Scheme 4. Synthesis of elongated sPRP oligosaccharides 2, 3, and 4. Reagents and conditions: (a) (i) TCA, DCM, RT; (ii) 12, PivCl, pyridine, 0 °C, then I2, pyridine/H2O, RT, 85% for 6, 83% for 7, 80% for 8 (over two steps); (b) (i) TCA, DCM, RT; (ii) 9, PivCl, pyridine, 0 °C, then I2, pyridine/H2O, RT; (iii) hydrazine acetate, DCM, RT, 74% for 27, 72% for 28, 72% for 29 (over three steps); (c) Pd/C, H2, EtOAc/MeOH/50% AcOH(aq.), 87% for 2, 80% for 3, 79% for 4. PivCl = pivaloyl chloride, TCA = trichloroacetic acid.
