The manufacturing route to venetoclax takes place by
coupling of three key structural subunits: azaindole 162,
sulfonamide 165, and piperazine 172. The first of these
subunits was generated in two steps from commercially
available 4-bromo-2-fluoro-1-iodo-benzene (159).
Grignard formation of iodide 159 (i-PrMgCl) followed by
quenching with Boc2O provided the desired tert-butyl ester 160
without the need for chromatographic purification. Aromatic
substitution of crude 160 with azaindole 161 provided access to
162 in 86% yield after recrystallization from EtOAc/heptane. Sulfonamide 165 was
formed in 91% yield and 99.9% purity via aromatic substitution
of commercially available 163 with amine 164 at 80 °C
(DIPEA, MeCN).
Synthesis of the third venetoclax subunit, piperazine amine
hydrochloride salt 172, began with commercial cyclohexanone
166. Vilsmeier-Haack formylation of the
sterically more accessible enol tautomer of 166 delivered vinyl
chloride 167 in quantitative yield. Coupling of this chloride
with commercial aryl boronate 168 gave rise to transient enal
169 in 87% assay yield, which was not isolated. Crude 169 was
then carried into a reductive amination reaction with
commercial N-Boc piperazine (170). Precipitation and
recrystallization from acetonitrile ultimately furnished piperazinyl
alkene 171 in 74% yield from 167. Finally, subunit 172
was obtained via Boc removal with concentrated HCl in IPA at
65 °C and subsequent filtration, conditions that provided a 95%
yield of high purity intermediate 172 (>99.5%).
The final approach to venetoclax involved a palladiumcatalyzed
coupling of amine 172 with aryl bromide 162, ester
hydrolysis, and coupling of the resulting carboxylic acid with
sulfonamide 165. In practice, Buchwald-Hartwig amination of 162 with 172 proceeded smoothly and
relied upon workup with cysteine to enable cleansing of
residual palladium from the reaction mixture. This reaction
gave rise to advanced intermediate 173 in 89% yield after
crystallization from cyclohexane. Treatment of 173 with t-
BuOK/H2O/2-MeTHF at 55 °C provided the corresponding
free acid, which was immediately activated with EDC/DMAP/
Et3N to promote coupling with sulfonamide 165 at room
temperature. The final drug target could be accessed by
crystallization from EtOAc and washing with 1:1 DCM/EtOAc,
yielding venetoclax (XVIII) in free base form in 71% over
the two final steps. This synthetic route was capable of
fashioning the drug target in 52% overall yield based on the
longest linear sequence (7 steps).