Browsing by Author "Bunnage, Mark E."
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Item Asymmetric synthesis of the cis- and trans-stereoisomers of 4-aminopyrrolidine-3-carboxylic acid and 4-aminotetrahydrofuran-3-carboxylic acid(2004) Bunnage, Mark E.; Davies, Stephen G.; Roberts, Paul M.; Smith, Andrew D.; Withey, Jonathan M.The diastereoselective conjugate addition of lithium (S)-N-benzyl-N-alpha-methylbenzylamide has been successfully applied to the first asymmetric syntheses of cis-(3S,4R)- and trans-(3R,4R)-4-aminotetrahydrofuran-3-carboxylic acids (26% and 25% overall yield respectively, > 98% d.e. and > 97% e. e. in each case). Furthermore, the most efficient asymmetric synthesis to date of cis-( 3R, 4R)- and trans-( 3R, 4S)-4-aminopyrrolidine carboxylic acids is delineated: for cis-( 3R, 4R), four steps, > 98% d.e., 52% overall yield; for trans-( 3R, 4S), five steps, > 98% d.e., 50% overall yield.Item Kinetic resolution of tert-butyl (RS)-3-alkylcyclopentene-1-carboxylates for the synthesis of homochiral 3-alkyl-cispentacin and 3-alkyl-transpentacin derivatives(2004) Bunnage, Mark E.; Davies, Stephen G.; Parkin, Richard M.; Roberts, Paul M.; Smith, Andrew D.; Withey, Jonathan M.High levels of stereocontrol are observed in the conjugate addition of lithium dibenzylamide to tert-butyl (RS)-3-alkylcyclopentene-1-carboxylates (alkyl = Et, Bn), with addition occurring exclusively anti- to the 3-alkyl substituent. Treatment of a range of tert-butyl (RS)-3-alkylcyclopentene-1-carboxylates (alkyl = Et, Bn, Pr-i, Bu-t) with lithium (RS)-N-benzyl-N-alpha-methylbenzylamide indicates that good enantiorecognition is observed (E > 80) in their mutual kinetic resolution. In these reactions, conjugate addition of the lithium amide occurs exclusively anti- to the 3-alkyl substituent, with subsequent C(1)-protonation occurring preferably anti- to the 2-amino group in the 3-Et, 3-Bn and 3-Pr-i cases, giving predominantly the corresponding 1,2-syn-2,3-anti-diastereoisomers. Conjugate addition to (RS)-3-tert-butyl cyclopentene-1-carboxylate results in exclusive 2,3-anti-addition and a reversal in C(1)-protonation selectivity, giving predominantly the 1,2-anti-2,3-anti-diastereoisomer. Furthermore, the kinetic resolution of the tert-butyl (RS)-3-alkylcyclopentene-1-carboxylates (alkyl = Et, Bn, Pr-i, Bu-t) with lithium (S)-N-benzyl-N-alpha-methylbenzylamide proceeds efficiently, giving, at between 47 and 51% conversion, the resolved 3-alkylcyclopentene-1-carboxylates in > 85 to > 98% ee and the beta-amino ester products of conjugate addition in high de, consistent with E > 80 in each case. Subsequent deprotection of the 1,2-syn-2,3-anti-3-alkyl-beta-amino esters ( alkyl = Et, Bn, Pr-i) by hydrogenolysis and ester hydrolysis gives the corresponding 1,2-syn-2,3-anti-3-alkylcispentacins in > 98% de and 98 +/- 1% ee. Selective epimerisation of the 1,2-syn-2,3-anti-3-alkyl-beta-amino esters (alkyl = Et, Bn, Pr-i, Bu-t) by treatment with (KOBu)-Bu-t in (BuOH)-Bu-t gives the corresponding 1,2-anti-2,3-anti-3-alkyl-beta-amino esters in quantitative yield and in > 98% de, with subsequent deprotection by hydrogenolysis and ester hydrolysis giving the corresponding 1,2-anti-2,3-anti-3-alkylcispentacin hydrochlorides in > 98% de.