The conversion of 3 to 8 is summarized in Scheme 2. The trityl group (too large
and too acid-sensitive for the ensuing steps) was removed from N and both N’s were
protected by Cbz (benzyloxycarbonyl) groups. Protection of the tertiary OH specifi -
cally as the robust TBS (t-butyldimethylsilyl) group was found to be necessary for
the sequence involving the electrophilic aromatic substitution step, 5 to 6, and the
Stille coupling steps (6 7 → 8).