Selective deprotection of 2′,6′-di-O-benzyl-2,3:5,6:3′,4′-tri-O-isopropylidenelactose dimethyl acetal

The reactivity order of O-deisopropylidenation of the three isopropylidene protecting groups of 2′,6′-di-O-benzyl-2,3:5,6:3′,4′-tri-O-isopropylidenelactose dimethyl acetal (2) with various reagents was established. The 5,6-acetal group was, although to a limited extent, more reactive as compared with the 3′,4′ group, while the 2,3-O-isopropylidene group was definitely less reactive. Conditions were determined for the direct preparation of the 5,6,3′,4′-tetraol 5 (60% aqueous acetic acid, room temperature, 48 h, 73% yield) and the 5,6-diol 4 (propylene glycol and p-toluenesulphonic acid in dichloromethane, 46% yield). The diacetonated derivative 3, formally arising from a selective 3′,4′-O-deisopropylidenation, was obtained in high yield (90%) through a selective acetonation with 2-methoxypropene of the tetraol 5.