Scheme 1. Synthesis of ribitol unit 18 and disaccharides 13, 14, and 16. Reagents and conditions: (a) (i) NIS, acetone, –78 °C; (ii) NaBH4, EtOH, 0 °C, 43% (six steps from 20); (b) (i) 19, BF3·OEt2, DCM, –78 °C, 80%; (ii) NaOMe, MeOH, RT, 90%; (c) Bu2SnO, toluene, reflux, 2 h, then BnCl, TBAI, NaH, DMF, 50 °C, 2 h, 50%; (d) LevOH, DMAP, DIC, DCM, RT, 93%; (e) Pd(PPh3)4, DMBA, MeOH, RT, 75% for 14, 75% for 24; (f) DMTrCl, DMAP, DCM, RT, 93%; (g) PCl3, Et3N, imidazole, DCM, 0 °C, 85%. NIS = N-iodosuccinimide, TBAI = tetrabutylammonium iodide, DIC = 1,3-diisopropylcarbodiimide, DMAP = 4-dimethylaminopyridine, DMBA = 1,3-dimethylbarbituric acid.
