摘要:

AbstractVarious 8‐substituted 2,8‐dihydro‐2‐thioxopteridin‐4(3H)‐ones (14–21) and 2‐(methylthio)pteridin‐4(8H)‐ones (27–32) have been synthesized by condensation of the appropriate 5‐amino‐6‐(substituted amino)‐1,2‐dihydro‐2‐thioxopyrimidin‐4(3H)‐ones (22–34) and 5‐amino‐6‐(substituted amino)‐2‐(methylthio)pyrimidin‐4(3H)‐ones (25,26), respectively, with glyoxal, biacetyl, and benzil. The presence of a quinonoid cross‐conjugated π‐electron system makes this type of compounds susceptible to nucleophilic additions in position 7, which leads to intramolecular (43,45) and intermolecular (44) covalent adducts. The newly synthesized compounds have been characterized by elemental analyses, pKadeterminations,1H‐NMR and UV spectra. UV‐Spectral changes in dependence of the pH are associated with the most appropriate molecular species including the

ISSN:0018-019X    

DOI:10.1002/hlca.19880710602

出版商:WILEY‐VCH Verlag GmbH    

年代:1988

数据来源: WILEY

摘要:

AbstractThe [2 + 2] Photocycloaddition of 2‐(trimethylsilyloxy)‐1,3‐butadiene to a number of 2‐cycloalkenones proved to be quite a general reaction leading to good yields of the cycloadducts(Table). This finding is surprising since dienes, in general, are better known as quenchers of enone triplets rather than as photochemical reactants. Both the high substrate concentrations, which can be employed in these cycloadditions, and the remarkable regio and stereoselectivity of the processes qualify them as valuable for syntheses. In a first application, the photoproducts1a, bwere transformed in three steps into a viable precursor4of the pentalenolactone‐G and ‐H

ISSN:0018-019X    

DOI:10.1002/hlca.19880710603

出版商:WILEY‐VCH Verlag GmbH    

年代:1988

数据来源: WILEY

摘要:

AbstractEpoxidation of (−)‐(1R,2R,4R)‐2‐endo‐cyano‐7‐oxabicyclo[2.2.1]hept‐5‐en‐2‐exo‐yl acetate ((−)‐5) followed by saponification afforded (+)‐(1R,4R,5R,6R)‐5,6‐exo‐epoxy‐7‐oxabicyclo[2.2.1]heptan‐2‐one ((+)‐7). Reduction of (+)‐7 with diisobutylaluminium hydride (DIBAH) gave (+)‐1,3:2,5‐dianhydroviburnitol ( = (+)‐(1R,2R,3S,4R,6S)‐4,7‐dioxatricyclo[3.2.1.03,6]octan‐2‐ol; (+)‐3). Hydride reductions of (±)‐7 were lessexo‐face selective than reductions of bicyclo[2.2.1]heptan‐2‐one and its derivatives with NaBH4, AlH3, and LiAlH4probably because of smaller steric hindrance toendo‐face hydride attack when C(5)

ISSN:0018-019X    

DOI:10.1002/hlca.19880710604

出版商:WILEY‐VCH Verlag GmbH    

年代:1988

数据来源: WILEY

摘要:

AbstractThe oxygen‐exchange reaction of V10O 286−with bulk water has been followed by time‐dependent17O‐NMR spectroscopy (buffered solutions, pH ∼ 5.5, [V10]total∼ 0.17m,T= 298 K). It is shown that all seven structurally different sites of O‐atoms are kinetically similar but, in contrast to earlier studies, not identical (6 h ⩽ ‘t1/2’ ⩽ 11 h). The kinetic similarity of the various structural sites implies the some (but not full) O scrambling is involved. Two possible mechanisms with a ‘half‐bonded’ and an ‘open’ intermediate are discussed in detail to interpret the experimental results. A computer simulation of the exchange reaction based on these models is presented. It is shown that the ‘half‐bonded‐intermediate’ mechanism is consistent with the experimental data and the following parameters are calculated: formation of the intermediate:k1= 5.8 · 10−3s−1,k−1= 6.7 · 10−2s−1, [intermediate]∞≈ 8%; all activated O‐atoms exchange within the lifetime of the intermediate (τ ∼ 15 s), and the calculated exchange rate of the intermediate (k2⩾ 0.60 s−1) is consistent with earlier assumptions (k2≈ 0.5 s−1). It is shown that a simulation based on the ‘open‐intermediate’ mechanism results in kinetic parameters which are not consistent with the kinetics of the formation of cyclic metavanadates ((VO 3−)n,n= 4,5) from decavanadate, since the required formation rate is by a factor ∼ 102too fast, and the equilibrium concentration of metavanadates is by a factor of ∼ 2 too large (under the conditions of the O‐exchange experiments of decavanadate (T= 298 K, [V10]total≈ 0.17m, pH ∼ 5.55) the total amount of metavanadates present is ∼ 8%, with [(VO 3−)4]/[(VO 3−)5] ∼ 4:1; a qualitative analysis of the kinetics of the formation of metavanadates (vokinetics; the exact mechanism of the back‐reaction (at least second‐order) is not known with certainty) leads tok1⩾ 4·10−5s−1). O exchange of decavanadatesviaequilibrated metavanadates would lead to full scrambling of the O sites and is not consistent with the observed differences in the exchange rates. From the qualitative kinetic parameters of the metavanadate formation kinetics, it c

ISSN:0018-019X    

DOI:10.1002/hlca.19880710605

出版商:WILEY‐VCH Verlag GmbH    

年代:1988

数据来源: WILEY

摘要:

AbstractThe chemical synthesis by classical methods in solution of a dimer of the sequence 1 to 24 of adrenocorticotropin(ACTH) is described. The two monomers were covalently linked through their C‐termini, using lysine amide as spacer. Dimerization did not improve significantly the potency of the agonist ACTH‐(1–24), while it strongly potentiated the antagonistic effect of the fragments ACTH‐(11–24) and ACTH‐(7–24) in the stimulation of steroidogenesis in isolated adrenal cells. The results seem to imply a microaggregation of the receptors at the adrenal

ISSN:0018-019X    

DOI:10.1002/hlca.19880710606

出版商:WILEY‐VCH Verlag GmbH    

年代:1988

数据来源: WILEY

摘要:

AbstractA new algorithm for the systematic generation of conformations of macrocyclic systems is presented. The procedure is based on the concept of generic shapes that are found in such structures. These shapes are characterized by a selection of harmonics which occur in an approximateFourierrepresentation of the atomic coordinates of the rings. Following a fixed protocol, a limited set of in‐plane and out‐of‐plane circular harmonics is used to define an ensemble of generic ring shapes. These generic shapes are used as start structures for energy minimizations by a given force‐field method. To account for the possibility of having several final conformations originating from the same generic shape, the corresponding initial structure is taken several times and subjected to a randomization step before minimization. The resulting conformations that fall within a preset low‐energy band are collected and screened for duplicates and enantiomers. The efficiency of this procedure (ratio between the number of accepted conformations and the total number of energy minimizations) depends on the flexibility of the macrocyclic system. The efficiency is generally quite high for very flexible rings. According to the proposed protocol, the number of generic shapes used as start structures grows as the square ofN(lnN), whereNis the ring size. The algorithm lends itself to conformational analyses of medium‐size and large rings as well as of loops spanned between fixed struc

ISSN:0018-019X    

DOI:10.1002/hlca.19880710607

出版商:WILEY‐VCH Verlag GmbH    

年代:1988

数据来源: WILEY

摘要:

AbstractThe complexestrans‐[PtXY(2] (X = H or Me; Y = OMe, OCHO, CO2H, and BH4;2= 2,11‐bis{bis[3‐(trifluoromethyl)phenyl]phosphinomethyl}benzo[c]phenanthrene) were prepared, and their decompositions totrans[PtHX(2)] were studied. Some binuclear hydrido‐bridged complexes,e.g.[(2)HPt(μ‐H)PtH(2)]+, were also obtained. The preparation of complexestrans‐[PtHX(28)2] (X = H or Me,28= bis[3‐(trifluoromethyl)phenyl]benzylphosphine) is also reported. The X‐ray crystal structure oftrans‐[PtHCl1)] (1= 2,11‐bis(diphenylphosphinomethyl)benzo[c]phenanthr

ISSN:0018-019X    

DOI:10.1002/hlca.19880710608

出版商:WILEY‐VCH Verlag GmbH    

年代:1988

数据来源: WILEY

摘要:

AbstractVariable‐pressure1H‐NMR Spectroscopy has been used to study the fluxionality of some five‐coordinated Fe complexes in solution. For [Fe(CO)21,3‐cyclooctadiene (PPh3)], the CO site exchange is known (by analogy with [Fe(CO)3(1,3‐cyclooctadiene)]) to be a non‐dissociative process, and an activation volume ofca.0 cm3.mol−1was indeed obtained. However, for [Fe(CO2){2,3‐η:O‐σ‐(7,7‐dimethoxybicyclo[2.2.1]hept‐2‐ene)}(PPh3)], the activation volume of +5 cm3mol−1suggests that an unprecedented dissociation process is responsible for the CO site exchange. The molecular structure of [Fe(CO)2(1,3‐cyclooctadiene)(PPh3)] was ascertained by single‐crystal X‐ray diffractometry. The crystals are triclinic, space groupP1,a= 9.606(3),b= 16.795(2),c= 7.743(8) Å, α = 97.83(4), β = 109.63(4), γ = 83.37(2)°. The structure determination has shown that the complex possesses a tetragonal pyramidal coordination, with the endocycli

ISSN:0018-019X    

DOI:10.1002/hlca.19880710609

出版商:WILEY‐VCH Verlag GmbH    

年代:1988

数据来源: WILEY

摘要:

Reactions of Methyl Coumalate and Coumalaldehyde with Ambident NucleophilesMethyl coumalate and coumalaldehyde show great diversity in their reactions with ambident nucleophiles both depending upon the 2H‐pyran‐2‐one derivative and the nature of the ambident nucleophile used. The products are either pyridine or pyrimidine deriva

ISSN:0018-019X    

DOI:10.1002/hlca.19880710610

出版商:WILEY‐VCH Verlag GmbH    

年代:1988

数据来源: WILEY

摘要:

AbstractAt 20°, [2.2.2]hericene ( = 2,3,5,6,7,8‐hexamethylidenebicyclo[2.2.2]octane;1) adds 2 equiv. of SO2to give successively mono‐sulfolene13and the bis‐sulfolene14(for thermodynamic and kinetic data, seeTables 1and2). Under forcing conditions (high concentration, long reaction time), no trace of tris‐sulfolene15could be detected. The inertia of bis‐sulfolene14is attributed to the ‘barrelene effect’ which destabilizes the tr

ISSN:0018-019X    

DOI:10.1002/hlca.19880710611

出版商:WILEY‐VCH Verlag GmbH    

年代:1988

数据来源: WILEY