Friedel-Crafts acylation of toluene (II) with 3-methoxybenzoyl chloride (I) gave benzophenone (III). Subsequent methyl group oxidation using KMnO4 afforded carboxylic acid (IV), which was further converted to amide (V) by activation with SOCl2, followed by treatment with diethylamine. Reduction of the keto group of (V) by means of NaBH4 yielded alcohol (VI). The required benzhydryl chloride (VII) was then prepared by treatment of alcohol (VI) with concentrated HCl.
The alkylation of trans-2,5-dimethylpiperazine (VIII) with allyl bromide gave rise to a racemic mixture of N-allylpiperazines, which was resolved using (-)-camphoric acid. The desired (2R,5S)-piperazine (IX) was then alkylated with benzhydryl chloride (VII) producing a mixture of the title compound and its benzhydryl epimer (X), which were separated by flash column chromatography.
In a different synthetic strategy, 4-formylbenzoic acid (XI) was first coupled to diethylamine by means of EDC to yield N,N-diethyl 4-formylbenzamide (XII). The intermediate iminium salt produced by condensation between aldehyde (XII) and the chiral piperazine (IX) was then isolated as the benzotriazole adduct (XIII). Displacement of the benzotriazolyl group by the Grignard reagent (XIV) produced the title diastereoisomer as the major product, which was further enriched by recrystallization from acetonitrile/water.