Helvetica Chimica Acta

doi: 10.1002/hlca.19890720101pmid: N/A

Milon, Alain; Nakatani, Yoichi; Kintzinger, Jean‐Pierre; Ourisson, Guy

doi: 10.1002/hlca.19890720102pmid: N/A

Cycloartenol (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 of 4 by molecular modeling and NMR. Molecular modeling suggests that two conformations I and II coexist, differing mainly at the level of ring C, and of nearly equal energy, I and II each having ring A and B in a chair and half‐chair conformation, respectively, with ring C 1,3‐diplanar in I (solid‐state structure as determined by X‐ray crystallography) and in chair conformation in II. A complete assignment of the 1H‐ and 13C‐NMR spectra of 4 and 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 conformations I and II. Low‐temperature NMR experiments show a fast equilibrium between two conformations, presumably I and II. It is concluded, therefore, that the cyclopropane ring of 4 produces a flexibility at the level of ring C which may be important for the membrane properties.

Oppolzer, Wolfgang; Swenson, Rolf E.; Pachinger, Werner

doi: 10.1002/hlca.19890720103pmid: N/A

Dienyl acetates 2 lacking a geminal disulfone or a malonate group cyclize selectively to (E)‐l‐methylidene‐2‐styrylcyclopentanes 4 in the presence of 7 mol‐% of [Pd(PPh3)4].

Kussler, Manfred; Balli, Heinz

doi: 10.1002/hlca.19890720104pmid: N/A

On the Influence of the Acceptor Part of the Chromophor of Dimethinemerocyanine Dyes upon the Aggregation Tendency in Aqueous Solution

Weber, Andreas; Stämpfli, Urs; Neuenschwander, Markus

doi: 10.1002/hlca.19890720105pmid: N/A

Substituted methylidenecyclopropanes 12a–d, being easily available from 1,1‐dibromo‐2‐(phenylthio)‐cyclopropane (9a), are attractive precursors of triafulvene (2‐methylidene‐1‐cyclopropene; 1). Both the sulfoxide 12b and the sulfone 12c react with an excess of alkoxides (t‐BuOK and NaOMe) to give 12e and 12f, respectively, while the sulfinyl group of 12b may be replaced by the PhCH2S substituent in the presence of PhCH2SH/t‐BuOK. These reactions (Scheme 4) may be explained by assuming 1 as a reactive intermediate, although an alternative sequence including carbene 20 (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) of 12a–c is the most acidic proton. Final evidence for 1 results from the reaction of 12d with cyclopentadienide (Scheme 7): the reaction of 1 with cyclopentadiene produces the expected [4 + 2]‐cycloaddition product 23, while some mechanistic insight results from the sequence 12d → 24 → 25.

Weber, Andreas; Galli, Roberto; Sabbioni, Gabriele; Stämpfli, Urs; Walther, Stefan; Neuenschwander, Markus

doi: 10.1002/hlca.19890720106pmid: N/A

Attempted Synthesis of Calicene from Trisubstitued Cyclopropanes and Cyclopentenone

Matusch, Rudolf; Schmidt, Gerhard

doi: 10.1002/hlca.19890720107pmid: N/A

Competition of Endoperoxide and Hydroperoxide Formation in the Reaction of Singlet Oxygen with Cyclic, Conjugated Dienes

Trifonov, Latchezar S.; Orahovats, Alexander S.

doi: 10.1002/hlca.19890720108pmid: N/A

The reaction between the chiral allenic acid (+)‐(S)‐1 and the carbodiimides 2a–d and the keten‐imine 6 gives, under mild conditions, the tricyclic compounds 3–5, 7, and 8. Low diastereoselectivity and a partial loss of optical activity are observed. A stepwise mechanistic pathway via a biradical intermediate is postulated.

Pindur, Ulf; Pfeuffer, Ludwig; Kim, Myung‐Hwa

doi: 10.1002/hlca.19890720109pmid: N/A

2‐Vinylindole (1a) and its donor‐ and acceptor‐substituted (E)‐derivatives 1b–e react highly locoselectively with dimethyl 1,2,4,5‐tetrazine‐3,6‐dicarboxylate (3) to form the novel (indol‐2‐yl)‐1,4‐dihydropyridazines 4a and 7 as well as the heterocyclic annellated pyridazines 4b, 5, and 6. The reactions of the structurally related 3‐vinylindoles 2a–e with 3 also gave rise to new indol‐3‐ylpyridazines 8, 9, and 10. The locoselectivities of these Diels‐Alder reactions were controlled mainly by steric effects.

Bally, Thomas; Roth, Kuno; Straub, Rolf

doi: 10.1002/hlca.19890720110pmid: N/A

The molecular cations of 1,3,5‐cyclooctatriene (COT+·) and bicyclo[4.2.0]octa‐2,4‐diene (BCO+·) can be observed intact in γ‐irradiated Freon glasses, as opposed to Ar matrices where ionization is accompanied by extensive rearrangements. This proves the superiority of the Freon‐glass method for the generation of metastable primary cations. The electronic absorption (EA) spectra reveal a) that a previous analysis of the BCO+· electronic structure on the basis of the ultraviolet photoelectron (UP( spectrum of BCO was incomplete, and b) 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 structure of COT+·.