摘要:

AbstractThe recently developed FAB/MSMS methodology (i.e.) ionization of an underivatized peptide by using fast‐atom‐bombardment (FAB) combined with tandem mass spectrometry (MSMS) is applied for the sequencing of the cyclotetrapeptide tentoxin (12). The scope and limitation of the strategy is discussed in detail. Possible resolutions to overcome problems related to (i) the resolution of isobaric fragment ions and (ii) the distinction of sequencevs. retro‐sequence are investigated. The novel strategy is compared with conventional techniques. Significant improvement of the presently used FAB/MSMS methodology can be achieved by combining this approach with accurate mass measure

ISSN:0018-019X    

DOI:10.1002/hlca.19870700302

出版商:WILEY‐VCH Verlag GmbH    

年代:1987

数据来源: WILEY

摘要:

AbstractThe tetradentate ligand 2,2′,2″‐nitrilotriphenol forms a boron complexIIIwith an intramolecular, transannular N→B dative bond of 1.68 Å in a strained tricyclo[3.3.3.0]undecane chelating system. The complex reacts with nitrogen bases L, such as pyridine, quinuclidine and others, to form complexesIII‐L, in which the intramolecular B–N bond is replaced by one betweenBand the external nucleophile. In solution, this displacement reaction is reversible. It was studied by temperature‐dependent NMR spectroscopy. The resulting reaction and activation parameters suggest that the reaction is a bimolecular nucleophilic subs

ISSN:0018-019X    

DOI:10.1002/hlca.19870700303

出版商:WILEY‐VCH Verlag GmbH    

年代:1987

数据来源: WILEY

摘要:

AbstractCrystal and molecular structures of a complex betweenB(III) and the tripod ligand 2,2′,2″‐nitrilotriphenol as well as of its pyridine and quinuclidine adducts are presented. The 2,2′,2″‐nitrilotriphenyl borate (III) molecule shows a strained, tricyclic chelate system with a centralN–Bdonor‐acceptor bond of 1.681(5) Å. In the adducts with pyridine (IV) and quinuclidine (III‐quin), this bond is broken, the N‐atom inverts and is pushed out of the coordination sphere (B–Nintern= 2.82 Å for Py and 2.85 Å for quin), and a new bond is formed between the boron atom and the external nucleophile (B–Nextern= 1.631 Å f

ISSN:0018-019X    

DOI:10.1002/hlca.19870700304

出版商:WILEY‐VCH Verlag GmbH    

年代:1987

数据来源: WILEY

摘要:

AbstractCrystal and molecular structures of three Al(III) complexes of the tripod ligand 2,2′,2″‐nitrilotriphenolate (I) are presented. They all show 5‐coordinate Al in approximately trigonal bipyramidal geometry, with an external nucleophile X occupying the second axial position. X is OH−in[Al(I)(OH)]−[Hquin]+(quin = quinuclidine), N in [Al(I)(py)] (py = pyridine), and one of the O‐atoms of a second molecule in the dimeric [(Al(I))2]. Correlated variations in the axial bond lengths of the trigonal bipyramid are observed: [(Al(I))2]: Al–Nint.= 2.094 Å, Al–Oext.= 1.850 Å; [Al(I)(py)]: Al–Nint.= 2.153 Å, Al–Next., = 1.992 Å; [Al(I)(OH)]−: Al–Nint.= 2.278 Å, Al–Oext.= 1.765 Å. They are interpreted in terms of a dissociative re

ISSN:0018-019X    

DOI:10.1002/hlca.19870700305

出版商:WILEY‐VCH Verlag GmbH    

年代:1987

数据来源: WILEY

摘要:

AbstractThe configuration of the spiro centre in polycyclic compounds containing a spiro[4.4]nonane substructure is analyzed in terms of symmetry coordinates. As revealed by X‐ray structures of such compounds, the local distortions around the quaternary C‐atom are dominated by a decrease of the ring‐bond angles at the spiro centre and a twist of one ring relative to the other. The planoid deformations given by the planarization indexPCare comparatively small for spiro[4.4]nonanes, which contain either no additional bridges between the rings or only α,β′–or β,β′‐polymethylene bridges. This indicates that the local strain in the spiro C(C)4fragment

ISSN:0018-019X    

DOI:10.1002/hlca.19870700306

出版商:WILEY‐VCH Verlag GmbH    

年代:1987

数据来源: WILEY

摘要:

AbstractStructures of α,α′‐bridged spiro[4.4]nonanes and [5.5.5.5]fenestranes, obtained from X‐ray analysis and MNDO calculations, are investigated in terms of symmetry‐deformation coordinates. The central C(C)4fragments of these molecules show strong preference for compression, which is due to opening of opposite bond angles. The planoid deformations and strain energies of the fenestranes are

ISSN:0018-019X    

DOI:10.1002/hlca.19870700307

出版商:WILEY‐VCH Verlag GmbH    

年代:1987

数据来源: WILEY

摘要:

AbstractN‐Acylnitroso derivatives6which were prepared byin‐situoxidation of the corresponding hydroxamic acids5reacted instantaneously and in high yields with dihydropyridine4. TheDiels‐Alderadducts8were formed regiospecifically with the acylnitroso dienophiles6a–c, whereas the dienophiles6d–fgave mixtures of both regioisomers7and8. These and some other results [2] were best explained by the FMO theory. TheDiels‐Alderadducts7and8gave the corresponding ‘anti’‐cis‐glycols when reacted with OsO4/N‐methylmorpholineN‐oxide. Hydrogenolysis of the N–O bond followed by peracetylation led to the expected aminolyxose derivatives14and16. A similar sequence, using4and the hydroxamic‐acid derivative18of (+)‐D‐mandelic acid led, with a poor asymmetric induction, to a mixture of the expected optically acti

ISSN:0018-019X    

DOI:10.1002/hlca.19870700308

出版商:WILEY‐VCH Verlag GmbH    

年代:1987

数据来源: WILEY

摘要:

AbstractThe phytochemical investigation of the dried fruits ofSwartzia madagascariensis(Leguminosae) afforded the five triterpenoid saponins1–5. They were shown to be glucuronides of oleanolic acid and of gypsogenin by chemical and spectral means (FAB‐MS,13C‐NMR,GC/MS). One of the isolated compounds, identified as3‐O‐[O‐α‐L‐rhamnopyranosyl‐(1→3)‐(β‐D‐glucopyranosyluronic acid)]oleanolic acid, was responsible for the high molluscicidal activity ofSwartzia madagascariensisfruits against the schistosomiasis‐transmitting snailsBiomphalari

ISSN:0018-019X    

DOI:10.1002/hlca.19870700309

出版商:WILEY‐VCH Verlag GmbH    

年代:1987

数据来源: WILEY

摘要:

AbstractCatalysts are obtained by exchange of the interstitial cations in the K10 montmorillonite. They are applied toFriedel‐Craftsalkylations with halides, alcohols, and olefins. They are quite effective even with unactivated hydrocarbons. Isomer distribution depends little on the catalyst used. Thermodynamic equilibration does not take place, the reactions appear to be kinetically controlled. Efficiency of the catalysts bears no apparent relation to that of the correspondingLewisacids under homogeneous conditions, and it depends on the nature of the alkylating agent. Zr(IV) and Ti(IV), in general, give the best result

ISSN:0018-019X    

DOI:10.1002/hlca.19870700310

出版商:WILEY‐VCH Verlag GmbH    

年代:1987

数据来源: WILEY

摘要:

AbstractThe title ester5is shown to undergo C–C bond cleavage under the conditions of basic ester hydrolysis (KOH/EtOH) with formation of potassium ethyl carbonate (6) and the tautomeric methylcyclopentadienes7and8. In contrast, porcine liver esterase (PLE,EC 3.1.1.1) cleanly hydrolyses5to give the isolable 1‐methyl‐2,4‐cyclopentadiene‐1‐carboxylic acid (13). The latter undergoes thermal dimerization with conservation of the geminal‐substitution pattern. The configuration of theDiels‐Alderadduct17is ascertained by it photochemical transformation into bishomocubane dicarboxylic acid12, easily distinguished by itsC2symmetry. Under the conditions of acid‐catalyzed hydrolysis, dimerization of ester5and polymerization prevail, unless low acid concentration is used. The dimer9of5has one ester function that is reluctant to undergo

ISSN:0018-019X    

DOI:10.1002/hlca.19870700311

出版商:WILEY‐VCH Verlag GmbH    

年代:1987

数据来源: WILEY