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  • 1
    Electronic Resource
    Electronic Resource
    Polymer- and Dendrimer-Bound Ti-TADDOLates in Catalytic (and Stoichiometric) Enantioselective Reactions: Are pentacoordinate cationic Ti complexes the catalytically active species? (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1710-1740 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: α,α,α′,α′-Tetraaryl-1,3-dioxolane-4,5-dimethanols (TADDOLs), containing styryl groups either at C(2) of the heterocyclic ring or in the α-position, were prepared in the usual way (18-22, 24, 25). These compounds were copolymerized with styrene and divinylbenzene in a suspension, yielding polymers (33-40, Scheme 3) as beads with a rather uniform particle-size distribution (150-45 μm), swellable in common organic solvents. HOCH2- and BrCH2-substituted TADDOLs were also prepared and used for attachement to Merrifield resin or to dendritic molecules (23, 26-32). The TADDOL moieties in these materials are accessible to form Ti (and Al) complexes (Scheme 4) which can be used as polymer- or dendrimer-bound reagents (stoichiometric) or Lewis acids (catalytic). The reactions studied with these new chiral auxiliaries are: enantioselective nucleophilic additions to aldehydes (of R2Zn and RTi(OCHMe2)3; Scheme 5, Table 1) and to ketones (of LiAlH4, Table 2); enantioselective ring opening of meso-anhydrides (Scheme 6); [4+2] and [3+2] cycloadditions of 3-crotonyl-1,3-oxazolidin-2-one to cyclopentadiene and to (Z)-N-benzylidenephenylamine N-oxide ( → 48, 49, Scheme 7, Tables 3, 4, and Fig. 5). The enantioselectivities reached with most of the polymer-bound or dendritic TADDOL ligands were comparable or identical to those observed with the soluble analogs. The activity of the polymer-bound Lewis acids was only slightly reduced as compared with that encountered under homogeneous conditions. Multiple use of the beads (up to 10 times), without decreased performance, has been demonstrated (Figs. 3 and 4). The poorer selectivity in the Diels-Alder reaction (Scheme 7a), induced by the polymer-bound Cl2Ti-TADDOLate as compared to the soluble one, is taken as an opportunity to discuss the mechanism of this Lewis-acid catalysis, and to propose a cationic, trigonal-bipyramidal complex as the catalytically active species (Fig. 6). It is suggested that similar cations may be involved in other Ti-TADDOLate-mediated reactions as well.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790621
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  • 2
    Electronic Resource
    Electronic Resource
    Preparation and reactions of silyl nitronates derived from 2,2,2-trifluoronitroethane. Diastereoselective synthesis of trifluoromethyl-substituted aminoethanols and -propanols (1995)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1995 (1995), S. 1193-1215 
    Details
    ISSN: 0947-3440
    Keywords: Silyl nitronates, trifluoromethyl-substituted, NMR studies of ; Nitroalkanes, fluorinated, α,α-doubly deprotonated ; Nitroaldol additions, diastereoselective ; [3 + 2] Cycloadditions ; Isoxazolidines, trifluoromethyl-substituted, conformational studies of ; Aminoethanols and -propanols, trifluoromethyl-substituted, X-ray studies of ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Silyl nitronates of 2,2,2-trifluoronitroethane and of 1,1,1-trifluoro-2-nitropropane [SiMe3, SiMe2(tBu), SiMe2CMe2-CHME2] were prepared and their reactions studied. - The TBDMS nitronate of the nitroethane is present at the (E) and (Z) isomer (2.3:1 at room temperature; EA = 9.3 kcal mol-1 for their isomerization). BU4NF-catalyzed silyl nitro aldol additions to aldehydes show reasonable selectivities for the formation of the syn diastereoisomers of 5-13 (a. 4:1; with the non-fluorinated analogs anti isomers prevailing!). The anti epimers (5-9, 12, 13, 17-20) in the trifluoromethyl series can be prepared by diastereoselective protonation of the corresponding O-silyl lithium nitronates (ca. 9:1, Scheme 3). Careful reduction of the NO2 groups (H2/Raney nickel) gives trifluoroamino alcohol derivatives (22-30, Scheme 4). -The trimethylsilyl nitronates are also used for 1,3-dipolar cycloadditions (to 1-hexene, styrene, methyl vinyl ketone, and acrylates) to give N-(silyloxy)isoxazolidines (2-40; Scheme 5). The configuration of the major products (2,3-cis, 3,5-trans) was derived from NMR measurements, X-ray structure determinations, and thermal equilibration studies. The major course of reactions was thus derived to be an exo approach of the dipolarophiles to the (Z)-silyl nitronates (Scheme 8).- Some conversions of the N-(silyloxy)isoxazolidines (treatment with acid, base, and Raney nickel) are also described (products 42-51).
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jlac.1995199507162
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