摘要:

AbstractPreparation and screening of twenty new ligands, all analogs of α,α,α′,α′‐tetraaryl‐1,3‐dioxolane‐4,5‐dimethanol (TADDOL), for the Ti‐catalyzed asymmetric addition of methyltri(isopropoxy)titanium and diethylzinc to benzaldehyde are described. These ligands have the dioxolane ring of the TADDOL's replaced by cyclobutane, cyclopentane, cyclohexene, cyclohexane, bicyclo[2.2.1]heptene and ‐heptane and bicyclo[2.2.2]octene and ‐octane moieties; several have H‐atoms or alkyl groups in place of the aryl groups, and nine of them haveC2symmetry. X‐Ray crystallography and molecular mechanics are used to analyze the structure of the ligands, and two structural features appear to correlate with selectivity: (i) the torsion angle for the chelating O‐atom and theortho‐C‐atom of the axial Ph group (a small,ca.19°, angle is optimum,Fig.8) and (ii) the “degree of perpendicularity” of the axial Ph group (Fig. 9). Competition experiments indicate that TADDOL1acatalyzes both the methyltitanium and diethylzinc additions ≥ 50 times faster than the related dioxolane analogs12a, 12c, and12e(Scheme 7), indicating that both axial and equatorial aryl groups (see Footnote 6) are necessary for ligand‐accelerated catalysis of these reactions. A refined mechanistic hypothesis is presented (Fig. 10) to explain the selectivities observed for these new ligands. Our analysis suggests that a combination of structural features appear necessary for good catalytic efficiency and high selectivity. These features, especially the rather subtle conformational effects, appear to be optimized

ISSN:0018-019X    

DOI:10.1002/hlca.19940770802

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractEnantiomerically pureC2‐symmetric 1,4‐diols embodying bicyclic C‐frameworks were synthesized by means of asymmetriccarbo‐Diels‐Alderreactions as key steps (Scheme 1). They were investigated as chiral ligands in the enantioselective addition of ZnEt2to aromatic aldehydes. In the presence of 20–40 mol‐% of the titanates formed from these diols and [Ti(i‐PrO)4] at −78°, the respective 1‐arylpropanols were obtained with enantiomer ratios up to 93

ISSN:0018-019X    

DOI:10.1002/hlca.19940770803

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractIn a theoretical investigation of the structure and reactivity of indole derivatives of tricarbonylchromium(0), we have used two different semi‐empirical quantum chemical models derived from the extendedHückelmolecular‐orbital (EHMO) formalism. The first one, based on the atom‐superposition and electron‐delocalization (ASED) method, is used to optimize the geometry of the systems; it is shown to lead to results in satisfactory agreement with experiment in the case of the complex for which X‐ray structural data are available, the average errors being 0.03–0.05 Å for bond distances and 5° for bond angles. The second one consists of a local reactivity index, made of the intermolecular interaction energy between the organometallic substrate and a model reactant. It is seen that this procedure is able to reproduce the experimental trends as to the most reactive regions of the systems investigated towards nucleophilic and electrophilic addi

ISSN:0018-019X    

DOI:10.1002/hlca.19940770804

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractThe oxidative transformation of synthetic (+)‐aristoteline ((+)‐6) into other metabolites which had been isolated fromAristoteliaspecies was investigated. Thus, treatment of (+)‐6with I2as the single oxidant furnished the naturally occurring indole alkaloids (+)‐makonine ((+)‐9),(+)‐aristotelinone ((+)‐11), or (+)‐11, 12‐didehydroaristoteline ((+)‐7) in good yields, the selectivity of the oxidation process depending on the chosen

ISSN:0018-019X    

DOI:10.1002/hlca.19940770805

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

Abstract1,3‐Thiazole‐5(4H)‐thione oxides2were prepared by oxidation of the corresponding 1,3‐thiazole‐5(4H)‐thiones1withm‐chloroperbenzoic acid (Table 1). Addition reactions of2with organolithium andGrignardreagents yielded 4,5‐dihydro‐4,4‐dimethyl‐1,3‐thiazol‐5‐yl methyl sulfoxides of type4viathiophilic attack (Table 2). Whereas the reaction with the organolithium compounds proceeded with fair‐to‐excellent yields, theGrignardreagents reacted only very sluggishly. The sulfoxides4could also be preparedviaoxidation of 4,5‐dihydro‐4,4‐dimethyl‐5‐(methylthio)‐1,3‐thiazoles of type3withm‐chloroperbenzoic acid, toge

ISSN:0018-019X    

DOI:10.1002/hlca.19940770806

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractL‐Aspartic acid by successiveN‐tosylation, anhydride formation, and reduction was converted into (3S)‐3‐(tosylamino)butano‐4‐lactone (4). Electrophilic methylation of4, subsequent iodo‐esterification and nucleophilic methylation, followed by saponification and deprotection, gave (2S, 3R)‐3‐amino‐2‐methylpentanoic acid (2) with an ee of>99% in seven steps and in an

ISSN:0018-019X    

DOI:10.1002/hlca.19940770807

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractReaction of the 3‐silylated β‐lactams1with glycoxalates gives bis‐lactam3, but the same reaction in the presence of 1 equiv. of Me3SiCl leads to the formation of the disilylated adducts5. The latter is desilylated by (Bu4N)F yielding the monocarboxylates7of 3‐methylidene‐β‐lactams, which, with oxidizing agents, give the spiro compound8. The structure of8is established by spectroscopic data and a crystal‐str

ISSN:0018-019X    

DOI:10.1002/hlca.19940770808

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractBased on the natural abundance of13C,in vivo13C‐NMR was used for the first time to monitor the metabolism of sucrose and hydroquinone (1) in cell suspensions of the plantRauwolfia serpentina(L.) BENTH. ex KURZ. Cells converted sucrose extracellularly into α‐D‐ and β ‐D‐glucose as well as into β ‐D‐fructofuranose and β ‐D‐fructopyranose, respectively. The sugar mixture was completely taken up by the cells after 4 days. Hydroquinone fed at that time resulted in optimum conversion into its β ‐D‐glucoside arbutin (2) within 10 h. A further metabolite, the primeveroside (3) of hydroquinone, appeared as a trace compound after 10 h. The formation of this diglycoside can be increased by f

ISSN:0018-019X    

DOI:10.1002/hlca.19940770809

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

Single‐Crystal Structures of a Donor‐Acceptor‐Substituted Cyclopentadiene, Its Cation and Its Anion Dimer: Different Distortion of a Five‐Membered Carbon Ring1,4‐Dimethyl‐2,3,4,6‐tetrahydro‐1H‐cyclopentapyrazine‐5,7‐dicarbonitrile, its molecular cation, and its dimer dianion, generated by hydride abstraction or deprotonation, and crystallized, differ remarkably in their structures: the five‐membered ring in the neutral compound is of (Z)‐butadiene‐type, whereas, in the tetrafluorborate salt, a peripheric NCCCCCN⊕ cyanine distortion occurs and in the sodium derivatives, solvated by either one or two 1,2‐dimethoxyethane molecules, a cyclopentadienyl anion is formed. The structural perturbations induced by changing molecular charges are rationalized by extensive MNDO calculations, which on geometry optimization, reproduce the experimental structures and, in addition, provide charge distributions, further confirming thenN/π and π/π interactions already recogniza

ISSN:0018-019X    

DOI:10.1002/hlca.19940770810

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractThe readily available title compounds4aand24react with N‐, O‐, S‐, and C‐nucleophiles in presence of MnO2to give the corresponding mono‐ or disubstituted 2H‐azabenzimidazoles ( = azaisobenzimidazoles),e.g.,11–18and26a–h, respectively, or 2,3‐dihydro‐1H‐azabenzimidazoles ( = dihydro‐azabenzimidazoles) such as9and10and27and28, respectively, by a 1, 4‐ or 1,6‐Michaeladdition (Schemes 2and4). The bromo‐dihydro‐1H‐azabenzimidazole4blost the Br‐atom when treated with piperidine or morpholine yielding the corresponding disubstituted 2H‐azabenzimidazole21(Scheme 3). Reductive ring opening of the substituted spiro compounds leads to mono‐ and disubstituted diaminopyridines which are intermediates for fused pyridine ring systems with substituents often not available by conventional routes and of potential pharmaceutical interest (see32–37).E.g., starting from4a, a three‐step synthesis of the analgesic flupirtine maleate (= ethyl {2‐amino‐6‐[(4‐fluorobenzyl)amino]pyridin‐3‐yl}carbamate maleate =Katadolon®;39) and of its non‐fluorinated derivativeD‐7195is described

ISSN:0018-019X    

DOI:10.1002/hlca.19940770811

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY