The synthesis of
2 (
Scheme 2) commenced with facile preparation of compounds
10 and
12 according to the reported procedures [
36,
49]. Disaccharide
11 containing a preassembled
α-(1→3) glucosidic bond was obtained by the regioselective ring opening of a benzylidene protected precursor (see the Supporting information for details) [
36]. Thereafter, the iterative one-pot coupling reactions between
10 –
13 were carried out through preactivation of glycosyl donor using TolSOTf,
in situ generated from TolSCl and AgOTf, in diethyl ether at −78 ℃ for 15 min, which was followed by addition of acceptor at the same temperature, and the reaction mixture was subsequently warmed up to room temperature and stirred for another 15 min to finish the glycosylation. The amount of each building block was pivotal for the smooth assembly of pentasaccharide
5. At first, 1.0 equiv. of donor
10 and 0.9 equiv. of
11 were utilized, and thereafter, 0.8 equiv. of
12 and 0.7 equiv. of
13 were sequentially added after activation of the resultant donors
14 and
8, respectively. Finally, the four-component one-pot glycosylation reactions afforded the linker equipped
5 in a 47% overall yield. All of its glycosyl linkages were identified as
α-configuration by the small anomeric coupling constants in
1H NMR spectrum (≤3.6 Hz). No other anomeric isomers were positively isolated or identified by NMR analysis, indicating that all the above three steps glycosylation were virtually
α-specific, possibly due to the synergistic
α-directing effects of the TolSCl/AgOTf promotion system and the steric
β-shielding effect of the bulky mono-, di-, and tetrasaccharide motifs (labelled in blue color) in glycosyl donors
10, 14, and
8, respectively. Thereafter, removal of the
tert-butyldimethylsilyl (TBS) group in
5 by using 3HF·Et
3N resulted in the pentasaccharide
9 in 90% yield. Finally, deprotection of the benzoyl (Bz) group under Zemplén condition (CH
3ONa/CH
3OH) was followed by removal of all the benzyl (Bn) groups and reduction of the azido group
via Pd(OH)
2-catalyzed hydrogenolysis to deliver the target
α-glucan
2 in 75% overall yield after purification on a G10 column and lyophilization. The structure of
2 was rigorously characterized with 1D, and 2D NMR and HR MS data. However, on the other hand, the convergent glycosylation between pentasaccharides
8 and
9 under the same reaction conditions gave compound
6, unexpectedly, with a low stereospecificity (
α: β = 1:1), albeit in a 76% yield. Desilylation of the mixture with 3HF·Et
3N led to
15α and
15β, which could be readily separated and characterized.