Chain Extension of Carbohydrates. Part 6. Synthesis of the Two C-6 Epimers of the 6-Acetylamino-4,6-dideoxyheptopyranosiduronic Acid Present in Amipurimycin by Means of Stereocontrolled Ethynylation of Methyl 2,3-Di-O-benzyl-4-deoxy-α