The behavior of 2-acetamido-2-deoxy-d-mannose with isopropenyl acetate in the presence of p-toluenesulfonic acid. II. Evidence bearing on the mechanism of the formation of 3,4,6-tri-O-acetyl-2-(n-acetylacetamido)-1,2-dideoxy-d-arabino-hex-1-enopyranose and of 2-(D-glycero-1,2-diacetoxyethyl)-4-(N-acetylacetamido)furan.