Reagents
and conditions: (a) ZnCl2, CF3COCH2CO2CH2CH3, ethanol, reflux, overnight
(74%); (b) BrCH2CO2tBu, DMF,
80 °C, overnight (65%); (c)
H2N(CH2)2NHCO2tBu, HBTU/DIPEA, DMF, rt, overnight (70%); (i) H2N(CH2)3CO2Et·HCl, HBTU/DIPEA,
DMF, rt, overnight (85%); (ii) KOH, THF/CH3OH/H2O, rt, (80%); (iii) TFA, rt, 2 h; (iv) HBTU/DIPEA, DMF, rt, overnight
(92%); (v) KOH, THF/CH3OH/H2O, rt (71%); (vi)
(a) HBTU/DIPEA, DMF, rt, 5 h; and (vii) mPEG-NH2 (av MW
≈ 1000), HBTU/DIPEA, DMF, rt, overnight (85% for 2 steps).