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  • Wiley-Blackwell  (2)
  • 1995-1999  (2)
  • 1965-1969
  • 1999  (2)
  • 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).
    Additional Material: 2 Ill.
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  • 2
    ISSN: 1434-1948
    Keywords: Niobium ; Tin ; Hydrides ; Group 5 ; Group 14 ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of niobocene trihydride [Cp2NbH3] with one equivalent of an alkyl- or aryltin halide XnSnR4-n (n = 1, 2) leads to products of the displacement of hydride by tin, i.e. [Cp2NbH2(SnX3-nRn)] [R = Me, n = 3 (1); R = Me, × = Cl, n = 2 (3); R = Ph, n = 3 (4); R = Ph, × = Cl, n = 2 (5); R = Et, × = Br, n = 2 (6)]. Use of an excess of the tin reagent results in a halodealkylation exchange reaction at the tin atom bound to the metal center. Reaction of the mono(silyl) complexes [Cp2NbH2(SiXMe2)] (X = Cl, Ph) with organotin halides XnSnR4-n gives products of the displacement of silicon by tin, whereas the bis(silyl) complex [Cp2NbH(SiPhMe2)2] does not undergo this reaction. The bis(tin)-substituted niobocene [Cp2NbH(SnMe3)2] has been prepared by the reaction of [Cp2NbH2(SnMe3)] with the tin amide Me3SnNMe2 under thermal conditions, as well as by the literature method of thermolysis of [Cp2NbH(η2-C2H3Ph)] in the presence of Me3SnH. Reaction of [Cp2NbH(SnMe3)2] with two equivalents of Cl2SnMe2 affords the bis(tin)-substituted complex [Cp2NbH(SnClMe2)2] (9). The possible presence of non-classical hypervalent H···Sn-Cl interactions in this compound has been assessed by means of X-ray structure analysis. The structural data for 9 are in accord with an essentially classical description as a tin-substituted dihydride complex without significant Sn···H interactions.
    Additional Material: 1 Ill.
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