The asymmetric synthesis of enantiopure C(5)-substituted transpentacins via diastereoselective conjugate additions to a β′-amino-α,β-unsaturated ester

© 2016 The asymmetric synthesis of a range of 1,2-anti-1,5-syn-transpentacins, bearing either alkyl, phenyl or hydroxymethyl substituents at the C(5)-position, has been achieved using the diastereoselective conjugate additions of Grignard reagents to an enantiopure β′-amino-α,β-unsaturated ester as the key step. In addition, the doubly diastereoselective conjugate addition of an enantiopure lithium amide reagent to the β′-amino-α,β-unsaturated ester provided access to the corresponding β,β′-diamino ester, which was subsequently converted to both (S,S)-(2,5-diaminocyclopent-1-yl)methanol and (S,S)-2,5-diaminocyclopentane-1-carboxylic acid. In each case, the final enantiopure products were obtained as single diastereoisomers (>99:1 dr) in good yields over five steps or fewer from commercially available starting materials.