Total synthesis of periploside A, a unique pregnane hexasaccharide with potent immunosuppressive effects

Periploside A is a pregnane hexasaccharide identified from the Chinese medicinal plant Periploca sepium, which features a unique seven-membered formyl acetal bridged orthoester (FABO) motif and potent immunosuppressive activities. Here, we show the synthesis of this molecule in a total of 76 steps with the longest linear sequence of 29 steps and 9.2% overall yield. The FABO motif is constructed via a combination of Sinaÿ’s and Crich’s protocol for the formation of orthoester and acetal glycosides, respectively. The 2-deoxy-β-glycosidic linkages are assembled stereoselectively with judicious choice of the glycosylation methods. The epimer at the spiro-quaternary carbon in the FABO motif has also been elaborated in a stereo-controlled manner. This epimer, as well as the synthetic analogues bearing the FABO motif, retain largely the inhibitory activities of periploside A against the proliferation of T-lymphocyte, indicating the importance of the chemical connection of the FABO motif to their immunosuppressive activity.


p-Methoxyphenyl 2-O-benzyl-α-D-fucopyranoside (S3)
To a solution of S2 (3.94 g, 9.84 mmol) in methanol (50 mL) and water (5 mL) was added an aqueous solution of 5% HCl (a few drops) until pH = 3. The mixture was heated to 50 °C and stirred for 10 h until the TLC showed complete conversion. The mixture was quenched with a saturated NaHCO 3 solution. The solvent was then removed. The residue was diluted with CH 2 Cl 2 , washed with saturated NaHCO 3 solution and brine, respectively, and was then dried over Na 2 SO 4 and concentrated.
After stirring for 3 h at rt, the mixture was diluted with CH 2 Cl 2 , washed with saturated NH 4 Cl solution and brine, and was then dried over Na 2 SO 4 and concentrated. The

Preparation of pregnane diol 7 and its derivative S14 for X-ray diffraction analysis
To a stirred solution of AD-mix-β 12

Pregnane disaccharide 3
To a solution of S20 (16.5 mg, 0.0193 mmol) in CH 2 Cl 2 /MeOH (1.5 mL/1.5 mL) was added NaOMe (20 mg, 0.37 mmol) at rt. After stirring for 40 h, the mixture was filtered through silica gel. The filtrate was evaporated in vacuo to give a residue, which was purified by flash chromatography (petroleum ether/ EtOAc = 2:1) to afford

Preparation of periploside analogs 41-43 C1'''-epi-periploside A (41)
To a solution of 40α (4.5 mg, 3.1 μmol) in pyridine/EtOH (1.0 mL/1.0 mL) was added thiourea (10 mg, 0.13 mmol) at rt. After stirring at 80 o C for 2 h, the mixture was concentrated in vacuo to give a residue, which was purified by flash chromatography (CHCl 3 /MeOH = 30:1) to afford a colorless syrup. The syrup was dissolved in THF/pyridine (1.5 mL/0.75 mL). HF·py (70% HF in pyridine, 0.10 mL) was added dropwise at 0 o C. After stirring at rt for 40 h, a saturated NaHCO 3 solution was added slowly to the mixture. The resulting mixture was diluted with CH 2 Cl 2 , washed with saturated NaHCO 3 solution, and was then extracted with CH 2 Cl 2 twice. The combined organic layer was washed with brine, dried over Na 2 SO 4 , and concentrated. The HRMS (ESI) calcd for C 35

Bioassay for the immunosuppressive activities 17
Preparation of spleen cells from mice BALB/C mice were sacrificed and spleens were removed aseptically. A single cell suspension was prepared after cell debris and clumps were removed. Erythrocytes were lysed using ammonium chloride buffer solution. The isolated lymphocytes were washed 3 times with PBS containing 2% FBS, and were re-suspended in RPMI 1640 medium at the indicated concentration.