摘要:

AbstractIn an ideal synthesis, the formation of the CC bonds as well as the creation of the chiral centers and of the final functionality would be carried out simultaneously in each step. Accordingly, stereoselective CC bond forming reactions are required. Instead of a pair of diastereomers, a synthesis should yield only one of the two possible diastereomers. Such selectivity is often achieved in the addition of crotylboron, ‐aluminum, ‐tin, ‐titanium, and ‐chromium compounds to aldehydes. Depending on whether one starts fromE‐ orZ‐crotyl compounds, one diastereomeric adduct is formed preferentially. In this case the newly formed CC bond carries a methyl substituent. However, the synthetic goal is often a compound carrying an oxygen function at the site of the new CC bond. Compounds of this type can be obtained through addition of heteroatom‐substituted allylboronic acid esters or analogous reagents

ISSN:0570-0833    

DOI:10.1002/anie.198205553

出版商:Hüthig&Wepf Verlag    

年代:1982

数据来源: WILEY

摘要:

Abstract“From Configurational Notation of Stereoisomers to the Conceptual Basis of Stereochemistry” is the title of a lecture [V. Prelog, ACS Symp. Ser. No. 12 (1975) 179] in which the chronologically and alphabetically third member of the CIP‐triumvirate discusses the history of the CIP‐system. The experience accumulated in the meantime, particularly the efforts to teach computers to use the system as described in 1966 by Cahn, Ingold and Prelog, has shown that certain of its aspects must be more strictly formulated and others modified. Questions dealing with stereoisomers (and stereotopicity), which previously have been treated predominantly in a pragmatic manner, are now discussed in a systema

ISSN:0570-0833    

DOI:10.1002/anie.198205671

出版商:Hüthig&Wepf Verlag    

年代:1982

数据来源: WILEY

摘要:

AbstractSince the stereoisomers of molecules with one or more asymmetric centers often exhibit different biological activities (e.g.thalidomide, pheromones), stereoselective synthesis as a method of preparative chemistry is rapidly attaining importance. Of the numerous drugs prepared by total synthesis that contain at least one asymmetric center, only about 20% have so far been used in sterically pure form. Amino acids constitute the greatest “chiral pool” of compounds whose enantiomers can be obtained commercially in large amounts; they are gradually being used more and more frequently as auxiliary agents or educts in asymmetric synthe

ISSN:0570-0833    

DOI:10.1002/anie.198205841

出版商:Hüthig&Wepf Verlag    

年代:1982

数据来源: WILEY

摘要:

AbstractIn any literature searh one must be able topredictwhich modes of expression have been used to represent the concepts of interest in the search file. It is these lingual expressions which will be looked up or will constitute the search parameters in a mechanized search. In the case ofgeneralconcepts and statements, uncontrolled natural language lacks the required representational predictability, as is demonstrated, for example, for classes of chemical compounds and for types of reactions. Hence, considerable loss of relevant information is bound to occur in searches for these concepts in free text files, regardless of how advanced a computer program may be. By restricting the modes of expression as is achieved by means of an indexing language and its reliable employment, predicability is increased and, consequently, information loss is drastically reduced.—In contrast, the modes of expression forindividualconcepts (e.g.individual chemical substances, authors, institutions) are, by their vary nature, often sufficiently predictable in uncontrolled natural language. In these cases, the value of translating them into indexing‐language expressions is often questionable. This holds true in particular if an indexing language does not (yet) provide adequate expressions to represent the concepts, and also in the case of ambiguous natural language expressions. Here, preserving the original, uncontrolled terms in the search file is advisable, at least in addition to an attempted translation into the indexing language.—The weaknesses of uncontrolled natural language files become apparent only at a rather late stage in the continual expansion of an information system. In its early stages the necessity of predictability is reduced through the effectiveness of human memory and the—initially—relatively high homogeneity of the experts' linguistic usage during that short period of time. Furthermore, loss of relevant information as a consequence of the lack of predictability will at first be minimal and will not yet be noticed as a steadily increasing source of search failure. Nor does the steady rise in costs for excogitating and phrasing queries with more and more alternatives become apparent at this stage. It is largely for these reasons that uncontrolled natural language systems have unjustifiably frequently been preferred. The typical weakness and strengths of both kinds of language suggest combining them so that their inherent strengths are retained and utilized as far as

ISSN:0570-0833    

DOI:10.1002/anie.198206081

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, 1360. DOI:10.1002/anie.198213600

ISSN:0570-0833    

DOI:10.1002/anie.198206171

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, 1392. DOI:10.1002/anie.198213920

ISSN:0570-0833    

DOI:10.1002/anie.198206182

出版商:Hüthig&Wepf Verlag    

年代:1982

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198206191

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, 1473. DOI:10.1002/anie.198214730

ISSN:0570-0833    

DOI:10.1002/anie.198206201

出版商:Hüthig&Wepf Verlag    

年代:1982

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.198206212

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, 1385. DOI:10.1002/anie.198213850

ISSN:0570-0833    

DOI:10.1002/anie.198206221

出版商:Hüthig&Wepf Verlag    

年代:1982

数据来源: WILEY