ISSN:0018-019X    

DOI:10.1002/hlca.19980810312

出版商:WILEY‐VCH Verlag GmbH    

年代:1998

数据来源: WILEY

摘要:

Abstract‘Fully chiral’ dendrimers, containing a stereogenic center at each and every branching point, have been prepared using a chiral core triol with aromatic elongating units (cf.27) and chiral branch diols (cf.8,12, and24) as building blocks. The biggest dendrimer prepared is of the 4th generation (33: 46 building blocks, 93 stereogenic centers, 1028possible stereoisomers), and has been obtained by a convergent growth approach in 32 steps starting from the biopolymer poly[(R)‐3‐hydroxybutanoic acid] (P(3‐HB)). All compounds were shown to be monodisperse by MALDI‐TOF mass spectrometry. Spin‐lattice relaxation‐time (T1) measurements and size‐exclusion chromatography show typical features of structurally related achiral dendrimers. The influence of the chiral building blocks on the shape of the whole dendrimer has been investigated by chiroptical measurements: the specific rotation can be considered as average of all chiroptical properties of its constituent chiral units, independent of the solvent, the concentration, and the temperature. On the other hand, regularity in the circular dichroism (CD) spectra is completely lost with variation of the solv

ISSN:0018-019X    

DOI:10.1002/hlca.19980810313

出版商:WILEY‐VCH Verlag GmbH    

年代:1998

数据来源: WILEY

摘要:

AbstractThe synthesis of new ‘bridged’ β‐cyclodextrin (β‐CD) ‘dimers’7–12was successfully achieved by two one‐pot reactions from β‐CD(3)and 6A‐azido‐6A‐deoxy‐β‐CD(4). The ‘phosphine imine’ reaction was shown to be a superior approach compared to theMitsunobureaction as coupling strategy for the preparation of these ‘dimers’. NMR Data, along with molecular‐modelling calculations, suggest a ‘helical‐like’ arrangement for the phenanthroline‐diyl‐linked ‘dimer’ derivative9. Complexation properties of9were established by UV‐VIS‐spectrophotometric titration toward four metals. Among them CuIIor EuIIIions were complexed selectively by9, but no complexation occurred with LaIIIand ZnII. In addition a specific and interesting esterase activity toward the phosphodiester bond of bis(4‐nitrophenyl)

ISSN:0018-019X    

DOI:10.1002/hlca.19980810314

出版商:WILEY‐VCH Verlag GmbH    

年代:1998

数据来源: WILEY

摘要:

AbstractA novel general synthesis of 2,4,5‐tri‐ and 2,4,5,6‐tetrasubstituted pyrimidines5a–dand7a, e, f, gby condensation of thiouronium salts of type3with (ethoxymethylidene)malononitrile(4)and [bis(methylthio)methylidene]malononitrile(6), respectively, was first established in solution (Scheme 1) and successfully transferred onto solid support by using the polymer‐bound thiouronium salt11(Scheme 3). Further investigations were directed toward a multidirectional cleavage procedure of the 2‐(alkylsulfinyl) intermediates, obtained from the 2‐(alkylthio)pyrimidines7a(Scheme 2) or12and14(Schemes 3and4), with different nucleophiles to form highly substituted pyrimidines. In addition, fused‐heterocycle derivatives22a‐‐h, 24a–c, and26a–ewere generated in good‐to‐excellent yields by condensation of7a, e, hwith versatile isocyanates and isothiocyanates, with subse

ISSN:0018-019X    

DOI:10.1002/hlca.19980810315

出版商:WILEY‐VCH Verlag GmbH    

年代:1998

数据来源: WILEY

摘要:

AbstractIntroduction of the non‐complexing chiral (S)‐2‐methylbutoxy substituent close to the complexing site of a bis[(bipyridinyl)methoxy]calixarene podand resulted in the induction of an enantiomeric excess ofca.30 % in the corresponding chiral CuIcomplex. Structural investigations by high‐resolution NMR studies led us to propose the left‐handed prohelical [CuI(bpy)2] substructure for the major e

ISSN:0018-019X    

DOI:10.1002/hlca.19980810316

出版商:WILEY‐VCH Verlag GmbH    

年代:1998

数据来源: WILEY

摘要:

AbstractPrevious work has indicated orbital‐symmetry effects upon forward electron transfer inbimolecular systems, with magnitude similar to that encountered in rigidmonomolecular systems. The present work, which employs back electron transfer, supports and extends these earlier finding

ISSN:0018-019X    

DOI:10.1002/hlca.19980810317

出版商:WILEY‐VCH Verlag GmbH    

年代:1998

数据来源: WILEY

摘要:

AbstractThe new, axially chiral bis(4,5‐dihydrooxazoles)4have been synthesized in a straightforward manner, starting from the substituted, racemic 1,1′‐biphenyl‐2,2′‐dicarboxylic acids1and optically active amino alcohols2. The adducts were resolved by medium‐performance liquid chromatography (MPLC; seeScheme 1). Formation of Cu1complexes of4was followed by1H‐NMR spectroscopy. The catalytic behavior of these complexes has been investigated by asymmetric cyclopropanation of styrene with ethyl diazoacetate. Beside the influence of steric factors, a significant electronic effect on asymmetric induction could also be observed (seeSc

ISSN:0018-019X    

DOI:10.1002/hlca.19980810318

出版商:WILEY‐VCH Verlag GmbH    

年代:1998

数据来源: WILEY

摘要:

AbstractThe title compounds D‐10and L‐10were prepared from1in eight steps and in a combined overall yield of 41

ISSN:0018-019X    

DOI:10.1002/hlca.19980810319

出版商:WILEY‐VCH Verlag GmbH    

年代:1998

数据来源: WILEY

摘要:

AbstractAccording toab‐initiocalculations, the CF…︁HO H‐bond in 1,3‐diaxial 3‐fluorocyclohexanol is characterized byd(F…︁H) = 2.08 Å, and ∠ (F…︁HO) = 138°, and by ΔEbetween 1.2 and 4.1 kcal/mol, depending upon the reference system. Relative to the OH stretching frequency of axial cyclohexanol, the OH stretching frequency of 1,3‐diaxial 3‐fluorocyclohexanol is shifted by Δω = 7 cm−1. The rigid fluoro diols D‐4and L‐4were prepared from tetrahydroxy‐p‐benzoquinone in 11 steps and 1% overall yield. The IR spectrum of4in CCl4soln. is characterized by Δν = 7 cm−1for the axial and Δν = 44 cm−1for the equatorial OH group. A relatively strong intramolecular CF…︁HO bond of4in CCl4is evidenced by the large through‐space coupling5J(F,HO) of 9.3 Hz. Nevertheless, this FHO bond is disrupted in ethereal solvents, while the bifurcated O…︁HO bond subsists. In CCl4, the carbene generated from the glucosylidene‐derived diazirine5reacted more rapidly with the axial OH group of D‐4and L‐4than with the equatorial one. This regioselectivity is in keeping with the weaker H‐bond of the axial OH group. The regioselectivity is lower in ethers, but its solvent dependence does not parallel solvent basicity. This is not satisfactorily explained by the differential interaction of the ether solvents with the axial and equatorial OH groups, as evidenced by the solvent dependence of their chemical shift, but must also reflect the different interaction of the solvents with the carbene derived from5, leading to ylides. The lower solvent dependence of the anomeric selectivity for the glycosidation of the equatorial OH group is a consequence of the coordination of the intermediate oxycarbenium ion with OC(1) and OC(3) rather than with the solvent. Under conditions of competitive glycosylation in CCl4, the fluoro alcohol D‐22reacted mor

ISSN:0018-019X    

DOI:10.1002/hlca.19980810320

出版商:WILEY‐VCH Verlag GmbH    

年代:1998

数据来源: WILEY

摘要:

AbstractThe reaction ofN,N'‐dimethyl‐2‐nitroethene‐1,1‐diamine(8)with α,ß‐unsaturated acyl isothiocyanates9affords 3,3‐diamino‐2‐nitroacrylthioamides10(Scheme 2) in moderate‐to‐good yields. Cyclization of10under acidic conditions gives 1,3‐thiazin‐4‐one derivatives of type11. Oxidative cyclization of10with diethyl azodicaboxylate leads to 4‐nitro‐1,2

ISSN:0018-019X    

DOI:10.1002/hlca.19980810321

出版商:WILEY‐VCH Verlag GmbH    

年代:1998

数据来源: WILEY