Derivatives S5a and S5b were synthesized according to already known procedures. Based on the procedure given by Nalluri et al. 34 S5c was synthesized. EtOAc. The filtrate and the organic washings were combined and washed with a 5% aqueous solution of KOH in brine. The aqueous layer was extracted with DCM (3 x 10 mL) and EtOAc