Scheme 5.

Reagents and conditions: a) TIPDSCl, imidazole, DMF, 79%; b) DMF-DMA, toluene, 60 °C, 91%; c) MeI, NaH, DMF, 0 °C; d) 85% hydrazine hydrate, CH₃CN, 30% over two steps; e) TBAF, THF, 0 °C to rt, 50%; f) isobutyryl chloride, TEA, DMAP, CH₂Cl₂, 72%; g) TBAF, THF, RT; h) TFA, THF, 61% over two steps; i) N-Boc-l-Valine, HOBT, EDCI, DMAP, CH₂Cl₂, 75%; j) 85% hydrazine hydrate, CH₃CN; k) TBAF, AcOH, THF, 64% over two steps; l) 4 M HCl/CH₃OH, 75%; m) TMSCl, pyridine, then pentyl chloroformate, CH₂Cl₂, 0 °C; n) TBAF, THF, RT, 48% over two steps.