Construction of a cis-Cyclopropane via Reductive Radical Decarboxylation. Enantioselective Synthesis of cis- and trans-1-Arylpiperazyl-2-phenylcyclopropanes Designed as Antidopaminergic Agents

Abstract

(1S,2S)-, (1S,2R)-, and (1R,2S)-1-(2,4-Dimethylphenyl)piperazyl-2-phenylcyclopropane (2a, 3, and ent- 3, respectively), which were designed as conformationally restricted analogues of haloperidol (1), a clinically effective antipsychotic agent, were synthesized from chiral epichlorohydrins using the Barton reductive radical decarboxylation as the key step. (1S,2R)-1-(tert-Butyldiphenylsilyloxy)methyl-2-carboxy-2-phenylcyclopropane (5), which was prepared from (S)-epichlorohydrin ((S)-7), was converted into its N-hydroxypyridine-2-thione ester 12, the substrate for the reductive radical decarboxylation. When 12 was treated with TMS₃SiH in the presence of Et₃B or AIBN, the decarboxylation and subsequent hydride attack on the cyclopropyl radical intermediate from the side opposite to the bulky silyloxymethyl moiety occurred, resulting in selective formation of the corresponding reductive decarboxylation product 4- cis with the cis-cyclopropane structure. From 4- cis, the cis-cyclopropane-type target compound 3 was readily synthesized. Starting from (R)-epichlorohydrin ((R)-7), ent- 3 was similarly synthesized. Epimerization of the cyclopropanecarboxamide ent- 16 -cis, a synthetic intermediate for ent- 3, on treatment with a base prepared from Bu₂Mg and i-Pr₂NH in THF occurred effectively to give the corresponding trans isomer 16- trans, which was converted into 2a with the trans-cyclopropane structure.