摘要:

AbstractSequences of directly adjacent stereogenic centers were first discovered for the monosaccharides, which therefore constituted the first focus of interest in Stereoselective synthesis. The structures of many polyketide‐derived natural products that have been elucidated in recent years often exhibit longer sequences of stereogenic centers in which hydroxy‐substituted secondary carbon atoms alternate with tertiary carbon atoms. This finding inspired the development of methods allowing the specific construction of first two and then—more importantly–three neighboring stereogenic centers. The diverse methodologies that have thereby emerged reveal common principles and stress the variety of approaches possible in stereoselective sy

ISSN:0570-0833    

DOI:10.1002/anie.198704893

出版商:Hüthig&Wepf Verlag    

年代:1987

数据来源: WILEY

摘要:

AbstractThe chemistry of the non‐metallic elements has in recent years passed through a period of rapid development, often referred to as its “renaissance”. To emphasize just one of the key facets: numerous short‐lived molecules containing multiple bonds to elements of the third and higher periods have been discovered, often accompanied by the planned synthesis of derivatives which are sterically shielded by bulky groups and thus kinetically stabilized. Thus today molecules such as silabenzenes H6C6−nSinand silaethenes H2SiCH2or R2SiCR2, disilenes R2SiSiR2and diphosphenes RPPR, silaphenylisonitrile H5C6NSi, or methylidyne‐phosphanes RCP, are all well‐known species. Sandwich compounds with P6rings or silicon centers demonstrate that there are now hardly any barriers to impede the imagination of the non‐metal chemist. In sharp contrast is our lack of knowledge regarding the “microscopic” pathways of chemical reactions: thus apart from information provided for example by molecular beam experiments, or from exact numerical calculations involving species consisting of only a few atoms, it remains largely unknown from which directions medium‐sized molecules must approach each other to successfully collide and form a “reaction complex”, in which way their structures are changed in such a process or which role is played by molecular dynamics in the energy transfer.–The pyrolysis of azides XN3, i.e. compounds which tend to explode violently when ignited in the condensed phase but can be heated in low‐pressure gas flow systems without much risk, illustrates that studies of reactive intermediates are of interest not only because novel molecules may be discovered and isolated, and thereby possibilities for synthesis expanded. Moreover, some aspects of the “microscopic” pathways of these azide pyrolyses can be described satisfactorily on the basis of calculated energy hypersurfaces, and the influence of molecular dynamics becomes experimentally visible in the “chemical activation” of

ISSN:0570-0833    

DOI:10.1002/anie.198705041

出版商:Hüthig&Wepf Verlag    

年代:1987

数据来源: WILEY

摘要:

AbstractThe enormous progress made in the field of medicine over the past few decades has been partly due to the introduction of new instruments but also a result of the use of new materials. It is impossible to imagine modern medicine without metals, alloys, sintered corundum, organic high polymers (also as composite materials), glassy carbon, etc. Bioglass ceramics open up new possibilities for medical treatment and constitute a new area of research in the natural sciences and medicine. Owing to their widely variable combinations of properties, bioglass ceramics can be more easily adapted to suit medical requirements than can customary implants. Two properties of bioglass ceramics are of primary importance: their biocompatibility, i.e., acceptance of the material by the tissues of the human body without irritation, rejection reactions, or toxic effects; and their bioactivity, i.e., the ability to establish firm intergrowths with tissues of the human body. This property is not shared by any of the classical biomaterials. A wide range of applications is envisaged for the bioglass ceramics that have so far been developed; some are still undergoing animal tests while others are being clinically tested in humans. Possible applications are the replacement of vertebrae and use in the middle ear, throat, nose, and eye, in the entire head region, in the shoulder and leg, and in dental prosthetics, in particular the replacement of dental roots (a hard tissue substitute in the broadest sense of the word). The question as to the behavior of a bone/bioglass ceramic contact or bond on a long‐term scale and on being exposed to varying mechanical stress has still not been satisfactorily answered, because interdisciplinary research in this field is still immature. All observations made so far indicate, however, that the materials do not cause any adverse effect

ISSN:0570-0833    

DOI:10.1002/anie.198705271

出版商:Hüthig&Wepf Verlag    

年代:1987

数据来源: WILEY

摘要:

AbstractThe selective enrichment of metals in nature is of great interest.[1]For example, the concentration of vanadium in the blood of tunicates is a millionfold higher than in sea water.[2–4]Fly agaric(Amanita muscaria)also concentrates vanadium.[5,6]The structure of the vanadium compound of tunicates has not yet been elucidated.[7,8]However, a structure (3) has been proposed[9]for the vanadium compound of fly agaric, amavadin.[9‐11]The structural elucidation and synthesis of the organic ligand1b[9b]allows, for the first time, the specificity of a naturally occurring vanadium ligand to be investigated. Amavadin can be prepared from VIVO salts and synthetic1b.[9b]Since the ligand1bwas previously unknown and since all attempts to obtain crystals of amavadin suitable for an X‐ray structure analysis failed, a detailed investigation of the complex formation of1bshould also provide an answer to the question of the arrangement of the ligands around the central atom in ama

ISSN:0570-0833    

DOI:10.1002/anie.198705451

出版商:Hüthig&Wepf Verlag    

年代:1987

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198705461

出版商:Hüthig&Wepf Verlag    

年代:1987

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198705481

出版商:Hüthig&Wepf Verlag    

年代:1987

数据来源: WILEY

摘要:

AbstractFor corrigendum see DOI:10.1002/anie.198708041

ISSN:0570-0833    

DOI:10.1002/anie.198705491

出版商:Hüthig&Wepf Verlag    

年代:1987

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198705511

出版商:Hüthig&Wepf Verlag    

年代:1987

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198705521

出版商:Hüthig&Wepf Verlag    

年代:1987

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198705541

出版商:Hüthig&Wepf Verlag    

年代:1987

数据来源: WILEY