Kinetic Studies of Acetyl Group Migration between the Saccharide Units in an Oligomannoside Trisaccharide Model Compound and a Native Galactoglucomannan Polysaccharide

Abstract Acyl group migration is a fundamental phenomenon in carbohydrate chemistry, recently shown to take place also between two non‐adjacent hydroxyl groups, across the glycosidic bond, in a β‐(1→4)‐linked mannan trisaccharide model compound. With the central mannoside unit containing acetyl groups at the O2 and O3 positions, the O2‐acetyl was in the earlier study shown to migrate to O6 of the reducing end. Potential implications of the general acyl migration process on cell signaling events and plant growth in nature are intriguing open questions. In the present work, migration kinetics in this original trisaccharide model system were studied in more detail together with potential interactions of the model compound and the migration products with DC‐SIGN lectin. Furthermore, we demonstrate here for the first time that similar migration may also take place in native polysaccharides, here represented by galactoglucomannan from Norway spruce.


General information
For following the migration process and for identification and characterization of the new compounds, a Bruker Avance-III spectrometer operating at 500.20 MHz ( 1 H) and 125.78 MHz ( 13 C) equipped with a Prodigy BBO CryoProbe was used. The characterization was performed using a standard set of 1D and 2D NMR spectroscopic techniques: 1 H, 13 C, 1D-TOCSY, DQF-COSY, Multiplicity edited HSQC (CH and CH 3 positive, CH 2 negative, both coupled and decoupled), and HMBC. The reported signals are referenced to an internal standard (TMS δH = 0.0 ppm, δC = 0.0 ppm) or residual solvent signal (MeOH δH = 3.31 ppm, δC = 49.00 ppm, CDCl 3 δH = 7.26 ppm, δC = 77.16 ppm). Chemical shifts are reported with two decimals for 1 H and one decimal for 13 C, where this is not sufficient for distinguishing two signals an additional decimal is given. Coupling constants are reported in Hz with one decimal and mentioned only the first time they are encountered. Accurate coupling constants and shifts were extracted from the 1 H spectra using the NMR simulation software ChemAdder/SpinAdder 1 . HRMS was recorded on a Bruker daltonics micro-ToF with ESI in positive mode as ionization source. TLC analysis was performed on Merck silica gel 60 F254 plates and the spots were visualized with UV light and charring with H 2 SO 4 /MeOH (1:4) and heating. All reactions were monitored by TLC. Column chromatography was carried out using silica gel 60 (0.040 -0.060 mm) as stationary phase and as eluents hexane:EtOAc or toluene:EtOAc were used. All chemicals were purchased from Sigma-Aldrich and used as such. Dry dichloromethane was obtained by distillation from a suspension of CaH 2 under argon. Dry MeOH and DMF were purchased and used as such. Reactions sensitive towards moisture and air were carried out under argon atmosphere.

Synthesis of model compounds
Compound 1a was synthesized according to an earlier reported method. 2

Synthesis of compound 1b
The synthesis of 1b started by making building block 4 from 3. 3 The selective ring opening of 3 was done using I 2 and NaBH 3 CN in acetonitrile. 4 The yield of 5 was fair using this selective ring opening method. The rest of the synthesis followed the same synthesis pattern as has previously been done for 1a. 2 A β-mannosylation according to Crich protocol 5,6 with donor 5 and acceptor 4 yielded 6 in a fair yield. The removal of PMB groups and the following benzoylation gave a good yield of 7. The selective ring opening using BF 3 •OEt 2 and Et 3 SiH also gave a good yield of 8.

S5
Migration study pH profiles Figure S1. The pH profile starting from 1a and 1b. Figure S2. The pH profile of the migration study in GGM.

Kinetic modeling
Kinetic modeling of the trisaccharide model compounds

Kinetic modeling of GGM
In figures S17-S19 the kinetic model and the experimental data is shown for migration in the GGM.

Methyl 2,3-di-O-benzyl-6-O-p-methoxybenzyl-α-D-mannopyranoside (4).
To a solution of 3 (1320 mg, 1 equivalent) in acetonitrile (12 ml) was 3 Å molecular sieves added under stirring. After 15 min NaBH 3 CN was added and the reaction mixture was cooled to 0 °C. I 2 was dissolved in acetonitrile (60 ml) and added dropwise to the reaction mixture. After 30 min the reaction was diluted with CH 2 Cl 2 (100 ml) and the mixture was filtered through celite. The solution was the washed with 10% Na 2 CO 3 solution (2 × 80 ml) and saturated NaCl solution (80 ml). The organic phase was dried with Na 2 SO 4 and the solvent was evaporated. The crude product was purified by column chromatography (hexane:EtOAc 2:1) to provide 4 as a clear oil. Yield: