摘要:

AbstractChiral triols1–3(‘tris(hydroxymethyl)methane’ derivatives), prepared from (R)‐3‐hydroxybutanoic acid and aldehydes, are used as center pieces of dendrimers. The triols may be employed as such or after attachment of spacers containing alkyl or aryl moieties (see5and7). The branches combined with the original or elongated triols are those first reported byFréchet(9–12, benzyl ethers of 3,5‐dihydroxybenzyl alcohol and bromide). In this way, 1st‐, 2nd‐, and 3rd‐generation chiral dendrimers without (13–15), or with aliphatic (16–18) or aromatic (19–21) spacers are prepared. The molecular weights range from 447 to 2716 Dalton. Two of the chiral triols,i.e.,2and3, are used as center pieces for chiral dendrimers containing 6 NH2, or 6 and 12 NO2groups on the periphery (22–27), with 3,5‐dinitrobenzoyl chloride as the branching unit. All compounds thus synthesized are of course monodisperse and are fully characterized. In some cases, the optical activity of the dendrimers indicates that conformationally chiral substructures might be present. The NH2‐ and NO2‐substituted compounds avidly clathrate smaller molecules; they are sorbents exchanging ho

ISSN:0018-019X    

DOI:10.1002/hlca.19940770702

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractHPLC Separation of higher fullerenes was compared on two different stationary phases, and the preparative isolation of pure C76is described. Higher‐fullerene derivatives1and2were prepared byDiels‐Alderreaction of C70and C76with anortho‐quinodimethane intermediate generatedin situ. Three out of four possible isomeric C70monoadducts,i.e.1a–c, and, for the first time, one isomeric C76monoadduct,i.e.2c, could be isolated in pure form and characterized by1H‐NMR,13C‐NMR, UV/VIS, and mass spectrometry. Five other C76isomersi.e.,2a,b,d–fwere obtained in partially separated product fractions. Coalescence temperatures and energy barriers were determined for the cyclohexene‐ring inversion in two of the isomeric C70derivatives. The structure of the C70adducts could be deduced unambiguously from symmetry considerations based on high‐ and low‐temperature1H‐NMR spectroscopy. A simple model for the qualitative evaluation of the local curvature of fullerene surfaces is presented and used for the prediction of addition sites in higher fullerenes. These predictions are compared to the experimental results mentioned above as well as to predictions resulting from π‐bond‐order considerations and from calculat

ISSN:0018-019X    

DOI:10.1002/hlca.19940770703

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractSynthesis of optically active sesquiterpenes with a eudesmane C‐skeleton from the chiral starting material thujone involves transformation of a tricyclic intermediate (1R,2R,4S)‐1,7‐dimethyl‐4‐(1‐methylethyl)tricyclo[4.4.0.02,4]dec‐6‐en‐8‐one (2) into the bicyclic compound β ‐cyperone (5). Hydroxylation of2at C(5) or C(11) permits subsequent opening of the cyclopropane ring and rearrangement to β ‐cyperone. In this publication, studies involving hydroxylation of2by fungal cultures are presented. The resultant products are useful intermediates in efficient synthesis of eudesmane sesquiterpenes. Of five fungi tested,Rhizopus oryzaeATCC 11145 proved most versatile. It hydroxylates at the exocyclic C(11) position in high yield (70%) and, to a lesser extent, at C(5) (5%). Enzymatic activity appears at the end of growth phase and at least 2.2 g of2per liter can be metabolized without significant loss of product yield. A second fungus,Cunninghamella echinulataATCC 9244, proved most useful for hydroxylation of derivatives of2for the preparation of derivatives of β ‐cyperone, although product yields were low (2–20%), some derivatives were nonreactive, and hydroxylation at C(9) occurred. The relationship between precursor structure an

ISSN:0018-019X    

DOI:10.1002/hlca.19940770704

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractIncubation of the synthetic18O‐labelled phytyl‐hydroquinone (O4‐18O)‐2with the tocopherol cyclase fromAnabaena variabilisKÜTZING(Cyanobacteria) in D2O furnished the doubly labelled γ‐tocopherol, (2R,3S,4′R,8′R)‐(1‐18O,3‐2H)‐1. The chirality at C(3) was determined by two independent routes providing interlocking evidence that the enzyme‐catalyzed ring closure proceeds bysi‐protonation of the double bond of2and concomitantre‐

ISSN:0018-019X    

DOI:10.1002/hlca.19940770705

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractThe pyrophosphoric‐acid‐analogue phosphonoformic acid (pfa) and the amino‐acid‐analogue (aminomethyl)phosphonic acid (ampa) both form, in the deprotonated state,i.e., as–OOC–PO 32−and H2NCH2PO 32−, respectively, five‐membered chelate rings with metal ions. pfa inhibits both phosphate transport and virus replication, while ampa is a metabolic product of the common herbicide glyphosate ( N‐(phosphonomethyl)glycine). The acidity constants of H2pfa–and H2ampa± as well as the stability constants of the [M(Hpfa)], [M(pfa)]–, [M(Hampa)]+, and [M(ampa)] complexes, where M2+ Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Cu(2,2′‐bipyridyl)2+, Cu(1,10‐phenanthroline)2+, Zn2+, or Cd2+, have been determined by potentiometric pH titrations in aqueous solution at 25° andI 0.1M(NaNO3). The structures of isomeric complexes and the connected intramolecular equilibria are deduced and evaluated based on the equilibrium constants measured and those calculatedviathe pKavalues of the above mentioned ligands and previously established logK vs.pKastraight‐line plots (H. Sigel et al., Helv. Chim. Acta1992,75, 2634) for a simple phosphonate‐M2+coordination. pfa forms stronger complexes than ampa with all the above mentioned metal ions, with the single exception of [Cu(ampa)] which is slightly more stable than [Cu(pfa)]–. In neutral solutions, more precisely at pH ofca.6, pfa complexes of alkaline‐earth‐metal ions retain one phosphonate‐bound proton, [M(Hpfa)], while those of the transition‐metal ions chelate with the trianionic ligand, pfa3–. In accord with increasing ligand‐basicity, the stability‐constant order for all metal‐ion complexes is oxalate>pfa>pyrophosphate but, owing to proton competition in pyrophosphate, in neutral solutions metal‐ion complexation of pfa3–competes with P2O 74−. With ampa alkaline‐earth‐metal ions interact only with the phosphonate group of even the dianionic ligand (though Mg2+appears to form a low fraction of a [Mg(ampa)] chelate) while transition

ISSN:0018-019X    

DOI:10.1002/hlca.19940770706

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractThe antivirally active 3′‐deoxyadenylyl‐(2′–5′)‐3′‐deoxyadenylyl‐(2′–5′)‐3′‐deoxyadenosine (cordycepin trimer core) was modified at the 2′‐ or 5′‐terminus, by attachment of cholesterolviaa carbonate bond (→15) or a succinate linker (→16and27) to improve cell permeability. The corresponding monomeric conjugates4,7, and21of cordycepin were prepared as model substances to study the applicability of the anticipated protecting groups – the monomethoxytrityl (MeOTr), the (tert‐butyl)dimethylsilyl (tbds), and the β ‐eliminating 2‐(4‐nitrophenyl)ethyl (npe) and 2‐(4‐nitrophenyl)ethoxycarbonyl (npeoc) groups – for the final deblocking steps without harming the ester bonds of the conjugate trimers. The syntheses were performed in solution using phosphoramidite chemistry. The fully protected trimer conjugates13,14, and26as well as all intermediates were characterized by elemental analyses, UV and1H‐NMR spectra. The deblocked conjugates15,16, and27were pure according to HPLC and showed the correct compositions by mass spectra. Comparative biological studies indicated that cordycepincholesterol conjugate trimers16and27were 333‐ and 1000‐fold, respectively, more potent inhibito

ISSN:0018-019X    

DOI:10.1002/hlca.19940770707

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractThe structure of peregrine (1), a norditerpenoid alkaloid isolated fromDelphinium peregrinumvar.elongatumBOISS., was revised on the basis of the1H‐COSY, HMQC, HMBC, and ROESY NMR spectra and of the X‐ray analysis of its parent alcohol2. Some of the13C‐NMR resonances of1and the related alkaloids peregrine alcohol (2), 14‐O‐acetylperegrine (3), bicoloridine (4), bicoloridine alcohol (5), 6‐O‐acetylbicolorine (6), bicolorine (7), and 14‐O‐acetylbicolorine (8), wer

ISSN:0018-019X    

DOI:10.1002/hlca.19940770708

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractThe crystal structure of the Sb2F11salt of the 2‐phenyladamant‐2‐yl cation,1· Sb2F11, was determined at 183 K (P21/c,R1 = 0.0652, σ(CC) = 0.02 Å), because earlier published results indicated a charge delocalization from the cationic C(2) into the σ framework (CC hyperconjugation) and a bending of the C(2) bridge. In the structure of1, a displacement of the C(2) bridge by 7.8(12)° from the symmetrical position and CC bond‐length deviations from expectation values were found which are in agreement with preferential CC hyperconjugation on one face of C(2). The interactions of1with two Sb2F11counterions nearest to C(2) also indicate different behaviour of the two faces of C(2). The benzylic reson

ISSN:0018-019X    

DOI:10.1002/hlca.19940770709

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractThe bicyclic monoselenoacetal7, easily obtained from (±)‐7‐oxabicyclo[2.2.1]hept‐5‐en‐2‐one (6)viaa radical addition‐acyl migration sequence, was converted to racemic 12‐epiprostaglandins3and4. The key intermediate was the all‐cis‐formyllactone2brelated toCoreylactone (see12;Scheme 1). The presence of a (tert‐butyl)‐dimethylsilyl protective group for the 11‐OH substituent (prostaglandin numbering) was found to be crucial in avoidingβ ‐elimination and epimerization during theWittig‐Hornerreaction (Scheme 2). Epimerization at C(12) at the formyllactone stage (see2b) was also possible and gave the known precursor1bof naturally occur

ISSN:0018-019X    

DOI:10.1002/hlca.19940770710

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

Abstract(Hydroxymethyl)bilane synthase (HMBS) catalyses the conversion of porphobilinogen (2) into the (hydroxymethyl)bilane derivative3, a linear tetrapyrrolic intermediate in the biosynthesis of haem, chlorophyll, and related pigments. The conversion involves the sequential formation of four intermediate covalent enzyme‐substrate complexes, before the product is released. We analysed the pre‐steady‐state kinetics of the formation of the complexes, taking advantage of their remarkable chemical stability allowing chromatographic separation. The experimental approach involved the generation of the complexes while HMBS was immobilised on an anion‐exchange column. A solution being 0.2Kmin substrate was pumped through the column during a time interval which was varied to sample the pre‐steady‐state period. Then, the enzyme and enzyme‐substrate complexes were eluted and their proportions evaluated. A computer simulation of the pre‐steady‐state time course, in combination with a χ2fitting to the experimental data, allowed the specificity constantskcat/Kmfor the individual steps of the process to be derived. By repeating the analysis with variants of HMBS in which specific amino acids were replaced by others, we demonstrated that it is possible to trace the consequences of amino‐acid replacements down to the individual steps of the reaction sequence. Since the positions of the amino acids concerned in the three‐dimensional structure were known, detailed structure‐function relationships be

ISSN:0018-019X    

DOI:10.1002/hlca.19940770711

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY