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Pierre Gilles – One of the best experts on this subject based on the ideXlab platform.

  • smi2 mediated cross coupling of nitrones with β silyl acrylates synthesis of Australine
    ChemInform, 2012
    Co-Authors: Pierre Gilles

    Abstract:

    The process affords the coupling products with nearly complete diastereoselection and can be used as key step in a new synthesis of (+)-Australine (VII).

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  • SmI2-Mediated Cross-Coupling of Nitrones with β-Silyl Acrylates: Synthesis of (+)-Australine.
    ChemInform, 2012
    Co-Authors: Pierre Gilles

    Abstract:

    The process affords the coupling products with nearly complete diastereoselection and can be used as key step in a new synthesis of (+)-Australine (VII).

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  • SmI2-Mediated Cross-Coupling of Nitrones with β-Silyl Acrylates: Synthesis of (+)-Australine
    Organic Letters, 2012
    Co-Authors: Pierre Gilles

    Abstract:

    The SmI2-mediated cross-coupling of nitrones with β-silyl-α,β-unsaturated esters, followed by zinc reduction, allows an efficient and highly diastereoselective preparation of β-silyl lactams, which are precursors of β-hydroxy lactams through Tamao–Fleming oxidation. By applying the method to a cyclic, carbohydrate-derived nitrone, a new synthesis of (+)-Australine has been realized in only 11 steps and in 21% overall yield from l-xylose.

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Peter Hrnciar – One of the best experts on this subject based on the ideXlab platform.

  • synthesis of polyhydroxylated pyrrolizidine alkaloids of the alexine family by tandem ring closing metathesis transannular cyclization Australine
    Journal of Organic Chemistry, 2000
    Co-Authors: James D White, Peter Hrnciar

    Abstract:

    Dienes 23 and 54, prepared from epoxy alcohol 9 via oxazolidinones 15 and 51, respectively, underwent ring-closing metathesis with Grubbs’s catalyst to give azacyclooctenes 26 and 55. Treatment of each azacyclooctene with m-chloroperoxybenzoic acid afforded β-epoxide 28 from 26 and α-epoxide 61 from 60. Basic hydrolysis of each of these oxazolidinones was accompanied by transannular attack at the epoxide by nitrogen, resulting in 2-(O-benzyl)-7-deoxyalexine (29) and 1,2-di-(O-benzyl)Australine (62). The latter was converted to the alkaloid Australine (3) upon hydrogenolysis. Attempts to effect ring-closing metathesis of dienes 37, 38, and 46 were unsuccessful, suggesting that, while one allylic oxygen substituent can be tolerated by Grubbs’s catalyst, two cannot.

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  • Synthesis of polyhydroxylated pyrrolizidine alkaloids of the alexine family by tandem ring-closing metathesis-transannular cyclization. (+)-Australine.
    The Journal of Organic Chemistry, 2000
    Co-Authors: James D White, Peter Hrnciar

    Abstract:

    Dienes 23 and 54, prepared from epoxy alcohol 9 via oxazolidinones 15 and 51, respectively, underwent ring-closing metathesis with Grubbs’s catalyst to give azacyclooctenes 26 and 55. Treatment of each azacyclooctene with m-chloroperoxybenzoic acid afforded β-epoxide 28 from 26 and α-epoxide 61 from 60. Basic hydrolysis of each of these oxazolidinones was accompanied by transannular attack at the epoxide by nitrogen, resulting in 2-(O-benzyl)-7-deoxyalexine (29) and 1,2-di-(O-benzyl)Australine (62). The latter was converted to the alkaloid Australine (3) upon hydrogenolysis. Attempts to effect ring-closing metathesis of dienes 37, 38, and 46 were unsuccessful, suggesting that, while one allylic oxygen substituent can be tolerated by Grubbs’s catalyst, two cannot.

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  • Configurational and conformational analysis of highly oxygenated pyrrolizidines: definitive identification of some naturally occurring 7a-epi-alexines
    Tetrahedron-asymmetry, 1998
    Co-Authors: Mark R. Wormald, Russell J. Molyneux, Robert J Nash, Peter Hrnciar, James D White, George W J Fleet

    Abstract:

    Abstract The NMR spectra of a number of naturally occurring alexines (tetrahydroxylated pyrrolizidine alkaloids) are analyzed and the consequences of changes in the configuration on the conformation of these bicyclic systems discussed. Unambiguous syntheses of Australine (7-epi-alexine) and of 7,7a-epi-alexine have now unequivocally established the structures of two natural products isolated from Castanospermum australe which were insecure due to erroneous NMR data. Chemical shift parameters are unreliable as a method of comparing different samples of identical compounds; however, 1H–1H three bond coupling constants (3JHH) provide easy direct comparison between samples and allow assignments of both the relative configurations for the ring protons and the conformation of the pyrrolizidine framework.

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James D White – One of the best experts on this subject based on the ideXlab platform.

  • synthesis of polyhydroxylated pyrrolizidine alkaloids of the alexine family by tandem ring closing metathesis transannular cyclization Australine
    Journal of Organic Chemistry, 2000
    Co-Authors: James D White, Peter Hrnciar

    Abstract:

    Dienes 23 and 54, prepared from epoxy alcohol 9 via oxazolidinones 15 and 51, respectively, underwent ring-closing metathesis with Grubbs’s catalyst to give azacyclooctenes 26 and 55. Treatment of each azacyclooctene with m-chloroperoxybenzoic acid afforded β-epoxide 28 from 26 and α-epoxide 61 from 60. Basic hydrolysis of each of these oxazolidinones was accompanied by transannular attack at the epoxide by nitrogen, resulting in 2-(O-benzyl)-7-deoxyalexine (29) and 1,2-di-(O-benzyl)Australine (62). The latter was converted to the alkaloid Australine (3) upon hydrogenolysis. Attempts to effect ring-closing metathesis of dienes 37, 38, and 46 were unsuccessful, suggesting that, while one allylic oxygen substituent can be tolerated by Grubbs’s catalyst, two cannot.

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  • Synthesis of polyhydroxylated pyrrolizidine alkaloids of the alexine family by tandem ring-closing metathesis-transannular cyclization. (+)-Australine.
    The Journal of Organic Chemistry, 2000
    Co-Authors: James D White, Peter Hrnciar

    Abstract:

    Dienes 23 and 54, prepared from epoxy alcohol 9 via oxazolidinones 15 and 51, respectively, underwent ring-closing metathesis with Grubbs’s catalyst to give azacyclooctenes 26 and 55. Treatment of each azacyclooctene with m-chloroperoxybenzoic acid afforded β-epoxide 28 from 26 and α-epoxide 61 from 60. Basic hydrolysis of each of these oxazolidinones was accompanied by transannular attack at the epoxide by nitrogen, resulting in 2-(O-benzyl)-7-deoxyalexine (29) and 1,2-di-(O-benzyl)Australine (62). The latter was converted to the alkaloid Australine (3) upon hydrogenolysis. Attempts to effect ring-closing metathesis of dienes 37, 38, and 46 were unsuccessful, suggesting that, while one allylic oxygen substituent can be tolerated by Grubbs’s catalyst, two cannot.

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  • Configurational and conformational analysis of highly oxygenated pyrrolizidines: definitive identification of some naturally occurring 7a-epi-alexines
    Tetrahedron-asymmetry, 1998
    Co-Authors: Mark R. Wormald, Russell J. Molyneux, Robert J Nash, Peter Hrnciar, James D White, George W J Fleet

    Abstract:

    Abstract The NMR spectra of a number of naturally occurring alexines (tetrahydroxylated pyrrolizidine alkaloids) are analyzed and the consequences of changes in the configuration on the conformation of these bicyclic systems discussed. Unambiguous syntheses of Australine (7-epi-alexine) and of 7,7a-epi-alexine have now unequivocally established the structures of two natural products isolated from Castanospermum australe which were insecure due to erroneous NMR data. Chemical shift parameters are unreliable as a method of comparing different samples of identical compounds; however, 1H–1H three bond coupling constants (3JHH) provide easy direct comparison between samples and allow assignments of both the relative configurations for the ring protons and the conformation of the pyrrolizidine framework.

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