摘要:

AbstractThe α‐D‐allo‐diol9possesses an intramolecular H‐bond (HOC(3) to OC(1)) in solution and in the solid state (Fig. 2). In solution, it exists as a mixture of the tautomers9aand9b(Fig. 3), which possess a bifurcated H‐bond, connecting HOC(2) with both OC(1) and OC(3). In addition,9apossesses the same intramolecular H‐bond as in the solid state, while9bis characterized by an intramolecular H‐bond between HOC(3) and OC(4). In solution, the β‐D‐anomer12is also a mixture of tautomers,12aand presumably a dimer. The H‐bonding in9and12is evidenced by their IR and1H‐NMR spectra and by a comparison with those of3–8, 10, and11. The expected regioselectivity of glycosidation of9and12by the diazirine1or the trichloroacetimidate2is discussed on the basis of the relative degree of acidity/nucleophilicity of individual OH groups, as governed by H‐bonding. Additional factors determining the regioselectivity of glycosidation by1are the direction of carbene approach/proton transfer by H‐bonded OH groups, and the stereoelectronic control of both the proton transfer to the alkoxy‐alkyl carbene (in the σ‐plane) and the combination of the thereby formed ions (π‐plane of the oxycarbenium ion). Glycosidation of9by the diazirine1or the trichloroacetimidate2proceeded in good yields (75–94%) and with high regioselectivity. Glycosidation of9and12by1or2gave mixtures of the disaccharides14–17and18–21, respectively (Scheme 2). As expected, glycosidation of12by1or by2gave a nearly 1:1 mixture of regioisomers and a slight preference for the β‐D‐anomers (Table 4). Glycosidation of the α‐D‐anomer9gave mostly the 1,3‐linked disaccharides16and17(α‐Dβ‐D) along with the 1,2‐linked disaccharides14and15(α‐D<β‐D, 1,2‐/1,3‐linked glycosidesca.1:4), except in THF and at low temperature, where the β‐D‐configurated 1,2‐linked disaccharide15is predominantly formed. Similarly, glycosidation of9with2yielded mainly the 1,3‐linked disaccharides (1,2‐/1,3‐linked productsca.1:3 and α‐D/β‐Dca.1:4). Yields and selectivity depend upon the solvent and the temperature. The regioselectivity and the unexpected stereoselectivity of the glycosidation of9by1evidences the combined effect of the above mentioned factors, which also explain the lack of regio‐complementarity in the glycosidation of9by1and by2(Scheme 3). THF solvates the intermediate oxycarbenium ion, as evidenced by the strong influence of this solvent on the regio‐ and stereoselectivity, particularly at low temperature

ISSN:0018-019X    

DOI:10.1002/hlca.19940770133

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractThe synthesis of four electropolymerizable 2,2′‐bipyridinium salts with tuned reduction potential (E1∘) is described (N,N′‐ethylene‐4‐methyl‐4′‐vinyl‐2,2′‐bipyridinium dibromide (4;E1∘–0.48 V), 4‐methyl‐N,N′‐(trimethylene)‐4′‐vinyl‐2,2′‐bipyridinium dibromide (5;E1∘ −0.66 V),N,N′‐ethylene‐4‐methyl‐4′‐[2‐(1H‐pyrrol‐1‐yl)ethyl]‐2, 2′‐bipyridinium bis(hexafluorophosphate) (6b;E1∘ −0.46 V), and 4‐methyl‐4′‐[2‐(1H‐pyrrol‐1‐yl)ethyl]‐N,N′‐(trimethylene)‐2,2′‐bipyridinium bis(hexafluorophosphate) (7b;E1∘ −0.66 V)).E1∘‐Tuning is based on the torsional angle C(3)–C(2)–C(2′)–C(3′), imposed by theN,N′‐ethylene andN,N′‐(trimethylene) bridge. The vinylic compounds4and5undergo cathodic, the pyrrole derivatives6band7banodic electropolymerization on glassy carbon electrodes from MeCN solutions, yielding thin, surface‐confined films with surface concentrations of redox‐active material in the range 5 · 10−9<Γ<2.10−8mol/cm2, depending on experimental conditions. The modified electrodes exhibit reversible ‘diquat’ electrochemistry in pure solvent/electrolyte. Copolymerization of6bor7bwith pyrrole yields most stable electrodes. Bi ayer‐film‐modified electrodes were prepared by sequential electropolymerization of the monomers. The assembly electrode/poly‐6b/poly‐7bbehaves as a switch, it transforms – as aSchmitttrigger – an analog input signal (the electrode potential) into a digital output signal (redox state of the outer polymer film). Forward‐(electrode/poly‐7b/poly‐6b) and reverse‐biased assemblies (electrode/poly‐6b/poly‐7b) were coupled to the electrochemical reduction of redox‐active solution species,e.g. N‐ (cyanomethyl)‐N′‐methyl‐4,4′‐bipyridinium bis(hexafluorophospha

ISSN:0018-019X    

DOI:10.1002/hlca.19940770134

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractSingle crystals of 1‐[2,4,6‐tri(tert‐butyl)phenyl]‐2‐phenylphosphaethene (PPPE) and of2D‐ and13C‐enrichedPPPEwere studied by ESR after X‐ray irradiation. Two phosphorus‐centered radicals were trapped in the crystals. The first one was characterized by its31P,1H‐, and13C‐hyperfine tensors, the second one exhibited coupling with31P only. Comparison of these parameters with those predicted byab initiocalculations on some phosphinyl species indicates that these two radicals probably result, on the one hand, from an addition of a H‐atom to the C‐atom of the PC bond and, on the other, from a cyclization of the parent molecule. The proposed mechanisms are consistent with the mutual orientations of the hyperfine eigenvectors and bond directions in the undamaged molecule. A C‐centered radical which results from an addition of a H‐atom to the P‐atom of the phos

ISSN:0018-019X    

DOI:10.1002/hlca.19940770135

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractNMR, potentiometric, and UV/VIS measurements were run to study the protonation and the In3+and Cu2+stability constants of 1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetic acid (do3a, L). The protonation of do3a follows the typical scheme with two high and several low logKHvalues. Between pH 11 and 13, the protonation mainly occurs at the N‐atom, which is not substituted by an acetate side chain. The In3+complex is not appreciably protonated even at low pH values (pH ⋐ 1.7), whereas [CuL] can add up to three protons in acidic solution to give the species [CuLH], [CuLH2], and [CuLH3], the stability of which was determined. The formation rates of the Y3+, Gd3+, Ga3+, and In3+complexes with do3a were measured using a pH‐stat technique, whereas that of Cu2+, being faster, was followed on a stopped‐flow spectrophotometer. In all cases, the reaction scheme implies the rapid formation of partially protonated intermediates, which rearrange themselves to the final product in the rate‐determining process. ([MLH])in, an intermediate, in which the metal ion probably is coordinated by two amino acetate groups, proved to be the reactive species for Y3+, Gd3+, and Ga3+. The formation of [Cu(do3a)] was interpreted by postulating that either ([CuLH])inor ([CuLH])in, and ([CuLH2])inare the reactive complexes. The rates of dissociation of the Y3+, Gd3+, and Cu2+complexes with do3a were studied spectrophotometrically. For Y3+and Gd3+, arsenazo III was used as a scavenger, whereas for Cu2+the absorption associated with d‐d* transition was followed. For [Y(do3a)] and [Gd(do3a)], the rate law follows the kinetic expressionkobsdk0+k1[H+]. The dissociation of [Cu(do3a)]goes through the proton‐independent dissociation of [CuLH3], which is the m

ISSN:0018-019X    

DOI:10.1002/hlca.19940770136

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractOligoribonucleotides containing isoguanosine (  1,2‐dihydro‐2‐oxoadenosine; isoG;1) were prepared. The building block2was synthesized using the (dimethylamino)methylidene residue as NH2protecting group. The monomethoxytrityl as well as dimethoxytrityl group were introduced at OH–C(5′) (→5and6). Silylation of5with triisopropylsilyl chloride formed the 2′‐O‐blocked derivative7almost exclusively. Reaction with PCl3/1,2,4‐1H‐triazole furnished the phosphonate2which was used in solid‐phase synthesis of the oligoribonucleotides10and11. RNAse T1hydrolyzed U‐A‐G‐U‐U‐isoG‐U‐U‐A‐G (10) at the 3′‐site of G but not of isoG. The self‐complementary oligomer (A‐U‐isoG‐U)3(11) fo

ISSN:0018-019X    

DOI:10.1002/hlca.19940770137

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractThe preparation of a macrocyclic diphosphine ligand5incorporating a binaphthyl unit and its NiIIand PdIIcomplexes,6and7, is described. A complete assignment of1H,‐13C‐, and31P‐NMR data on the basis of 2D NMR spectroscopy (1H,1H‐,13C,1H‐,31P,1H‐,31P,13C‐COSY) was possible for5, and also – in

ISSN:0018-019X    

DOI:10.1002/hlca.19940770138

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

ISSN:0018-019X    

DOI:10.1002/hlca.19940770139

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

ISSN:0018-019X    

DOI:10.1002/hlca.19940770140

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

ISSN:0018-019X    

DOI:10.1002/hlca.19940770101

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY