On treatment with boron trifluoride–diethyl ether in benzene solution, tri-O-acetyl-D-glucal dimerises, and from the products the crystalline C–C linked disaccharide derivative 1,3,4,6-tetra-O-acetyl-2-C-(4,6-di-O-acetyl-2,3-dideoxy-α-D-erythro-hex-2-enopyranosyl)-2-deoxy-β-D-glucopyranose has been isolated. Dimerisation is believed to occur by the trans-addition of one molecule of 1,4,6-tri-O-acetyl-2,3-dideoxy-D-erythro-hex-2-enopyranose to the double bond of another, and in agreement with this, evidence has been obtained that tri-O-acetyl-D-glucal initially rearranges in the presence of the catalyst (or thermally) to give mainly the 2,3-unsaturated isomer.