摘要:

AbstractDetailed structural studies of five pairs of diastereomeric salts of ephedrine and halogen‐substituted chiral cyclic phosphoric acids indicate a correlation between the efficiency of the resolution process and the lattice energy difference between the salts of a diastereomeric pair. Attempts to model such a correlation quantitatively by lattice energy calculations with Molecular Mechanics (MM) methods have not yet been successful. The MM techniques proved unable to reproduce the crystal structures with sufficient accuracy. However, the results of the analyses qualitatively support the presumed correlation, and show that the five resolutions are directed by different acid‐base interactions in the hydrophobic lay

ISSN:0165-0513    

DOI:10.1002/recl.19921110301

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

Abstract5β‐Ethylthebaine(3), together with 5β‐butylthebaine(4), was obtained from thebaine(1)after reaction with butyllithium and ethyl iodide. With butyllithium and diethyl sulfate, thebaine yielded 5β‐ethylthebaine(3), 5β, 10α‐diethylthebaine(5), and 5β, 10α, 10β‐triethylthebaine(6). The ratio of the compounds (3:4and3:5:6) depended on the excess of reagents used. Employing the reversed sequence of addition of reagents, that is, first diethyl sulfate and then butyllithium, we prepared 5β, 10α‐diethylthebaine(5)and 5β‐methyl‐10α‐ethylthebaine (8) from 5β‐ethylthebaine (3) and 5β‐methylthebaine (7), respectively. These alkylation reactions are discussed in terms ofPearson's hard‐soft acid‐base theory.Diels‐Alder reaction of3, 5and8with ethyl acrylate gave exclusively 7α‐substituted 6α, 14α‐ethenoisomorphinans9, 10and11, respectively. Esters of the cycloadducts9and11were converted into their tertiary alcohols12and13with methylmagnesium bromide. The 3‐methoxy group in12and13was hydrolyzed with a solution of potassium

ISSN:0165-0513    

DOI:10.1002/recl.19921110302

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractIron(III)‐exchanged sepiolites exhibit dual behaviour as catalysts in the reactions of α‐acetoxystyrenes1a,b; operating as Lewis acids (to give2–5) in thermolysis reactions under reduced pressure or as single‐electron acceptors (to give 7) in photolysis under aerobic co

ISSN:0165-0513    

DOI:10.1002/recl.19921110303

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractMandelic and lactic acids are converted to the 1,3‐dioxolan‐4‐ones by treatment with acetone dimethyl acetal. Deprotonation followed by treatment with an allyl acetate and a catalytic amount (1 mol %) of palladium catalyst afforded the allylated dioxolanones, which could be hydrolyzed to the corresponding α‐allyl α‐hydroxy acids. The lithium enolate of the dioxolanone of mandelic acid was also coupled with methallyl, cinnamyl, geranyl and (E)‐1‐methyl‐2‐butenyl acetates. The zinc enolate of the dioxolanone of lactic acid reacted smoothly with allyl acetate in a catalyzed reaction whereas no detectable reaction was observed when the lithium enolate was used. This appears to be the result of complications arising from the enhanced basicity of the lithium compared to zinc enolate.Various attempts were made to achieve enantioselectivity using chiral ligands on the palladium catalyst. The zinc enolates were found to provide better results although the enantioselectivity was only modest, about 30% enantiomeric excess being the best result obtained. Chiraphos proved to be the best optically active ligand

ISSN:0165-0513    

DOI:10.1002/recl.19921110304

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractA theoretical and NMR‐spectroscopic conformational analysis is presented of the 3‐methyl‐1,3,5‐hexatrienes and of (Z)‐3‐tert‐butyl‐1,3,5‐hexatriene. It is shown that (E)‐3‐methyl‐1,3,5‐hexatriene exists mainly as thetEtrotamer and (Z)‐3‐methyl‐1,3,5‐hexatriene as thetZtrotamer. Indications are found for steric strain involving the methyl group of theEisomer. In contrast to earlier theoretical predictions, this strain does not lead to detectable amounts ofs‐cisrotamers.Ab‐initiocalculations reported here agree with this experimental finding, but MMP2 calculations underestimate the energy of thes‐cisform.1H NMR relaxation rates and Nuclear Overhauser Enhancements (NOE) are shown to allow a reliable determination of the predominant ground‐state conformation of alkyl‐substituted hexatrienes.NOE experiments on (Z)‐3‐ter‐butyl‐1,3,5‐hexatriene reveal that atert‐butyl group changes the preferred conformation fromtZttocZtbut, surprisingly, is unable to completely counteract the strong intrinsic preference for a spatially extendeds‐trans,s‐transconformation

ISSN:0165-0513    

DOI:10.1002/recl.19921110305

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractThe diastereogenic addition of (phenyldimethylsilyl)magnesium chloride to methyl 3‐O‐allyl‐2,4‐di‐O‐benzyl‐α‐D‐manno‐hexodialdo‐1,5‐pyranoside gave a L‐glycero‐D‐manno‐heptopyranoside (L,D‐Hepp) derivative having at C‐7 the phenyldimethylsilyl function as a hydroxy‐masking group. The L,D‐Hepp) derivative was converted into two acceptors which could be applied successfully for the preparation of the trisaccharides L‐α‐D‐Hepp)‐(1→3)‐O‐[L‐α‐D‐Hepp‐(1→7)]‐L‐

ISSN:0165-0513    

DOI:10.1002/recl.19921110306

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

Abstract(12,13‐13C2)Retinal, (13,14‐13C2)retinal, (19‐13C)retinal and (20‐13C)retinal (1) were prepared in a simple fashion in high yield via a consecutive strategy. The key step is the reaction of aN‐methoxy‐N‐methylamide with an alkyllithium or a Grignard reagent. The preparation of the requiredN‐methoxy‐N‐methylamide is discussed. In this scheme, only three commercially available13C‐labeled starting materials (ethyl bromoacetate, acetonitrile and methyl iodide) are sufficient to construct retinals with any possible combination of13C labeling in the conjugated tail end. This strategy is applicable to the preparation of many other conjugated systems, such as retinoids, car

ISSN:0165-0513    

DOI:10.1002/recl.19921110307

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractAn A‐ring‐derived analog of the natural germination stimulant, Strigol (1), has been prepared from citral in an unambiguous manner. This analog,2, a γ‐hydroxy aldehyde for which a high stimulant activity was claimed in the literature, has been re‐evaluated as a germination stimulant for seeds of parasitic weeds and was found to be inactive. It was also shown that analog2is rather stable in aqueous solution, in contrast to other reports.In addition, some related A‐ and AB‐ring‐derived analogs have been prepared and biologically evaluated. The γ‐hydroxy acid12was obtained in one step from a mixture of α‐ and β‐cyclocitral in 58% yield. The dihydroxy analog14was prepared by reduction of the γ‐hydroxy ester13. The AB‐ring analog15has been synthesized in five steps from ester13following published procedures. All analogs have been evaluated usingStrigaandOrobancheseeds; none of the compounds induced germination. It was concluded that the actiphore of strigol does not reside in

ISSN:0165-0513    

DOI:10.1002/recl.19921110308

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

ISSN:0165-0513    

DOI:10.1002/recl.19921110309

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractCondensation of R‐(+)‐1‐(2‐chlorophenyl)‐2,2‐dimethyl‐propan‐1,3‐diol (2) with a derivative (1) of 1‐formylcyclohexene substituted in the 3‐position with an acetoxy group proceeded with absolute stereospecificity at the acetal center. Catalytic alkylation of the π‐allyl complex obtained upon reaction with Pd with the anion of dimethylamalonate proceeded at the 3‐position of the cyclohexyl ring w

ISSN:0165-0513    

DOI:10.1002/recl.19921110310

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY