The development of a continuous-flow
sequence for the synthesis
of an important drug candidate precursor is reported. Abediterol is
a β2-adrenoceptor agonist that has undergone phase
IIa clinical trials for the treatment of respiratory disease. A flow
sequence is developed for the preparation of the lipophilic amine
tail portion of abediterol. The sequence comprises of a phase-transfer-catalyzed
liquid/liquid O-alkylation, a rhodium-catalyzed hydroformylation,
and a ruthenium-catalyzed reductive amination. The reactions were
optimized separately within continuous-flow environments to identify
important parameter effects. The strongly basic O-alkylation operates
with greater than 90% conversion within a 23 min residence time. The
hydroformylation uses 1 mol % Rh(acac)(CO)2 (acac = acetylacetone)
as a catalyst and 6 mol % Xantphos as a ligand with 1.1 equiv of hydrogen
and carbon monoxide. The optimized O-alkylation and hydroformylation
telescoped flow process was successfully operated over 6 h. The protocol
is shown to be high yielding for the desired linear aldehyde (75%
gas chromatography yield, ∼2.5 g/h). The sequence requires
a solvent switch prior to the reductive amination. The final step
is a high-pressure (40 bar) and high-temperature (150 °C) Ru-catalyzed
reductive amination using ammonia and hydrogen to afford the amine
tail. The solution yield for the formation of the amine tail was 78%.
The yield of the reductive amination with an unoptimized isolation
was 50%, resulting in an overall isolated yield for the three-step
sequence of 38%. This compares favorably against the batch yield of
26% using a different synthetic route.