.beta.-D-Allopyranose, 2-deoxy-4-O-2-deoxy-2-(1,3-dihydro-1,3-dioxo-2H-isoindol-2-yl)-3-O-(phenylmethyl)-4,6-O-(phenylmethylene)-.beta.-D-allopyranosyl-2-(1,3-dihydro-1,3-dioxo-2H-isoindol-2-yl)-3,6-bis-O-(phenylmethyl)-