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81.
6-exo-Methylbicyclo[3.2.0]hepten-7-ones and their 2-alkylidene analogues are readily prepared from dialkyl squarates. These compounds undergo facial oxy-Cope ring expansions upon treatment with vinyllithium; the former leads to bicyclo[4.2. 1]non-1(4)-en-6-ones and the latter to the first examples of bicyclo[5.2.1]dec-1(10)-en-5-ones, compounds having exceptionally strained bridgehead double bonds. The transformations are controlled by the 6-exo-methyl group in the starting material along with the substituent at position-1 (bridgehead) which force attack of the lithium reagent from the concave face of the starting material, thus allowing the cyclopentenyl or alkylidene groups to participate in the sigmatropic event.  相似文献   
82.
[formula: see text] This paper describes a new tandem reaction sequence leading to angularly fused polyquinanes from squaric acid-derived bicyclo[6.3.0]-undecadienediones. Such compounds undergo a dual Michael addition. The enolate form in the first intermolecular addition undergoes the second intramolecular transannular addition to give the angular polyquinanes. A particularly interesting example is a catalytic transformation of cis-13-methylyricyclo[10.3.0.0]pentadeca-4(5),12(13)-diene-3 ,14-dione to (3R*,3aS*,5aR*,9aR*,11aR*)-3-methyl-1,2,3,5,5a,6 ,7,10,11,11a-decahydro-4H- pentaleno[6a,1-c]indene-2,10-dione, a compound having the tetracyclic ring system found in the natural product waihoensene. The mechanism and synthetic scope of these reactions are discussed.  相似文献   
83.
Riboflavin was irradiated anaerobically in aqueous EDTA solutions over the pH range 2.5–10. In other dye systems (Bonneau and Pereyre, 1975), only the trivalent anion of EDTA was found to have significant reactivity for photoreduction. For riboflavin, the reactivity begins with monoanionic EDTA, and the reactivity is markedly increased as the charge increases. This suggests that the charge on the reductant is more important to the electron transfer process for riboflavin than the formation of a nonhydrogen bonded nitrogen site on EDTA. At high concentrations of EDTA in the pH range 4–8, quenching of the photoreduction occurs, which can be explained by an energy transfer between the excited singlet state of riboflavin and trianionic EDTA, possibly as an association complex. The rate constants for the photoreduction of riboflavin by the monovalent, divalent, and trivalent anions of EDTA are 1.0 times 107M-1 s-l, 4.8 times 10′M-1 s-l, and 2.0 times 108M-1s-1, respectively. The rate constant for the singlet state quenching by trianionic EDTA is 3 times 109M-l s-1, and the limiting quantum yield for intersystem crossing for riboflavin in aqueous solution is 0.50 ± 0.05.  相似文献   
84.
Several NMR screening techniques have been developed in recent years to aid in the identification of lead drug compounds. These NMR methods have traditionally been used for protein targets, and here we examine their applicability for an RNA target. We used the SHAPES compound library to test three different NMR screening methodologies: the saturation transfer difference (STD), the 2D trNOESY, and the WaterLOGSY experiments. We found that the WaterLOGSY experiment was the most sensitive method for our RNA target, the P4P6 domain of the Tetrahymena thermophila Group I intron. Using the WaterLOGSY experiment, we found that 23 of the 112 SHAPES compounds interact with P4P6. To identify which of these 23 hits bind through nonspecific interactions, we counterscreened with a linear duplex RNA control and identified one of the SHAPES compounds as interacting with P4P6 specifically. We thus demonstrated that the WaterLOGSY experiment in combination with the SHAPES compound library can be used to efficiently find RNA binding lead compounds.  相似文献   
85.
Studies on the biosynthesis of cylindrospermopsin (1), a potent hepatotoxin associated with the cyanobacterium Cylindrospermopsis raciborskii, indicate that 1 is an acetogenin with guanidinoacetic acid serving as the starter unit of the polyketide chain. Feeding experiments show that C14 and C15 of 1 are derived from C1 and C2 of glycine, respectively, and C4 through C13 arise from five contiguous acetate units attached head to tail. The methyl carbon on C13 originates from the C(1) pool. The starter unit, established by the incorporation of [guanidino-(13)C,alpha-(15)N]-guanidinoacetic acid into N16 and C17 of 1, does not appear to be formed from glycine by known amidination pathways. The origin of the NH-CO-NH segment in the uracil ring is also unknown.  相似文献   
86.
Two syntheses of some optically active 2-benzyl-2,3-dihydro-4H-benzopyrans and benzopyran-4-ones are presented. An asymmetric synthesis starting from D- and L-phenylalanine was used to provide both enantiomers of 2-benzyl-6-(methoxycarbonyl)-2,3-dihydro-4H-benzopyran-4-one 19. Phenylalanine was diazotized in aqueous sulfuric acid to 2-hydroxy-3-phenylpropionic acid 6 which was converted in four steps to 1-bromo-2-(4-methoxycarbonylphenoxy)-3-phenylpropane 11. (4R,S)-Benzamido-2-benzyl-2,3-dihydro-6-(methoxycarbonyl)-4H-1-benzopyran-4-carboxylic acid 16 was prepared from 11 by amidoalkylation with α-hydroxyhippuric acid in methanesulfonic acid solution followed by spiroalkylation to (4R,S)-2-benzyl-2,3-dihydro-6-(methoxycarbonyl)spiro[4H-benzopyran-4,4′-2′-phenyloxazolidin]-5′-one 15. After the phenyloxazolidin-5-one 15 was hydrolyzed to the spirobenzamido carboxylic acid 16 , oxidative decarboxylation with sodium hypochlorite yielded optically active 2-benzyl-6-(methoxycarbonyl)-2,3-dihydro-4H-benzopyran-4-one 19. The ketone in 19 was reduced by hydrogenation over palladium on carbon to a methylene group and the ester was converted to the aldehyde to give both isomers of the desired intermediate 2-benzyl-6-(formyl)-2,3-dihydro-4H-benzopyran 25. The second synthesis relied on an enzymatic hydrolysis of ethyl 2,3-dihydrobenzopyran-2-carboxylate 27 with the lipase from P. fluorescens to provide the desired 2R-ester. The ester group in (R)- 27 was converted to the triflate (R)- 29. Displacement of the triflate group with phenylmagnesium bromide and cuprous bromide as catalyst gave 2R-benzyl-2,3-dihydro-4H-benzopyran (R)- 30. Formylation of (R)- 30 provided 2R-benzyl-6-(formyl)-2,3-dihydro-4H-benzopyran (R)- 25 identical with that from the first synthesis. These optically active intermediates are used in the preparation of the hypoglycemic agent englitazone.  相似文献   
87.
We present joint theoretical and experimental results which provide evidence for the selectivity of V(x)O(y)(+) clusters in reactions toward ethylene due to the charge and different oxidation states of vanadium for different cluster sizes. Density functional calculations were performed on the reactions between V(x)O(y)(+) and ethylene, allowing us to identify the structure-reactivity relationship and to corroborate the experimental results obtained by Castleman and co-workers (Zemski, K. A.; Justes, D. R.; Castleman, A. W., Jr. J. Phys. Chem. A 2001, 105, 10237). The lowest-energy structures for the V(2)O(2)(-)(6)(+) and V(4)O(8)(-)(10)(+) clusters and the V(2)O(3)(-)(6)(+)-C(2)H(4) and V(4)O(10)(+)-C(2)H(4) complexes, as well as the energetics for reactions between ethylene and V(2)O(4)(-)(6)(+) and V(4)O(10)(+) are presented here. The oxygen transfer reaction pathway was determined to be the most energetically favorable one available to V(2)O(5)(+) and V(4)O(10)(+) via a radical-cation mechanism.The association and replacement reaction pathways were found to be the optimal channels for V(2)O(4)(+) and V(2)O(6)(+), respectively. These results are in agreement with the experimental results reported previously. Experiments were also conducted for the reactions between V(2)O(5)(+) and ethylene to include an energetic analysis at increasing pressures. It was found that the addition of energy depleted the production of V(2)O(4)(+), confirming that a more involved reaction rather than a collisional process is responsible for the observed phenomenon. In this contribution we show that investigation of reactions involving gas-phase cationic vanadium oxide clusters with small hydrocarbons is suitable for the identification of reactive centers responsible for selectivity in heterogeneous catalysis.  相似文献   
88.
89.
90.
Photoinduced charge separation and recombination in a carotenoid-porphyrin-fullerene triad C-P-C(60)(1) have been followed by multifrequency time-resolved electron paramagnetic resonance (TREPR) at intermediate magnetic field and microwave frequency (X-band) and high field and frequency (W-band). The electron-transfer process has been characterized in the different phases of two uniaxial liquid crystals (E-7 and ZLI-1167). The triad undergoes photoinduced electron transfer, with the generation of a long-lived charge-separated state, and charge recombination to the triplet state, localized in the carotene moiety, mimicking different aspects of the photosynthetic electron-transfer process. Both the photoinduced spin-correlated radical pair and the spin-polarized recombination triplet are observed starting from the crystalline up to the isotropic phase of the liquid crystals. The W-band TREPR radical pair spectrum has allowed unambiguous assignment of the spin-correlated radical pair spectrum to the charge-separated state C(.+)-P-C(60)(.-). The magnetic interaction parameters have been evaluated by simulation of the spin-polarized radical pair spectrum and the spin-selective recombination rates have been derived from the time dependence of the spectrum. The weak exchange interaction parameter (J = +0.5 +/- 0.2 G) provides a direct measure of the dominant electronic coupling matrix element V between the C(.+)-P-C(60)(.-) radical pair state and the recombination triplet state (3)C-P-C(60). The kinetic parameters have been analyzed in terms of the effect of the liquid crystal medium on the electron-transfer process. Effects of orientation of the molecular triad in the liquid crystal are evidenced by simulations of the carotenoid triplet state EPR spectra at different orientations of the external magnetic field with respect to the director of the mesophase. The order parameter (S = 0.5 +/- 0.05) has been evaluated.  相似文献   
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