摘要:

AbstractThe reactions that occur during the cooking, baking, and preservation of foods of all kinds are of great importance for the production of aroma, taste, and color. However, more recently it has been shown that these reactions may be accompanied by a reduction in nutritive value and the formation of toxic compounds. For these reasons, the very complex reactions between reducing sugars and the free amino groups of amino acids or proteins, known as non‐enzymatic browning or the Maillard reaction, have again caught the interest of chemists. The Maillard reaction came to be seen in a new light as it was realized that it actually occurred in the human body. As a general rule, the longer the half‐life of a protein, the larger the amount of its Maillard products found, i.e., important factors are the ‘age’ or persistence of the protein in the body and the glucose concentration, particularly in diabetics. Many of the symptoms developed by diabetics resemble those of premature aging, which leads to the possibility that glucose, because of its reactivity towards proteins, is fundamentally involved in the normally slow progress o

ISSN:0570-0833    

DOI:10.1002/anie.199005653

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractWithin the last ten to twenty years, radioastronomers have discovered the existence of almost 100 different molecules in interstellar space. Ranging in complexity from two to thirteen atoms, these molecules are found in cold, rarefied regions called interstellar clouds, which are giant accumulations of gas and dust located in our galaxy as well as many others. Interstellar clouds are also the birthplaces of future generations of stars and are of great interest to astronomers. The observation of the large sample of gaseous molecules, detected mainly via their rotational spectral patterns, tells astronomers about the detailed physical conditions in interstellar clouds, and tells chemists about the extent of molecular synthesis possible under the seemingly harsh conditions of low temperature and density. The molecules are mainly organic in nature and comprise species known to be both stable and common in the laboratory as well as those both unstable and uncommon under terrestrial conditions, including radicals and molecular ions. Although the gas phase of interstellar clouds is well studied via spectroscopic techniques, the dust particles are much more poorly characterized via their scattering and absorption of visible radiation as well as some broad resonances in the ultraviolet and infrared regions of the spectrum. It is normally thought that these submicron‐sized particles consist of cores that are composites of silicate and carbonaceous materials with mantles that contain material deposited from the gas such as ices of water, ammonia, and methane. In addition to the dust particles and gaseous molecules, there is some evidence for very large aromatic molecules (the so‐called polycyclic aromatic hydrocarbons or PAH's) which occupy a nether region in between large gas‐phase species and small dust particles. As our understanding of the chemical processes in interstellar clouds increases, it may be possible to speculate how large interstellar molecules can come into existence and whether or not there is a clear connection between interstellar chemistry and the start of life on

ISSN:0570-0833    

DOI:10.1002/anie.199005951

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractConvergency is a highly serviceable aspect of organic synthesis, particularly when its application is accompanied by high levels of chirality transfer and regiocontrol. A reaction that is notably well suited to these objectives is the oxy‐Cope rearrangement. In addition, the benefits that can accrue from suitable condensation of β γ‐unsaturated ketones with a vinyl organometallic and subsequent [3,3] sigmatropic shift are substantive enhancement in structural complexity and the ability to generate a myriad of polycyclic frameworks. Another profitable feature is the complementary regeneration of the carbonyl group in a new structural context. In view of the pivotal role played by the carbonyl functionality in synthetic transformations, reacquisition of such a reaction site translates into heightened chemical versatility. The trajectories followed by vinyl organometallics as they engage in 1,2 addition to ketones are gradually being understood. Proper diastereoselective control of this bond‐making process, when coupled with full awareness of the chair or boat topography adopted during the ensuing oxy‐Cope electronic reorganization, provides for exciting new strategy‐level dimensions not often available in other contexts. The use of this two‐step sequence as a prelude to a third chemical event clearly serves to expand the possibilities for molecular construction still more. While a considerable amount of the work to date has focused on the preparation of rather elaborate carbocyclic molecules, principally with a view to gaining access to complex natural products, the possibilities for heterocyclic synthesis are currently being accorded increa

ISSN:0570-0833    

DOI:10.1002/anie.199006091

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractMetal complexes are capable of accomplishing almost anything, provided they contain the proper metal/ligand combinations. A host of essential biochemical transformations—but also a great many industrially significant reactions—occur within the coordination spheres of metal ions. For instance, the particular arrangement of ligands in the zinc‐containing enzyme carboanhydrase is responsible for an acceleration in the hydration of CO2by a factor of 109. It is the ligands that determine whether an iron atom will transfer molecular oxygen, as in the case of hemoglobin, or electrons, as with the cytochromes. By varying the ligands it is possible to establish in advance whether a metal ion in the presence of synthesis gas will cause an olefin to be hydrogenated or hydroformylated. Stated more generally, it is the ligands that stabilize the particular oxidation states of a metal and determine how substrate molecules will be coordinated and undergo reaction. The synthesis of new ligands that confer specific reactivity on metal ions is thus an important challenge for the coordination chemist. The following article describes organometallic compounds of the type [CpCo{P(O)R′R″}3]⊖, which have developed from an extremely unreactive laboratory curiosity into versatile oxygen‐containing ligands whose steric and electronic properties promise a series of interesting

ISSN:0570-0833    

DOI:10.1002/anie.199006271

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199006381

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199006391

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199006411

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199006431

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199006451

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.199006471

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY