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  • Amino acids  (2)
  • Dihydrogen complexes  (1)
  • Wiley-Blackwell  (3)
  • 1
    ISSN: 1434-193X
    Keywords: Amino acids ; Spiro compounds ; Michael addition ; Copper reagents ; Curtius degradation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Racemic spiropentylglycine (8) has been synthesized by sodium borohydride reduction of benzyl (E/Z)-2-chloro-2-spiropentylideneacetate (5-Bn), nucleophilic substitution of the chlorine in the product 6 with azide and hydrogenolytic deprotection of the resulting 7 (overall yield 15%). An alternative approach to 8 consisted of the coupling of the higher-order cuprate 10, generated by halogen-metal exchange from bromospiropentane (9), with the electrophilic glycine equivalent 11 followed by deprotection (overall yield 47%). Enantiomerically pure (1′-aminospiropentyl)acetic acid [(R)-16] (overall yield 16% from 5-Me) and 1-aminospiropentanecarboxylic acid [(R)-23] (29% from 5-Me) were obtained from the Michael adduct 14-Me of (4R,5S)-4,5-diphenyloxazolidin-2-one (13) and methyl (E/Z)-2-chloro-2-spiropentylideneacetate (5-Me). Racemic 1-aminospiropentanecarboxylic acid (R/S-23) was prepared by rhodium-catalyzed addition of dimethyl diazomalonate to methylenecyclopropane and subsequent Curtius degradation of the halfester 28 via the azide 29 (overall yield 14%). Upon standing in aqueous solution, 23 underwent complete rearrangement to the new 1-amino-2-methylenecyclobutanecarboxylic acid (24). The interesting derivative of azabicyclo[3.1.0]hexane-1-carboxylate 34 with an annelated spiropentane moiety and a β-amino acid fragment was incidentally obtained in a one-step intermolecular domino reaction starting with the addition of lithium benzylamide to methyl 2-chloro-2-cyclopropylideneacetate (32, 41% yield).
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  • 2
    ISSN: 1434-193X
    Keywords: Amino acids ; Bicyclopropylidene ; Methylenespiropentane ; Spiro compounds ; Organolithium derivatives ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Racemic bicyclopropylidenyl- (rac-11) and methylenespiropentyl- (rac-17) substituted alanines have been synthesized by iodination of bicyclopropylidenyl- and methylenespiropentylmethanols 7, 13, nucleophilic substitution of the iodine in 8, 14 with the enolate of ethyl α-(diphenylmethyleneamino)acetate (O'Donnell's glycine equivalent) and deprotection of 9, 15 in 24 and 18% overall yield, respectively. N-Methylbicyclopropylidenylalanine rac-22, was obtained from the Michael adduct of (bicyclopropylidenyl)magnesium bromide 18 to enamine 19 and deprotection of the carbamate 20 (23% overall yield). Racemic (1-amino-2-methylenespiropentane)- (37), (1-amino-2-methylenecyclopropane)- (3), and (1-aminobicyclopropylidene)carboxylic acid (39) were prepared as hydrochlorides by tert-butoxycarbonylation of the lithiated methylenespiropentane (6), methylenecyclopropane (4), or bicyclopropylidene (5) intermediates with di-tert-butyl pyrocarbonate (Boc2O), repeated lithiation of the tert-butyl esters 29, 30, and 33 with LDA and subsequent carboxylation, Curtius degradation of the half esters 31, 32, and 34 followed by deprotection in 11, 45, and 4% overall yields, respectively. Compound 37 was also prepared from bicyclopropylidene (5) following the same procedure, but with rearrangement in the last but one step, in 19% overall yield. An attempted Hofmann degradation of the bicyclopropylidenecarboxamido ester 40 with NBS failed and gave only bromohydrin 44 (27%), but with bis(acetoxy)iodobenzene provided carbamate 46a, b in 76 amd 79% yield, respectively. Along this route with subsequent deprotection of 46b, the amino acid 39 could be prepared in up to 10% overall yield from bicyclopropylidene.
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  • 3
    ISSN: 0749-1581
    Keywords: T1 method ; Dihydrogen complexes ; Dipole-dipole relaxation ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The unusual species [ReH5(H2){P(p-tolyl)3}2], for which a neutron diffraction study shows an intermediate H—H bond distance of 1.357 Å, is studied by variable-temperature 1H NMR spin-lattice (T1) relaxation time measurements. The T1 relaxation rate goes through a maximum at low temperature and the resulting T1 (min) value is 66 ms. This is within the range of T1(min) values found for classical ReH7L2 complexes, so H2 complexes with such long H—H bond distances are undetectable by the T1 method. The observed T1(min) value is compared with that calculated from the neutron diffraction data. We consider not only the usual proton-proton dipole-dipole (HHDD) contributions to the relaxation but also the metal-hydride dipole-dipole (MHDD) contributions. The latter are shown to be negligible for most transition metals, but not for Nb, V, Re, Mn, Co and Ta. Inclusion of the MHDD contributions leads to better agreement between the calculated and observed T1(min) values for a series of rhenium polyhydride complexes. The MHDD relaxation also accounts for most of the disparity previously noted between the calculated and observed T1(min) values for Re2H8(PEt2Ph)4.
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