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  • 1
    Electronic Resource
    Electronic Resource
    Historical changes in sediments of Pyramid Lake, Nevada, USA: consequences of changes in the water balance of a terminal desert lake (1994)
    Springer
    Journal of paleolimnology 12 (1994), S. 87-101 
    Details
    ISSN: 1573-0417
    Keywords: Great Basin ; climatic variations ; productivity ; organic matter ; nitrogen ; phosphorus ; hardwater lake
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Geosciences
    Notes: Abstract Sediment cores from the shallow and deep basins of Pyramid Lake, Nevada, revealed variations in composition with depth reflecting changes in lake level, river inflow, and lake productivity. Recent sediments from the period of historical record indicate: (1) CaCO3 and organic content of sediment in the shallow basin decrease at lower lake level, (2) CaCO3 content of deep basin sediments increases when lake level decreases rapidly, and (3) the inorganic P content of sediments increases with decreasing lake volume. Variations in sediment composition also indicate several periods for which productivity in Pyramid Lake may have been elevated over the past 1000 years. Our data provide strong evidence for increased productivity during the first half of the 20th Century, although the typical pattern for cultural eutrophication was not observed. The organic content of sediments also suggests periods of increased productivity in the lake prior to the discovery and development of the region by white settlers. Indeed, a broad peak in organic fractions during the 1800's originates as an increase starting around 1600. However, periods of changing organic content of sediments also correspond to periods when inflow to the lake was probably at extremes (e.g. drought or flood) indicating that fluctuations in river inflow may be an important factor affecting sediment composition in Pyramid Lake.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00678089
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  • 2
    Electronic Resource
    Electronic Resource
    Nutrient cycling and productivity in a desert saline lake: observations from a dry, low-productivity year (1992)
    Springer
    Hydrobiologia 246 (1992), S. 213-229 
    Details
    ISSN: 1573-5117
    Keywords: Nutrient cycling ; productivity ; internal loading ; seasonal variations
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Seasonal variability of nutrients and productivity were examined in Pyramid Lake, a hyposaline, N-deficient, terminal desert lake, during a dry period. River inflow and N-fixation during 1990 were minimal allowing internal nutrient cycling to be more closely studied. Nutrient cycling was strongly affected by seasonal thermal stratification that was typical for a warm monomictic lake. Concentrations of nitrate, phosphate, and silicate in surface waters were highest during winter mixing and decreased rapidly in the spring due to a diatom bloom. Maximum average chlorophyll concentration in surface waters was 2.7 ± 1.2 µg 1−1 and occurred in April while surface nutrients were being depleted. In contrast to chlorophyll, maximum particulate carbon in surface waters occurred in July–August when areal productivity was highest (367–398 mg C m−2 day−1). Concurrent with spring nutrient depletion in surface waters was increasing N-deficiency in the plankton. After the spring bloom dissipated in May, particulate matter (POM) became increasingly N-deficient reaching maximum elemental C : N of 〉 18 during summer-fall. Profiles of the C : N ratio of POM were nearly constant with depth for individual sampling dates suggesting that the residence time of POM in the water column was short (〈 1 month). While surface waters were nutrient depleted during summer stratification, nutrient concentrations of bottom waters progressively increased, presumably through the oxidation of POM sinking to the bottom (103 m). Converting the rate of oxygen depletion in bottom waters to carbon equivalents of POM suggests that 42 % of mean annual phytoplankton production in overlying waters during 1990 was mineralized in bottom waters.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00005699
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  • 3
    Electronic Resource
    Electronic Resource
    C2-Symmetrical Pyrrolidine Derivatives Chiral Auxiliaries in Radical Chemistry (1993)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 76 (1993), S. 441-450 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Fumaramides 3b and 3c bearing the C2-symmetrical pyrrolidine moieties (2R,5R)-2,5-bis(methoxymethyl)pyrrolidine (2b) or 1,3:4,6-di-O-benzylidene-2,5-dideoxy-2,5-imino-L-idit (2c), respectively, as a chiral auxiliary lead to high diastereoselectivities in radical reactions (‘tin method’;Scheme 1). Removal of the chiral auxiliaries affords the corresponding alkylated fumaric acids Scheme 2. Single-crystal X-ray structures of 3b and 3c support arguments that lead to the model of 1,4-stereoinduction.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19930760128
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  • 4
    Electronic Resource
    Electronic Resource
    Synthesis and Reactivity of a p-Methoxyphenyl-Substituted Enediyne. A Case of Electronic Influence on the Rate of the Bergman Cycloaromatization (1992)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1992 (1992), S. 855-861 
    Details
    ISSN: 0170-2041
    Keywords: Enediynes ; 1,5 Diynes ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Bicyclo[7.3.1]diynes 19a and 19b were prepared by using dibromo olefin 11 and ketone 12 as building blocks. The key step of the synthetic sequence is an intramolecular Nicholas reaction of 17 to give the bicyclo[7.3.1]diyn-10-one dicobalthexacarbonyl adducts 18a and 18b (66% total yield). Oxidative decomplexation of 18a and 18b with cer(IV) ammonium nitrate gave the diynes 19a and 19b, respectively. Both diynes 19a and 19b could be oxidatively converted into the enediyne 20 by using DDQ. In contrast to the unsubstituted enediyne 4, compound 20 can be isolated at room temperature Quantitative kinetic measurements of the rate of the Bergman cyclization of 20 gave ΔG# (37°C) = 111 kJ mol-1. This value is 12.3 kJ mol-1 higher than that of 4. The difference in the free activation energy between 20 and 4 is attributed to electronic effects.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jlac.1992199201140
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  • 5
    Electronic Resource
    Electronic Resource
    Diastereoselectivity in the halolactonization of 2-alkoxy-4-alkenoic acids and-4-alkenamides (1990)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1990 (1990), S. 323-330 
    Details
    ISSN: 0170-2041
    Keywords: 1,3-Asymmetric induction ; Butyrolactones ; Halolactonization ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Iodolactonization of 2-alkoxy- and 2-hydroxy-4-alkenoic acids 3a-d under kinetic control gives mixtures of cis- and trans-lactones 6a-d and 7a-d with cis-selectivity up to 5.5:1.0. The bromolactonization of carboxamides 4 a, 4 c, and 5 c shows moderate 1,3-asymmetric induction, favoring the trans-lactones 9 with a selectivity of up to 2.5:1.0.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jlac.199019900161
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  • 6
    Electronic Resource
    Electronic Resource
    Synthesis of 11-Membered Enediyne Ketones by the Intramolecular Nicholas Reaction (1993)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1993 (1993), S. 1009-1016 
    Details
    ISSN: 0170-2041
    Keywords: Enediynes ; Nicholas reaction ; Macrocycles ; Eleven-membered rings ; Calculations, force-field ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An efficient route to 11-membered enediyne ketones 13 and 22 was developed. This route is based on an intramolecular Nicholas reaction of 11 and 20, respectively. In the course of the preparation of the cyclization substrates 11 and 20, it was important to use the acetylenic alcohols 6 and 15 as building blocks for the construction of the cis-enediynes 8 and 17, respectively, instead of the corresponding ketones which were converted into unstable acyclic enediynes. Some transformations of enediyne ketone 22 were also studied. For example, 22 could be transformed into the 2-formyl compound 26 via the silyl enol ether 24. In addition, 24 was used for the preparation of the 2-amino ketone 29.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jlac.1993199301160
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