1,6-Anhydro-β-maltotriose: Preparation from pullulan, and regioselective partial-protection reactions

Abstract

1,6-Anhydro-β-maltotriose ( 4) was prepared from the microbial polysaccharide pullulan in 33% overall yield by a combination of enzymic degradation and subsequent chemical treatments. Reaction of 4 with α,α-dimethoxytoluene and with 2,2-dimethoxypropane, respectively, in the presence of an acidic catalyst afforded the 4″,6″- O-benzylidene and 4″,6″- O-isopropylidene derivatives in good yields. Employement of an excess of α,α-dimethoxytoluene resulted in the 3′,2″:4″,6″-di- O-benzylidene derivative in 62% yield; it is characterized by having an eight-membered interglycosidic acetal ring. A few partially benzylated derivatives were prepared from these cyclic acetals as intermediates. Discrimination among three sets of hydroxyl pairs, namely, 2:3, 2′:3′, and 2″:3″, of the 4″,6″- O-benzylidene-6′- O-trityl derivative was achieved by treatment with one equimolar proportion of 1,3-dichloro-1,1,3,3-tetraisopropyldisiloxane, which selectively reacts with the 2′:3′ hydroxyl pair, leaving the diaxial (2:3) and one of the diequatorial (2″:3″) hydroxyl pairs unaffected.