Glycals and their [2,3]-dehydrosugar derivatives have commonly been used in synthetic chemistry as electrophiles. Here we report a Pd-catalyzed polar inversion (umpolung) of this reaction, where the glycals and isomers can be used as nucleophiles. The reaction showed high regio- and stereoselectivity in the presence of numerous aromatic and aliphatic aldehydes. The synthetic utility of this reaction was demonstrated by the short synthesis of the tetrahydropyran fragment of the anticancer natural product mucocin.