It was a significant challenge to keep these conjugated molecules
approaching the Ro5, and in all cases lipophilicity increased
compared to astemizole. Clearly this would need to be addressed
in further design cycles, due to the potential for high lipophilicity
to increase both off-target pharmacology and the potential for adverse
toxicological outcomes.14 However, it was felt the designed
set of compounds 5–8 would allow for proof-of-principle of this
hybrid approach to be established, which could then trigger further
work to improve drug-like properties
The synthetic route employed to access the target compounds
5–8 is outlined in Scheme 1. Alkylation of 2-chlorobenzimidazole
9 was effected with 4-fluorobenzyl bromide 10
conditions
to afford 11 in excellent yield. Attempted palladium-catalysed
Buchwald–Hartwig type amination of 11 under standard
conditions only yielded starting materials, but 12 could be obtained
in good yield after treatment of 11 with excess piperazine
in the absence of catalyst under microwave irradiation. Final coupling
of 12 with 4,7-dichloroquinoline 13 afforded target compound
5.15 Amination of 13 with propanolamine 14 gave
intermediate 15 in excellent yield. Compound 6 was then obtained
by subsequent triflation of 15 to yield 16, followed by nucleophilic
substitution of the triflate with 12