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61.
Total synthesis of polycavernoside A, a lethal toxin of the red alga Polycavernosa tsudai 总被引:1,自引:0,他引:1
Blakemore PR Browder CC Hong J Lincoln CM Nagornyy PA Robarge LA Wardrop DJ White JD 《The Journal of organic chemistry》2005,70(14):5449-5460
[structure: see text] Two approaches to the synthesis of the aglycon 120 of polycavernoside A (1) were developed, only one of which was completed. The successful "second-generation" route assembled the aglycon seco acids 102 and 106 via Nozaki-Hiyama-Kishi coupling of aldehyde 70, prepared from methyl (S)-3-hydroxy-2-methylpropionate (72) and (S)-pantolactone (73), with vinyl bromide 71. The latter was obtained from a sequence which commenced from the silyl ether 24 of 3-hydroxypropionaldehyde and entailed cyclization of (Z)-zeta-hydroxy-alpha,beta-unsaturated ester 82. Regioselective Yamaguchi lactonization of trihydroxycarboxylic acids 102 and 106 and subsequent functional-group adjustments led to macrolactone 120, to which the fucopyranosylxylopyranoside moiety was attached. Stille coupling of the glycosidated aglycon 128 with dienylstannane 129 furnished polycavernoside A in a synthesis for which the longest linear sequence was 25 steps. The overall yield to lactone 120 was 4.7%. 相似文献
62.
Ottenberg CM McDonald FC O'Donnell ET Skrabal SA White DP 《Inorganic chemistry》2002,41(26):7151-7158
Increased preorganization can be achieved by immobilizing ligands on solid supports. Photoluminescent porous silicon, which can undergo facile hydrosilylation, was used as a support for open chain neutral N- and O-donor ligands. The abilities of these ligands to bind the divalent metal ions Ni(2+), Cu(2+), Zn(2+), and Pb(2+) are examined. Immobilized ligands selectively complexed Cu(II) over the other metal ions studied. Ligands immobilized on photoluminescent porous silicon also removed a significant amount, up to 98%, of Cu(II) from copper(II)-spiked, organic-rich, seawater samples. 相似文献
63.
Ring-opening/ring-closing metathesis on cyclobutene-containing substrates with angular oxygen functionality provides a stereospecific introduction of 1,5-bis-dienes required for an anion-accelerated oxy-Cope rearrangement. The reaction sequence offers generally a stereocontrolled preparation of a variety of medium ring-containing bicyclic ring systems, while rearrangement to the bicyclo[7,3,0]dodecane (9-5) system leads to a mixture of olefin isomers. 相似文献
64.
M. S. Platz W. R. White D. A. Modarelli S. Celebi 《Research on Chemical Intermediates》1994,20(2):175-193
The photochemistry of diazirines and diazo compounds is not as simple as nitrogen extrusion and carbene formation. The C-H
bonds adjacent to the diazo and diazirine moieties can migrate in the excited state and produce stable products without the
benefit of a relaxed carbene intermediate. Additionally, cyclobutyl substituted systems exhibit carbon migration. It is unfortunate
that the products of photochemical rearrangement of precursor excited states are identical to the products of thermal rearrangement
of carbenes. This has prevented accurate measurement of the yield and absolute reactivity of alkylcarbenes. That pyridine
reacts selectively with carbenes and not with the excited states of their nitrogenous precursors has allowed the separation
of these two pathways and an appreciation of their relative importance with structural variation. 相似文献
65.
Abstract— The chemical steps and the products of the bioluminescent and chemiluminescent oxidations of firefly luciferin are elucidated. The colors of firefly bioluminescence can be explained in terms of different ionic excited states and spectral shifts due to changes in molecular environment. Firefly luciferase undergoes conformational changes during catalysis. There are two sites for light production per 100,000 mW. A regulatory mechanism involving dehydro-luciferin is proposed for control of firefly flashing. 相似文献
66.
An in situ procedure has been developed for the separation of tumor necrosis factor (TNF) alpha directly from the fluid in the human gingival space. Paramagnetic beads coated with anti-TNF monoclonal antibodies were introduced into the gingival space of the subject with a polypropylene-tipped calibrated delivery system and retrieved using a permanent magnet designed to fit into the space. After retrieval, the amount of immunoadsorbed TNF was quantified using an immunochemical assay called the "cluster assay". The results indicate that following the appropriate preparation of the site, over 95% of the beads could be recovered. With this method we found that 62% of those cavities sampled contained TNF and that the values ranged from 0.10 to 13.0 ng/ml with a mean value of 1.7 ng/ml. A comparison of these values with those obtained from the same space using other methods suggests that the immunomagnetic method was more effective in retrieval of TNF. Because the separation is performed in situ we have named the procedure "chromatobiosis". 相似文献
67.
Chlorosulfonyl isocyanate adds to α-pinene in concerted fashion to give the unrearranged N-chlorosulfonyl-β-lactam 2 (75%). Thermolysis of cycloadduct 2 affords the rearranged N-chlorosulfonyl-γ-lactam 6 (60%) via a sequence of transient carbonium ions (45). Reductive hydrolysis of 2 and 6 gave, respectively, the NH-β-lactam 3 (70%) and the NH-γ-lactam 7 (70%). Proof of structure of the rearranged γ-lactams 6 and 7 are provided, respectively, by X-ray crystallographic analysis and the application of the Eu(dpm)3 NMR shift reagent. 相似文献
68.
John H. Bowie Peter Y. White Thomas Blumenthal 《Journal of mass spectrometry : JMS》1987,22(8):541-544
Deprotonated 2-phenyl-1,3-dithiane undergoes competitive losses of H˙, C3H6 C2H4S, C3H4S, C7H6S, C10H10 and C10H10S upon collisional activation. The elimination of H occurs from the phenyl ring. The loss of C3H6 occurs by simple cleavage of the dithiane ring. All other processes involve specific proton transfer followed by either cleavage or internal nucleophilic displacement. 相似文献
69.
D. J. White 《The Journal of the Operational Research Society》1984,35(8):759-767
The following assignment problem is considered. There are n activities to be assigned to n personnel. The cost of assigning activity i to person j is c ij . It is required to find all the efficient assignments, i.e. those for which there exists no other assignment which has at least as small costs for each person and strictly smaller costs for at least one person. The main results are as follows. In Theorem 1 it is shown that whereas, for many integer problems, the standard scalar weighting factor approach will not produce all the efficient solutions, in this case it will. In Theorem 2 it is shown that when each efficient vector is determined by a single assignment solution, the efficient set is identical to the set of efficient vertices of the convex hull of the assignment solution set. 相似文献
70.
E L White M S Uhrig T J Johnson B M Gordon R D Hicks M F Borgerding W M Coleman J F Elder 《Journal of chromatographic science》1990,28(8):393-399
Eight compounds from a Kentucky 1R4F reference cigarette smoke condensate have been determined by selected ion monitoring-mass spectrometry (SIM-MS) to confirm the validity of multidimensional gas chromatography (MDGC) as a quantitative tool in complex mixture analyses. Four electrostatically precipitated smoke condensate samples of 100 cigarettes each are dissolved individually in 25 mL of 2-propanol. The 2-propanol contains two methyl esters (C8 and C14) and seven deuterium-labeled compounds used as internal standards (IS). Analysis of the compounds of interest, pyridine; acetamide; acrylamide; phenol; o-, m-, and p-cresol; and quinoline, is accomplished by using two heartcuts. Heartcut times of the MDGC analysis are selected such that at least one IS is transferred with each group of compounds being analyzed. This study shows that the MDGC technique previously developed and described can be used for quantitative analyses. A comparison is made between the two types of internal standards. The results obtained for both types of internal standards agree within 20% of each other, on the average, with higher standard deviations for approximately 60% of the compounds where methyl esters are used as internal standards. 相似文献