Influence of the solvent in low temperature glycosylations with O-(2,3,5,6-tetra-O-benzyl-β-d-galactofuranosyl) trichloroacetimidate for 1,2-cis α-d-galactofuranosylation

Stereoselective Synthesis of a Tetrasaccharide Fragment from Cellulosome Produced by Clostridium thermocellum

“…The newly formed galactofuranosidic linkage was proved to be α, judging from its H-1/H-2 coupling constant ( 3 J H-1/H-2 4.0 Hz). [13] Then, selective hydrolysis of the levulinate group in 14 was completed under mild basic conditions (NH 2 NH 2 ·HOAc, CH 2 Cl 2 /CH 3 OH, rt), providing disaccharide 15 (80% yield) with a free hydroxy group at the C3' position. Glycosidic condensation of this compound with 4,6-Obenzylidene protected Galp donor 4 mediated by NIS/TfOH (cat.)…”

Synthesis of α-d-Glcp-(1→3)-α-d-Galf-(1→2)-α-l-Rhap constituent of the CPS of Streptococcus pneumoniae 22F. Effect of 3-O-substitution in 1,2-cis α-d-galactofuranosylation

“…Based in the high diastereoselectivities obtained in the construction of the a-D-Galf-(1/2)-L-Rha linkage by the use of tetra-O-benzylated galactofuranosyl imidate 1 as donor, 37 the construction of trisaccharide 3 was envisioned from the reducing end to the non-reducing end (Scheme 1). Retrosynthetic analysis indicated the stereoselective introduction of a 1,2-cis a-D-Glcp unit on the galactofuranosyl 3-OH of disaccharide 4, and this could be performed by the use of 2,3,4,6-tetra-Obenzyl-D-glucopyranosyl trichloroacetimidate (5) 51,52 as donor.…”

“…as catalyst was added (Method A). 37 Surprisingly, the target a-disaccharide was not detected even as a minor product. Instead, the b-product of glycosylation lacking the PMB group, allyl 2,5,6-tri-O-benzyl-b-Dgalactofuranosyl-(1/2)-3,4-di-O-benzyl-a-L-rhamnopyranoside (15) was isolated (24%, Scheme 3).…”

“…The 1 H NMR spectrum of 19 was similar to that from the fully benzylated analogue. 35,37 Lowering the amount of the TMSOTf avoided the removal of the PMB group; however, the yield of the glycosylation was moderate with high amount of transposition by-product (Table 1, entry 1). Highly armed donors and poor nucleophilic acceptors usually favor the rearrangements of the imidate, which are avoided employing the inverse procedure of glycosylation, i.e.…”

“…Interestingly, high stereoselectivity (a/b 10 : 1) has been observed in the construction of the a-D-Galf-(1/2)-L-Rha linkage using allyl 3,4-di-O-benzyl-a-L-rhamnopyranoside (2) as acceptor at À78 C in CH 2 Cl 2 with TMSOTf as catalyst, whereas same conditions in acetonitrile gave the b conguration as a single diasteromer. 37 By taking advantage of the above result we envisioned the synthesis of the trisaccharide constituent of Streptococcus pneumoniae 22F capsular polysaccharide (CPS) 16 (Fig. 1), a-D-Glcp-(1/3)-a-D-Galf-(1/2)-L-Rhap (3).…”