Synthesis of Two Tetrasaccharide Pentenyl Glycosides Related to the Pectic Rhamnogalacturonan I Polysaccharide

Abstract

The synthesis of two protected tetrasaccharide pentenyl glycosides with diarabinan and digalactan branching related to the pectic polysaccharide rhamnogalacturonan I is reported. The strategy relies on the coupling of N-phenyl trifluoroacetimidate disaccharide donors to a common rhamnosyl acceptor. The resulting trisaccharide thioglycosides were finally coupled to an n-pentenyl galactoside acceptor to access the two protected branched tetrasaccharides.

Glycosylation of n-pentenyl glycosides using bromodiethylsulfonium salt as an activator: interception of the glycosyl intermediate by chloride ion transfer

“…The crude mixture was purified by column chromatography using gradient elution with 5-15% ethyl acetate in hexanes as the eluent. 13 C{ 1 H} NMR (126 MHz, CDCl 3 ) δ C 177.0, 138.0, 137.9, 137.9, 128.5, 128.5, 128.0, 128.0, 127.9, 127.8, 127.5, 102.3 (C-1β), 84.8, 77.5, 75.5, 75.2, 75.1, 73.6, 72.5, 68.6, 64.9 (q, J = 33.9 Hz), 39.7, 27.1. 19 The glycosylation using β-pinene was carried out as described in general procedure B, using n-pentenyl 2-O-pivaloyl-3,4,6-tri-O-benzyl-β-D-glucopyranoside (11) (59 mg, 0.10 mmol), BDSB (59 mg, 0.11 mmol), glycosyl acceptor 6 (35 mg, 0.076 mmol), silver trifluoromethanesulfonate (51 mg, 0.20 mmol), β-pinene (76 μL, 0.49 mmol), and dichloromethane (1.0 mL).…”

“…In addition, the formation of vicinal dibrominated NPG side products was also observed during activation. [9][10][11] Other research groups have recognized the sluggish reactivity of certain NPGs and have converted them to more reactive glycosyl donors, including glycosyl trichloroacetimidates, 12 glycosyl trifluoroacetimidates, 13 glycosyl 1,2-orthoesters, 14 and glycosyl phosphates. 15 Recently, the development of glycosyl donors equipped with more reactive 4-pentenyl analogs has been pursued, such as gem-dimethyl NPGs (Andrade group), 8 2-allyloxy phenyl glycosides (Demchenko group), 16 and 4-npentenyl-1,5-enynyl glycosides (Chai group), 17 as shown in Fig.…”