Enantiomerically pure 7-oxabicyclo[2.2.1]hept-5-en-2-yl derivatives (naked sugars) as synthetic intermediates. 18. Synthesis of .alpha.-(1.fwdarw.2)-, .alpha.-(1.fwdarw.3)-, .alpha.-(1.fwdarw.4)-, and .alpha.-(1.fwdarw.5)-C-linked disaccharides through 2,3,4,6-tetra-O-acetylglucopyranosyl radical additions to 3-methylidene-7-oxabicyclo[2.2.1]heptan-2-one derivatives