摘要:

AbstractCycloartenol (4), a natural plant sterol, was shown to be an effective membrane reinforcer; this was attributed to its conformation. We now present a conformational analysis of4by molecular modeling and NMR. Molecular modeling suggests that two conformationsIandIIcoexist, differing mainly at the level of ring C, and of nearly equal energy,IandIIeach having ring A and B in a chair and half‐chair conformation, respectively, with ring C 1,3‐diplanar inI(solid‐state structure as determined by X‐ray crystallography) and in chair conformation inII. A complete assignment of the1H‐ and13C‐NMR spectra of4and the entire coupling network in rings A and B is determined by various modern NMR techniques. The conformation of rings A and B thus determined is in agreement with conformationsIandII. Low‐temperature NMR experiments show a fast equilibrium between two conformations, presumablyIandII. It is concluded, therefore, that the cyclopropane ring of4produces a flexibility at the level of ring C which may be important for the membra

ISSN:0018-019X    

DOI:10.1002/hlca.19890720102

出版商:WILEY‐VCH Verlag GmbH    

年代:1989

数据来源: WILEY

摘要:

AbstractDienyl acetates2lacking a geminal disulfone or a malonate group cyclize selectively to (E)‐l‐methylidene‐2‐styrylcyclopentanes4in the presence of 7 mol‐% of [P

ISSN:0018-019X    

DOI:10.1002/hlca.19890720103

出版商:WILEY‐VCH Verlag GmbH    

年代:1989

数据来源: WILEY

摘要:

On the Influence of the Acceptor Part of the Chromophor of Dimethinemerocyanine Dyes upon the Aggregation Tendency in Aqueous SolutionA series of novel long‐chain alkyl‐dimethinemerocyanine dyes containing sulfonate groups is synthesized in good yield and their aggregation tendency in H2O and in organic solvent/H2O mixtures investigated. The merocyanine dyes11and15only exhibit an intense, red‐shifted absorption band due to J‐aggregation in H2O without addition of salt. The influence of cationic and anionic tensides and other compounds as urea, glucose, and polysaccharides upon the J‐aggregation tendency is inv

ISSN:0018-019X    

DOI:10.1002/hlca.19890720104

出版商:WILEY‐VCH Verlag GmbH    

年代:1989

数据来源: WILEY

摘要:

AbstractSubstituted methylidenecyclopropanes12a–d, being easily available from 1,1‐dibromo‐2‐(phenylthio)‐cyclopropane (9a), are attractive precursors of triafulvene (2‐methylidene‐1‐cyclopropene;1). Both the sulfoxide12band the sulfone12creact with an excess of alkoxides (t‐BuOK and NaOMe) to give12eand12f, respectively, while the sulfinyl group of12bmay be replaced by the PhCH2S substituent in the presence of PhCH2SH/t‐BuOK. These reactions (Scheme 4) may be explained by assuming1as a reactive intermediate, although an alternative sequence including carbene20(Scheme 6) is not completely ruled out.D‐labelling experiments (Scheme 5) do not give conclusive evidence due to D scrambling, but deprotonation/methylation sequences show that HC(2) of12a–cis the most acidic proton. Final evidence for1results from the reaction of12dwith cyclopentadienide(Scheme 7):the reaction of1with cyclopentadiene produces the expected [4 + 2]‐cycloaddition product23, while some mechanistic insight results

ISSN:0018-019X    

DOI:10.1002/hlca.19890720105

出版商:WILEY‐VCH Verlag GmbH    

年代:1989

数据来源: WILEY

摘要:

Attempted Synthesis of Calicene from Trisubstitued Cyclopropanes and CyclopentenoneThe Li carbenoids4, prepared by treatment of substituted 1,1‐dihalocyclopropanes with BuLi, are reacted with cyclopent‐2‐enone under thermodynamic and kinetic control (Scheme 1). In general, the latter procedure gives better yields of cyclopropylcyclopentenols5a–e, but the reaction seems to be controlled mainly by the steric and electronic properties of the substituent Y. So, with4band4e, the main reaction is the attack of the carbenoid at C(1) of cyclopent‐2‐enone, while4a(Y = PhS) predominantly deprotonates the ketone (Scheme 4). Whereas5dand5ecan easily be converted to the dihydrocalicenes6dand6e(Scheme 6), the attempted elimination of H2O from5a–cleads to the rearranged products13–2due to the opening of the cyclopropane ring (Scheme 5). Finally, the generation of the parent compound2from the silylated precursor6dis attempted: treatment with MeO−gives the addition products18A/18B, while the reaction with Br2provides19by a bromination/dehydrobromination se

ISSN:0018-019X    

DOI:10.1002/hlca.19890720106

出版商:WILEY‐VCH Verlag GmbH    

年代:1989

数据来源: WILEY

摘要:

Competition of Endoperoxide and Hydroperoxide Formation in the Reaction of Singlet Oxygen with Cyclic, Conjugated DienesRose‐bengal‐sensitized photooxygenation of (−)‐(R)‐α‐phellandrene (1) in MeOH at room temperature yielded a complex mixture of products, contrary to previous reports describingcis‐(3S, 6R)‐epidioxy‐p‐menthene (2) andtrans‐(3R, 6S)‐epidioxy‐p‐menthene (3) as the only products. The mixture was separated by prep. HPLC (silica gel, pentane/Et2O 9:1). Besides the known endoperoxides2(yield 39%) and3(26%), all those hydroper‐oxides, which can be deduced from an ene reaction of1O2with1, were isolated,i.e.4β‐p‐mentha‐2,5‐dien‐1β‐yl hydroperoxide (4) (14%), 4β‐p‐mentha‐2,5‐dien‐1α‐yl hydroperoxide (5) (9%), (2R, 4R)‐p‐mentha‐1(7), 5‐dien‐2‐yl hydroperoxide (6) (2,1%), (2S, 4R)‐p‐mentha‐1(7),5‐dien‐2‐yl hydroperoxide (7) (1,5%) and (1R)‐p‐mentha‐3,6‐dien‐yl hydroperoxide (8; 1,5%;Scheme 1). Furthermore, the constantcis/transratio for all diastereoisomeric pairs (2/2,4/2,6/2) was striking. With the help of the two possible conformers1aand1bof the starting material a model of a common first step for endoperoxide as well as for hydroperoxide formation is developed. A photooxygenation at −50° supports this model. The absolute

ISSN:0018-019X    

DOI:10.1002/hlca.19890720107

出版商:WILEY‐VCH Verlag GmbH    

年代:1989

数据来源: WILEY

摘要:

AbstractThe reaction between the chiral allenic acid (+)‐(S)‐1and the carbodiimides2a–dand the keten‐imine6gives, under mild conditions, the tricyclic compounds3–5,7, and8. Low diastereoselectivity and a partial loss of optical activity are observed. A stepwise mechanistic pathwayviaa biradical intermediate is p

ISSN:0018-019X    

DOI:10.1002/hlca.19890720108

出版商:WILEY‐VCH Verlag GmbH    

年代:1989

数据来源: WILEY

摘要:

Abstract2‐Vinylindole (1a) and its donor‐ and acceptor‐substituted (E)‐derivatives1b–ereact highly locoselectively with dimethyl 1,2,4,5‐tetrazine‐3,6‐dicarboxylate (3) to form the novel (indol‐2‐yl)‐1,4‐dihydropyridazines4aand7as well as the heterocyclic annellated pyridazines4b,5, and6. The reactions of the structurally related 3‐vinylindoles2a–ewith3also gave rise to new indol‐3‐ylpyridazines8,9, and10. The locoselectivities of theseDiels‐Alderreactions were co

ISSN:0018-019X    

DOI:10.1002/hlca.19890720109

出版商:WILEY‐VCH Verlag GmbH    

年代:1989

数据来源: WILEY

摘要:

AbstractThe molecular cations of 1,3,5‐cyclooctatriene (COT+·) and bicyclo[4.2.0]octa‐2,4‐diene (BCO+·) can be observed intact in γ‐irradiatedFreonglasses, as opposed to Ar matrices where ionization is accompanied by extensive rearrangements. This proves the superiority of theFreon‐glass method for the generation of metastable primary cations. The electronic absorption (EA) spectra reveala) that a previous analysis of the BCO+·electronic structure on the basis of the ultraviolet photoelectron (UP( spectrum of BCO was incomplete, andb) that COT undergoes planarization upon ionization, in accord with qualitative expectations and MNDO calculations. A previously proposed LCBO model was used to assess the influence of the above geometry change on the electronic struc

ISSN:0018-019X    

DOI:10.1002/hlca.19890720110

出版商:WILEY‐VCH Verlag GmbH    

年代:1989

数据来源: WILEY

摘要:

AbstractMechanistic evidence for the light‐induced ring opening of thiophen‐2(5H)‐ones 1 in alcohols affording α, β‐unsaturated mercapto esters2is presented. Regio‐and stereochemical aspects of the ring closure of alkenylthio (type3) radicals15and17to S‐heterocycles16and18, of 3‐thiahex‐5‐enyl radicals4to (tetrahydrothien‐3‐yl)methyl radicals6and of (2,3‐dihydrothien‐3‐yl)methyl radicals30(type 7, but‐3‐enyl radicals) to cyclopropane‐methyl radicals29are discussed. Irradiation (λ 350 nm) of 1 in cyclohexane in the presence of 2,3‐dimethylbut‐2‐ene affords [2 +

ISSN:0018-019X    

DOI:10.1002/hlca.19890720111

出版商:WILEY‐VCH Verlag GmbH    

年代:1989

数据来源: WILEY