To a solution of compound 16-21, S4, S5, S7 (1 eq.) in methanol/ethanol (10 mL), Pd/C (5% palladium on carbon, 0.05 eq.) was added. The flask was evacuated and flushed with H2 and the mixture was stirred at room temperature under H2 atmosphere overnight. The completion of the reaction was monitored by HPLC and additional Pd (OH)2/C (0.05 eq.) was added into the system if the conversion was not complete 50 . The resulting reaction solution was filtered over Celite® and the solvents removed under reduced pressure. The subsequent purification was carried out utilizing reverse phase flash column chromatography (0→100% ACN in water, 0-30 min) or preparative HPLC.