摘要:

AbstractFour‐membered rings can be cleaved thermally, Photochemically, or catalytically into two π bonded fragments. Theoretical calculations, kinetic studies, and investigations of stereo‐ and regioselectivity have been undertaken to clarify the question of whether the reaction involves one or two steps and to permit predictions on its course. [2 + 2]‐Cycloreversions have been used to clarify the structure of four‐membered rings, to prepare highly reactive π electron‐systems and–in combination with a [2 + 2]‐cycloaddition–to protect double bonds. The combination of a cycloaddition and‐reversion can be used to convert a carbonyl group into an olefin. Starting with compounds containing annelated four‐membered rings, compounds with two functional groups or large ring systems can be prepared. [2 + 2]‐Cycloreversions have also been discussed in connection with

ISSN:0570-0833    

DOI:10.1002/anie.198202253

出版商:Hüthig&Wepf Verlag    

年代:1982

数据来源: WILEY

摘要:

AbstractMany natural products, such as the plant insecticides rotenone and ageratochromene (precocene), some fungitoxic phytoalexins, hashish constituents, vitamin E, and plant pigments, contain the chroman ring system as common structural unit. Some years ago we discovered a particularly simple entry to this class of heterocyclic compounds: The condensation ofo‐hydroxyacetophenones with aliphatic aldehydes and ketones in the presence of pyrrolidine leads to 4‐chromanones in good yields. The scope of application and limitations as well as modification of this versatile synthesis are outlined; the accompanying tables give a good indication of the wealth and variety of possible substituents. The reaction ofo‐hydroxyacetophenone,e. g.with glyoxylic acid or with α,β‐unsaturated ketones, proceeds atypically. The majority of the new chromanones can be used,inter alia, for the preparation of chromones, chromones and chromons, and for the synthesis of tricyclic and larger rin

ISSN:0570-0833    

DOI:10.1002/anie.198202471

出版商:Hüthig&Wepf Verlag    

年代:1982

数据来源: WILEY

摘要:

AbstractIn many cases the electrolytic synthesis of organic compounds is rather unselective. This is because the primary radical ions and/or radicals are very often so reactive that they are able to react competitively with several of the different species present in the electrolyte. The aim is to increase the selectivity by rationally altering the process parameters; this demands a knowledge,inter alia, of electrosorption effects and of the relationships between mass transfer and chemical reaction, at or in front of the electrodes (in the so‐called reaction layer). Four examples will be used to illustrate this theme:1The anodic synthesis of triarylsulfonium salts,2The addition of anodically generated N3‐radicals to olefins, producing “monomeric” 1,2‐diazides and “dimeric” 1,4‐diazides,3The cathodic synthesis of optically active 1‐(2‐pyridyl)ethanol,4The synthesis of 1,2‐ and 1,4‐diol ethers by anodic oxidation of vinyl compounds

ISSN:0570-0833    

DOI:10.1002/anie.198202561

出版商:Hüthig&Wepf Verlag    

年代:1982

数据来源: WILEY

摘要:

AbstractOwing to their particular crystallographic properties, ferrimagnetic hexagonal ferrites exhibit a far greater coercive force than the conventional magnetic pigments. They therefore appear to be suitable for use in magnetic information storage procedures, some of which are novel and are at the development stage. Thus, magnetic tapes of high coercive force containing barium ferrite could be used as master tapes for copying magnetic information or for producing forgery‐proof magnetic cards, if magnetic heads having high‐order write fields were successfully developed. Moreover, platelet‐like ferrite pigments in which the preferred direction of magnetic orientation is perpendicular to the plane of the platelet are of great interest for perpendicular magnetic recording.In this progress report, the crystal structures, magnetic characteristics of hexagonal ferrites, and chemical processes for their production are discussed. In particular, reactions in salt melts or under hydrothermal conditions produce finely divided pigments whose particles have a pronounced hexagonal, plate‐like habit, a narrow particle size distribution, and advantageous magnetic properties. The magnetic properties of the pigments crystallized from salt melts may be adjusted by cation e

ISSN:0570-0833    

DOI:10.1002/anie.198202701

出版商:Hüthig&Wepf Verlag    

年代:1982

数据来源: WILEY

摘要:

AbstractThe complete manuscript of this communication appears in:Angew. Chem. Suppl. 1982, 722. DOI:10.1002/anie.198207220

ISSN:0570-0833    

DOI:10.1002/anie.198202821

出版商:Hüthig&Wepf Verlag    

年代:1982

数据来源: WILEY

摘要:

AbstractThe complete manuscript of this communication appears in:Angew. Chem. Suppl. 1982, 582. DOI:10.1002/anie.198205820

ISSN:0570-0833    

DOI:10.1002/anie.198202831

出版商:Hüthig&Wepf Verlag    

年代:1982

数据来源: WILEY

摘要:

AbstractThe complete manuscript of this communication appears in:Angew. Chem. Suppl. 1982, 755. DOI:10.1002/anie.198207550

ISSN:0570-0833    

DOI:10.1002/anie.198202842

出版商:Hüthig&Wepf Verlag    

年代:1982

数据来源: WILEY

摘要:

AbstractThe complete manuscript of this communication appears in:Angew. Chem. Suppl. 1982, 718. DOI:10.1002/anie.198207180

ISSN:0570-0833    

DOI:10.1002/anie.198202851

出版商:Hüthig&Wepf Verlag    

年代:1982

数据来源: WILEY

摘要:

AbstractThe complete manuscript of this communication appears in:Angew. Chem. Suppl. 1982, 640. DOI:10.1002/anie.198206400

ISSN:0570-0833    

DOI:10.1002/anie.198202862

出版商:Hüthig&Wepf Verlag    

年代:1982

数据来源: WILEY

摘要:

AbstractThe complete manuscript of this communication appears in:Angew. Chem. Suppl. 1982, 660. DOI:10.1002/anie.198206600

ISSN:0570-0833    

DOI:10.1002/anie.198202871

出版商:Hüthig&Wepf Verlag    

年代:1982

数据来源: WILEY