Reagents and conditions: (a)
H2, Pd/C, MeOH, rt, 35–60 min; (b) Boc2O, Et3N, DMF, rt, 1–2.5 h; (c) benzyl alcohol or
substituted benzyl alcohol, DCC, DMAP, DMF, rt, 3.5 h–overnight,
43–82% (3 steps); (d) 4 M HCl/dioxane, rt, 1 h, then RP-HPLC
purification, 23–55%. R4 substituents of derivatives 13a–e are shown in Table 2.