Asymmetric synthesis of (1R,2S,3R)-3-methylcispentacin and (1S,2S,3R)-3-methyltranspentacin by kinetic resolution of tert-butyl (±)-3-methylcyclopentene-1-carboxylate

Abstract

Conjugate addition of lithium dibenzylamide to tert-butyl (±)-3-methylcyclopentene-1-carboxylate occurs with high levels of stereocontrol, with preferential addition of lithium dibenzylamide to the face of the cyclic α,β-unsaturated acceptor anti- to the 3-methyl substituent. High levels of enantiorecognition are observed between tert-butyl (±)-3-methylcyclopentene-1-carboxylate and an excess of lithium (±)-N-benzyl-N-α-methylbenzylamide (10 eq.) (E > 140) in their mutual kinetic resolution, while the kinetic resolution of tert-butyl (±)-3-methylcyclopentene-1-carboxylate with lithium (S)-N-benzyl-N-α-methylbenzylamide proceeds to give, at 51% conversion, tert-butyl (1R,2S,3R,αS)-3-methyl-2-N-benzyl-N-α-methylbenzylaminocyclopentane-1-carboxylate consistent with E > 130, and in 39% yield and 99 ± 0.5% de after purification. Subsequent deprotection by hydrogenolysis and ester hydrolysis gives (1R,2S,3R)-3-methylcispentacin in >98% de and 98 ± 1% ee. Selective epimerisation of tert-butyl (1R,2S,3R,αS)-3-methyl-2-N-benzyl-N-α-methylbenzylaminocyclopentane-1-carboxylate by treatment with KO^tBu in ^tBuOH gives tert-butyl (1S,2S,3R,αS)-3-methyl-2-N-benzyl-N-α-methylbenzylaminocyclopentane-1-carboxylate in quantitative yield and in >98% de, with subsequent deprotection by hydrogenolysis and ester hydrolysis giving (1S,2S,3R)-3-methyltranspentacin hydrochloride in >98% de and 97 ± 1% ee.