disaccharide‐derived glycal and hydroxyglycal esters. An assessment of the scope of the method is given, a total of 17 examples being ample evidence for its preparative utility and apparent generality. Substantiation of the mechanism presented – BF3‐promoted removal of the allylic acyloxy group, seizure by MCPBA of the allylcarboxonium ion generated, and fragmentation of the resulting hex‐2‐enopyranose 1‐(3‐chloro)perbenzoates on quenching – is provided by characterization of the perester intermediates. Conducting the reaction of glycal esters at room temperature a glycoltype C‐1 – C‐2 bond cleavage occurs instead with the generation of acylated 4,5‐dihydroxypentenals.