The synthetic route to alectinib as reported by Chugai
begins with 7-methoxy-2-tetralone (1). Bis-methylation
with tetrabutylammonium hydrogen sulfide (TBAHS)/aq KOH/MeI
followed by bromination with N-bromosuccinimide (NBS) provided
the bromo-tetralone 2 in 67% yield over the two steps. Further
reaction of 2 with 3-hydrazinobenzonitrile/trifluoroacetic acid (TFA) led to formation of the desired Fischer indole product,
albeit as a 1:1 mixture of regioisomers (3/4), which were carried
forward as a mixture to oxidation with 2,3-dichloro-5,6-dicyano-
1,4-benzoquinone (DDQ). It is important to note that although representative
procedures are published describing the conversion of
2 to alectinib (I), no yields were provided for these transformations.
Following oxidation, the desired product 5 could be isolated
as a single isomer via precipitation from the crude reaction mixture.
Installation of the 4-morpholino-piperidine moiety took place
in three transformations from 5, beginning with 1-dodecanethiol/
N-methyl-2-pyrrolidone (NMP)/NaOMe-facilitated methyl cleavage.
The corresponding phenol was then readily converted to the
triflate intermediate and displaced with 4-(piperidin-4-yl)morpholine
(6) at elevated temperature, providing intermediate 7. Crosscoupling
of the bromide 7 with ethynyl triisopropylsilane under
Pd-catalyzed cross-coupling conditions (Pd(CH3CN)2Cl2/2-dicyclohexylphosphino-
20,40,60-triisopropylbiphenyl (XPhos), reflux) followed
by cleavage of the resulting alkylsilane with
tetrabutylammonium fluoride (TBAF) yielded the ethynyl precursor
to alectinib. Hydrogenation of this unsaturated system under
standard conditions (H2, Pd/C) followed by HCl salt formation furnished
the final drug target alectinib hydrochloride (I).