摘要:

AbstractThe title compound is a condensation product of four guanidinium ions formed during crystallization of guanidinium nitrate. The crystal structure consists of infinite molecular ribbons stacked into layers which are rotated 58.5° with respect to each other and connected by two different networks of hydrogen bonds

ISSN:0018-019X    

DOI:10.1002/hlca.19940770602

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractThe title products are preparedviaan efficient three‐step synthesis which involves hydroalumination of methyl propiolate andLewis‐acid‐promoted reaction with acetone in the presence of BF3followed by two highly selectiveSN2′ reactions. The key step is the reaction of 2‐(chloromethyl)acrylates with R2CuLi/ZnCl2reagents which takes place with complete allylic rear

ISSN:0018-019X    

DOI:10.1002/hlca.19940770603

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractOligodeoxyribonucleotides containing 1‐deaza‐2′‐deoxyadenosine ( = 7‐amino‐3‐(2‐deoxy‐β‐D‐erythro‐pentofuranosyl)‐3H‐imidazo[4, 5‐b]pyridine;1b) formHoogsteenduplexes.Watson‐Crickbase pairs cannot be built up due to the absence of N(1). For these studies, oligonucleotide building blocks – the phosphonate3aand the phosphoramidite3b– were prepared from1bvia4aand5, as well as theFractosil‐linked6b, and used in solid‐phase synthesis. The applicability of variousN‐protecting groups (see4a–c) was also studied. TheHoogsteenduplex d[(c1A)20] · d(T20) (11 · 13;Tm15°) is less stable than d(A20) · d(T20) (12 · 13; Tm60°). The block oligomers d([c1A)10–;T10] (14) and d[T10–(c1A)10] (15) containing purine and pyrimidine bases in the same strand are also able to form duplexes with e

ISSN:0018-019X    

DOI:10.1002/hlca.19940770604

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractTheDiels‐Alderadduct of furan and 1‐cyanovinyl (1′R)‐camphanate was converted into methyl [(tert‐butyl)‐dimethylsilyl 5‐deoxy‐2, 3‐O‐isopropylidene‐β‐L‐ribo‐hexofuranosid] uronate ((+)‐4). Reduction with diisobutyl‐aluminium hydride gave the corresponding aldehyde which was condensed with the ylide derived from triphenyl‐(propyl)phosphonium bromide to give (1R, 2S, 3S, 4S)‐1‐[(tert‐butyl)dimethylsilyloxy]tetrahedro‐2, 3‐(isopropyl‐idenedioxy)‐4‐[(Z)‐pent‐2′ ‐enyl]furan ((+)‐7). Removal of the silyl protective group gave a mixture of the corresponding furanose that underwentWittigreaction with the ylide derived from [8‐(methoxycarbonyl)‐octyl]triphenylphosphonium bromide to yield methyl (11R, 12S, 13S, 9Z, 15Z)‐13‐hydroxy‐11, 12‐(isopropylidene‐dioxy)octadeca‐9, 15‐dienoate ((−)‐9). Acidic hydrolysis, then saponification affo

ISSN:0018-019X    

DOI:10.1002/hlca.19940770605

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

Structures and Molecular Properties of Charge‐Pertubed Molecules. 2, 3‐Diphenylquinoxaline Radical Anions in Solution and in CrystalsThe Na⊕ and K⊕ radical‐ion salts of 2, 3‐diphenylquinoxaline seem to be (according to a structural database search) among the first ones of N‐heterocyclic radical anions in crystals. The one‐electron reduction in aprotic 1, 2‐dimethoxyethan (DME) solution at metal mirrors and the crystallization under Ar have been preceded by cyclovoltammetric (CV) and ESR/ENDOR measurement. The first electron insertion at −1.63 V proves to be reversible, whereas the irreversible second step, which is accompanied by an overcrossing of the CV line, can be rationalized by an ‘ECE‐DISP’ mechanismviaa dianion redox disproportionation. The ENDOR spectrum resolves four1H couplings and allows to simulate the ESR spectrum including the14N hyperfine splittings. Both dark‐blue single crystals of the radical ion salts\documentclass{article}\pagestyle{empty}\begin{document}$[2,3{\rm - diphenylquinoxaline}^{{.} \ominus} {\rm Met}^ \oplus ({\rm DME})]^.$\end{document}show unexpected similarities for Met⊕ = Na⊕, K⊕ despite the 36‐pm difference in their ionic radii. The largest structural changes inflicted by the one‐electron reduction of the N‐heterocyclic molecule are observed in the vicinity of the N‐centers bearing the highest effective nuclear charge. The DME‐chelated metal cations coordinate at the N electron pairs and form Met⊕(DME)‐bridged polymer chains of the radical anion, which are differently ondulated in the Na⊕ and K⊕ radical anion salts. The take‐home lesson suggests that many more N‐heterocyclic molecules might be analogously reduced und

ISSN:0018-019X    

DOI:10.1002/hlca.19940770606

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractCyclo(‐D‐Leu‐L‐MeLeu‐D‐Leu‐L‐MeLeu‐D‐Leu‐L‐MeLeu‐) (1) and cyclo(‐L‐Leu‐D‐Leu‐L‐MeLeu‐D‐Leu‐L‐Leu‐D‐Leu‐) (2) were synthesized and used for a study of the interaction of β ‐rings in solution. In appropriate solvents,1and2formed dimers consisting of two β ‐rings connected through six interannular H‐bonds and having theN‐Me group(s) on the solvent‐exposed face. The study also afforded indications that2formed different dimers of this kind, differing in the relative orientation of the two β ‐rings. These experimental observations provide strong support to the idea that unsubstitutedD,L‐alternating cyclooligopeptides, such as cyclo(hexaleucine)

ISSN:0018-019X    

DOI:10.1002/hlca.19940770607

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractThe present work describes a new and efficient method for the preparation of either racemic or enantiomerically pure carbocyclic 2′‐deoxyribonucleosides1. Key steps are the efficient assembly of the racemic carbocyclic 2′‐deoxyribose core (±)‐12, its enzymatic resolution, and a new approach to covalently link the purine and pyrimidine bases with the cyclopentane moietyviathe cyclic sulfate (+)‐19. This total synthesis of enantiomerically pure and racemic carbocyclic 2′‐deoxyribonucleosides1represents one of the most efficient approaches reported to date. Starting from cyclopentadiene, the four carbocycles corresponding to the naturally occurring 2′‐deoxyribonucleosides could be prepared in 12 steps and 9–12% overall yield. For the corresponding racemic compounds, 10 steps were used with overall yie

ISSN:0018-019X    

DOI:10.1002/hlca.19940770608

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractThe excited electronic states of 2, 2‐dimethylisoindene (1) have been studied by electron‐energy‐loss spectroscopy. Its vertical gas‐phase triplet (13B2), and singlet (11B2) excitation energies are 1.61 and 3.19 eV, respectively. The excited states are thus lowered by 0.49 eV and 1.21 eV, respectively, when compared to the corresponding states of (all‐E)‐octatetraene, which serves as a reference compound. These shifts are partially reproduced by ZINDO calculations. The spectra give no evidence for a 21Agstate below the 11B2state, but this lack of observation does not exclude its existence. The lowest triplet state T1(1) was further characterized by flash photolysis. T1(1) was observed as a transient intermediate, λ ≤ 350 nm, with a lifetime of 8 m̈s in degassed hexane. The adiabatic excitation energy of T1(1) was bracketed to the range of 1.1 ± 0.1 eV by energy‐transfer experiments. Relationships between the energies of the lowest excited singlet and triplet states of1and the lowest excited doublet state of its radical cation\documentclass{article}\pagestyle{empty}\begin{document}${1}^{+\kern0pt {.}}$\end{document}– essentially a non‐Koopmans'

ISSN:0018-019X    

DOI:10.1002/hlca.19940770609

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractThe oxidation reactions of a series of quinoxaline derivatives, using KMnO4in the presence or absence of NaOH, are described. Neutral oxidation of 2‐chloro‐ and 2, 3‐dichloroquinoxalines2–4afforded the corresponding chloro‐ and dichloropyrazinedicarboxylic acids13and14in good yield. On the other hand, oxidation of quinoxalin‐2(1H)‐one and 1, 4‐dihydroquinoxaline‐2, 3‐dione derivatives in alkaline medium gave different products, with the quinoxalin‐2(1H)‐one (5) forming 1, 4‐dihydroquinoxaline‐2, 3‐dione (9), while various substituted quinoxalin‐2, 3‐dione derivatives (see9–11) gave a new type of dimeric products. The structural assignments for the new compoun

ISSN:0018-019X    

DOI:10.1002/hlca.19940770610

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY

摘要:

AbstractThe gas‐phase basic hydrolysis of clavulanic acid (a) was studied by using the AM1 semi‐empirical method. The results obtained show that the hydroxyethylidene side chain at C(2) is pivotal to the stability of the different reaction products involved. The products with an open oxazolidine ring are more stable than those with a closed ring fused to the β‐lactam ring. This behaviour differs from that of penicillins and cephalosporins where the most stable degradation products are those with an intact thiazolidine or dihydrothiazine ring, respectively, fused to the β‐lactam ring. The different chemical reactivity of clavulanic acid relative to penicillins and cephalosporins could explain the disparate behaviour of the latter two types of compound towards β‐lactamases. Once the acyl‐enzyme intermediate of clavulanic acid has been formed, it can evolve with cleavage of the oxazolidine ring to form a difficult to deac

ISSN:0018-019X    

DOI:10.1002/hlca.19940770611

出版商:WILEY‐VCH Verlag GmbH    

年代:1994

数据来源: WILEY