Synthesis of a series of omega-dimethylaminoalkyl substituted ethylenediamine ligands for use in enantioselective catalysis
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2015 Elsevier Ltd. All rights reserved. The title compounds H2NCH((CH2)nNMe2)CH2NH2 L1-L4 (n = 1-4) are efficiently synthesized in enantiopure form. The commercial starting materials, l-asparagine, (S)-5-hydroxymethyl-2-pyrrolidinone, and (S)-6-(((benzyloxy)carbonyl)-amino)-2-((tert-butoxycarbonyl)amino)hexanoic acid, are elaborated in 6-9 standard steps to give L1 (18% overall), L2 (13%), L3 (36%) and L4 (38%) or the corresponding tris(hydrochloric acid) salts [H3NCH((CH2)nNHMe2)CH2NH3]3+ 3Cl-, which are preferable for long term storage. The sequences make use of isobutyl carbamate, Cbz, and Boc protecting groups and Hofmann type rearrangements; the dimethylamino groups are introduced at late stages, either via reductive dimethylations or nucleophilic displacements involving mesylates and HNMe2. L1-L4 chelate to [Co(en)2]3+ fragments to give octahedral complexes that catalyze numerous enantioselective reactions.