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  • Organic Chemistry  (4)
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  • 1
    Electronic Resource
    Electronic Resource
    α-Alkylation of Amino Acids without Racemization. Preparation of Either (S)- or (R)-α-Methyldopa from (S)-Alanine (1985)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 68 (1985), S. 144-154 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Enantiomerically pure cis- and trans-5-alkyl-1-benzoyl-2-(tert-butyl)-3-methylimidazolidin-4-ones (1, 2, 11, 15, 16) and trans-2-(tert-butyl)-3-methyl-5-phenylimidazolidin-4-one (20), readily available from (S)-alanine, (S)-valine, (S)-methionine, and (R)-phenylglycine are deprotonated to chiral enolates (cf. 3, 4, 12, 21). Diastereoselective alkylation of these enolates to 5,5-dialkyl- or 5-alkyl-5-arylimidazolidinones (5, 6, 9, 10, 13a-d, 17, 18, 22) and hydrolysis give α-alkyl-α-amino acids such as (R)- and (S)-α-methyldopa (7 and 8a, resp.), (S)-α-methylvaline (14), and (R)-α-methyl-methionine (19). The configuration of the products is proved by chemical correlation and by NOE 1H-NMR measurements (see 23, 24). In the overall process, a simple, enantiomerically pure α-amino acid can be α-alkylated with retention or with inversion of configuration through pivaladehyde acetal derivatives. Since no chiral auxiliary is required, the process is coined ‘self-reproduction of a center of chirality’. The method is compared with other α-alkylations of amino acids occurring without racemization. The importance of enantiomerically pure, α-branched α-amino acids as synthetic intermediates and for the preparation of biologically active compounds is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19850680118
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  • 2
    Electronic Resource
    Electronic Resource
    Stereoselektive Alkylierung an C(α) von Serin, Glycerinsäure, Threonin und Weinsäure über heterocyclische Enolate mit exocyclischer DoppelbindungZughörige Kurzmitteilungen: [1] (Weinsäre) [2] (Threonin) [3] (Serin) [4] (Glycerinsäure).Siehe auch die Diskussionen in den vor kurzem erschienenen Übersichtsartikeln [5][6]. (1987)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 70 (1987), S. 1194-1216 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Steroselective Alkylation at C(α) of Serine, Clyceric Acid, Threonine, and Tartaric Acid Involving Heterocyclic Enolates with Evocyelic Double BondsThe chiral, non-racemic title acids are converted to methyl dioxolane-(cf. 13), oxazoline-(4) and oxazolidinecarboxylates (cf. 9). Deprotonation by Li(i-Pr) 2N at dry-ice temperature gives solutions of the lithium enolates A-D With exocyclic enolate double bonds. These are stable crough with respect to β-elimination (Scheme 1) to be alkylated with or without cosolvents such as HMPA or DMPU The products are formed in good to excellent yields and, with the exception of the tartrate-derived acetonlde (see Scheme 2), with diastereoselectivities above 90%. While the tartrate-and threonine-derived enolates (A and B, resp.) are chiral due to the second stereogenic center of the precursors, the serine- and glyceric-acid-derived enolates (Aand B, resp.) are chiral due to the second sterogenic center of the precursors, the serine-nd glyceric-acid-derived enolates are non-racemic due to a tert butyl-substituted (pivalaldehyde-derived) acetal center (C and D, resp.). The products of alkylation can be hydrolyzed to give α-branched tartaric acid (Scheme 2), allothreonine (Scheme 3), serine (Scheme 4), and glyceric-acid derivatives (Scheme 5) with quaternary stereogenic centers. The configurations of the products are determined by NOE-NMR measurements and by chemical correlation. These show that the dioxolane-derived enolates A and D are alkylated preferentially from that face of the ring which is already substituted (‘syn’-attack), while the dihydrooxazol-and oxazolidine-derived enolates B and C are alkylated from the opposite face (‘anti’-attack). The ‘syn’-attack is postulated to arise from strong folding of the heterocyclic ring due to electronic repulsion between the enolate π-system and non-bonding electron pairs on the heteroatoms (see Scheme 6).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19870700426
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  • 3
    Electronic Resource
    Electronic Resource
    Enantioselektive α-Alkylierung von Asparagin- und Glutaminsäure über die Dilithium-enolatocarboxylate von 2-[3-Benzoyl-2-(tert-butyl)-1-methyl-5-oxoimidazolidin-4-yl]essigsäure und 3-[3-Benzoyl-2-(tert-butyl)-1-methyl-5-oxoimidazolidin-4-yl]propionsäure (1985)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 68 (1985), S. 1507-1518 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Overall Enantioselective α-Alkylation of Aspartic and Glutamic Acid through Dilithium Enolatocarboxylates of 2- [3-Benzoyl-2-(tert-butyl)-1-methyl-5-oxoimidazolidin-4-yl]acetic and 3-[3-Benzoyl-2-(tert-butyl)-1-methyl-5-oxoimidazolidin-4-yl]propionic Acid, respectivelyThe pure methyl esters 10 of the heterocyclic carboxylic acids specified in the title were prepared in several steps by known methods from aspartic and glutamic acid, with overall yields of ca. 20%. The corresponding heterocyclic acids 11 were doubly deprotonated by LiNEt2/BuLi or LiN(i-Pr)2/BuLi to give enolatocarboxylates (3). The latter were reacted with electrophiles (MeOD, Mel, C6H5CH2Br) to give the crystalline products 14-21 diastereoselectively. Hydrolysis of the imidazolidinone ring of three such products gave the corresponding α-branched aspartic and glutamic acids 22-24 of known absolute configuration, thus establishing the stereochemical course of the overall enantioselective alkylations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19850680603
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  • 4
    Electronic Resource
    Electronic Resource
    Enantiomerenreine Synthesebausteine aus verzweigter Äpfelsäure (1983)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1983 (1983), S. 2114-2126 
    Details
    ISSN: 0170-2041
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Enantiomerically Pure Building Blocks for Syntheses from Branched MalatesIsocitric acid lactone (13) and the chiral building blocks (2S,3R)-4-bromo-1,2-epoxy-3-methylbutane (21), cis- and trans-2,3-epoxy-2-methylsuccinates (23, 24), and the singly protected (2R)-2-methyl-, -2-allyl- and -2-benzyl-1,3-propanediols (32a, b, c) are readily accessible in diastereomerically and enantiomerically pure forms from malates (8, 16, 25a, b, c) which are alkylated in position 3.
    Notes: Ausgehend von in 3-Stellung alkylierten, enantiomerenreinen Äpfelsäureestern (8, 16, 25a-c) wurden (+)-Isocitronensäurelacton (13) sowie die Synthesebausteine (2S,3R)-4-Brom-1,2-epoxy-3-methylbutan (21), cis- und trans-2,3-Epoxy-2-methylbernsteinsäureester (23, 24) und die einfach geschützten (2R)-2-Methyl-, -2-Allyl- und -2-Benzyl-1,3-propandiole (32a, b, c) diastereomeren- und enantiomerenrein hergestellt.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jlac.198319831207
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