Browsing by Author "Smith, Andrew D."

Now showing 1 - 7 of 7
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    Asymmetric synthesis of (1R,2S,3R)-gamma-methyl-cis-pentacin by a kinetic resolution protocol
    (2002) Bailey, Simon; Davies, Stephen G.; Smith, Andrew D.; Withey, Jonathan M.
    The asymmetric synthesis of (1R,2S,3R)-3-methyl-2-amino-cyclopentane carboxylic acid has been achieved via kinetic resolution of racemic tert-butyl 3-methyl-cyclopentene-1-carboxylate with homochiral lithium (S)-N-benzyl-N-alpha-methylbenzylamide.
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    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.
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    Kinetic resolution and parallel kinetic resolution of methyl (±)-5-alkyl-cyclopentene-1-carboxylates for the asymmetric synthesis of 5-alkyl-cispentacin derivatives
    (2005) Davies, Stephen G.; Garner, A. Christopher; Long, Marcus J. C.; Morrison, Rachel M.; Roberts, Paul M.; Savory, Edward D.; Smith, Andrew D.; Sweet, Miles J.; Withey, Jonathan M.
    Conjugate addition of lithium dibenzylamide to methyl 5-isopropyl, 5-phenyl- and 5-tert-butyl-cyclopentene-1-carboxylates occurs with high levels of substrate control (>88% de), with preferential addition to the face of the cyclic α,β-unsaturated acceptor anti- to the stereodirecting 5-alkyl substituent. Treatment of a range of methyl (±)-5-alkyl-cyclopentene-1-carboxylates with both lithium (±)-N-benzyl-N-α-methylbenzylamide and lithium (±)-N-3,4-dimethoxybenzyl-N-α-methylbenzylamide indicates significant enantiorecognition in their mutual kinetic resolutions, with preferential addition anti- to the 5-alkyl substituent, giving the 1,2-syn-1,5-anti-arrangement (E >16) after enolate protonation anti- to the amino functionality. The kinetic resolution of a range of methyl (±)-5-alkyl-cyclopentene-1-carboxylates with lithium (S)-N-benzyl-N-α-methylbenzylamide, and their efficient parallel kinetic resolution with a pseudoenantiomeric mixture of lithium (S)-N-benzyl-N-α-methylbenzylamide and lithium (R)-N-3,4-dimethoxybenzyl-N-α-methylbenzylamide are also demonstrated, giving a range of 5-alkyl-cispentacin derivatives in >98% de and high ee after N-deprotection.
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    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.
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    Parallel kinetic resolution of tert-butyl (RS)-3-alkyl–cyclopentene-1-carboxylates for the asymmetric synthesis of 3-alkyl–cispentacin derivatives
    (2004) Davies, Stephen G.; Garner, A. Christopher; Long, Marcus J. C.; Smith, Andrew D.; Sweet, Miles J.; Withey, Jonathan M.
    The double mutual kinetic resolution of tert-butyl (RS)-3-benzyl-cyclopentene-1-carboxylate with a 50 : 50 mixture of lithium (RS)-N-benzyl-N-alpha-methylbenzylamide and lithium (RS)-N-3,4-dimethoxybenzyl-N-alpha-methylbenzylamide gives, after protonation with 2,6-di-tert-butylphenol, a 50 : 50 mixture of the readily separable N-benzyl-(1SR,2RS,3RS,alphaRS)- and N-3,4-dimethoxybenzyl-(1SR,2RS,3RS,alphaRS)-beta-amino esters in >98% de in each case. This product distribution indicates that these amides react at very similar rates and with no mutual interference to furnish readily separable products, and are thus ideal for parallel kinetic resolution. The efficient parallel kinetic resolution ( E > 65) of a range of tert-butyl (RS)-3-alkyl-cyclopentene-1-carboxylates with a pseudoenantiomeric mixture of homochiral lithium (S)-N-benzyl-N-alpha-methylbenzylamide and lithium (R)-N-3,4-dimethoxybenzyl-N-alpha-methylbenzylamide gives, after separation and N-deprotection, a range of carboxylate protected 3-alkyl-cispentacin derivatives in >98% de and >95% ee.
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    Preparation of methyl (1R,2S,5S)- and (1S,2R,5R)-2-amino-5-tert-butyl-cyclopentane-1-carboxylates by parallel kinetic resolution of methyl (RS)-5-tert-butyl-cyclopentene-1-carboxylate
    (2003) Davies, Stephen G.; Díez, David; El Hammouni, Mohamed M.; Garner, A. Christopher; Garrido, Narciso M. ; Long, Marcus J. C.; Morrison, Rachel M.; Smith, Andrew D.; Sweet, Miles J.; Withey, Jonathan M.
    Comparison of the kinetic and parallel kinetic resolutions of methyl (RS)-5-tert-butyl-cyclopentene-1-carboxylate allows for the efficient synthesis of both (1R,2S,5S)- and (1S,2R,5R)-enantiomers of methyl 2-amino-5-tert-butyl-cyclopentane-1-carboxylate.
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    Strategies for the construction of morphinan alkaloid AB-rings: Regioselective Friedel-Crafts-type cyclisations of γ-aryl-β-benzoylamido acids with asymmetrically substituted γ-aryl rings
    (2016) Davies, Stephen G.; Goddard, Euan C.; Roberts, Paul M.; Russell, Angela J.; Smith, Andrew D.; Thomson, James E.; Withey, Jonathan M.
    The regioselectivity of the Friedel-Crafts-type cyclisation of a range of -aryl--benzoylamido acids, bearing oxy substituents at the C(3)- and C(4)-positions of the -aryl ring, has been investigated. In all of the cases examined (with 3,4-dimethoxy, 3,4-methylenedioxy and 3-hydroxy-4-methoxy substituents) the Lewis acid promoted cyclisation proceeds with exclusive regioselectivity for attack at the C(6)-position rather than at the C(2)-position, and furnishes the corresponding, N- and O-protected 3-amino-6,7-dihydroxy-1-tetralone derivatives. This inherent regioselectivity can be overturned by the regioselective introduction of chlorine as a blocking group for the C(6)-position; subsequent Lewis acid promoted cyclisation then proceeds with exclusive regioselectivity for attack at the C(2)-position to deliver the corresponding N- and O-protected 3- amino-5-chloro-7,8-dihydroxy-1-tetralone derivative. These complementary cyclisation protocols represent useful methods for the preparation of these benzo-fused carbocyclic ring systems, which are the functionalised AB-rings of a range of morphinan alkaloids.