The reaction of the propargyl tetrahydropyranyl ether (I) with TBDMS-Cl and BuLi in THF gives the silylated acetylene (II), which is treated with Br2 and PPh3 to yield the silylated propargyl bromide (III). The reaction of (III) with the pyranopyridone (IV) by means of NaH and LiBr in DMF/DME affords the alkylated pyranopyridone (V), which is cyclized with 4-acetoxyphenyl isocyanide (VI) by means of hexamethyldistannane in refluxing benzene to provide the silylated acetoxycamptothecin (VII). Finally, this compound is deacetylated with K2CO3 in methanol.
The reaction of camptothecin (I) with Tbdms-H, (t-BuO)2 and t-BuSH in refluxing dioxane gives the 7-silylcamptothecin (II), which is treated with H2O2 in hot AcOH to give the N-oxide (III). Finally, this compound is isomerized by photolysis in dioxane/H2SO4 to afford the target silylated 10-hydroxycamptothecin.
The reaction of 10-hydroxycamptothecin (IV) with Ac2O and pyridine gives the diacetate (V), which is silylated with Tbdms-H, (t-BuO)2 and Tis-SH in refluxing dioxane/TFA to give the silylated diacetate (VI). Finally, this compound is deacetylated by reaction with NaOMe in methanol or HCl in ethanol to afford the target silylated 10-hydroxycamptothecin.