Intramolekulare Diels‐Alder‐Additionen in 6‐(But‐3‐enyl)‐6‐methyl‐cyclohexa‐2,4‐dien‐1‐on‐Systemen; eine neue Synthese von Twistanderivaten

Abstract

Me 15 (357;) Verhaltnis ' 85 : 15. 17 koiinte in Sproz. Ausbeute isoliert werden; weitere Reaktionsprodukte (> 0,2%) liessen sicli gas-chromatographisch nicht nachweisen. 16 17 (SO,;) A logoy: + B with thc constants A = 0.788 and B = 0,108 and a correlation coefficient of 0.983.Introduction. -Bimolecular reactions between anionic nucleophiles and neutral substrates are known to be accelerated by several orders of magnitude in certain dipolar aprotic compared with protic solvents [l] [ Z ] . Two factors must be considered

Radical Annulation vs the 3-Exo-Trig Cyclization via Fine Tuning of the Substrate Structure1

“…The structure of the annulated product 7 was established from its spectral data (see the Experimental Section). The exo,exo stereochemistry at the carbons C-4 and C-2 bearing the methoxycarbonyl and aryl groups was assigned on the basis of the weak coupling (2.6 and 0 Hz) of the C-2 and C-4 endo protons with the bridgehead proton at C-3 . The generation of the isotwistane 7 is totally in agreement with our reasoning that only an o -methyl group is enough (instead of the naphthyl group) to change the course of the reaction and to bring about the radical annulation reaction.…”

Aryl Cation and Carbene Intermediates in the Photodehalogenation of Chlorophenols

“…Preparation of 2,6‐dimethyl‐4‐methoxyphenyl tert‐ butyldimethylsilyl ether (12) and 2,6‐dimethyl‐4‐trifluoroethoxyphenyl tert‐ butyldimethylsilyl ether (12′) : Authentic samples of these compounds for comparison with the photochemically obtained products were prepared from 2,6‐dimethyl‐4‐methoxy‐ (or 4‐trifluoroethoxy)‐phenol. In turn, these were obtained from 2,6‐dimethylhydroquinone, obtained by sodium dithionite reduction of 2,6‐dimethyl‐1,4‐benzoquinone,27 followed by selective acid catalyzed methylation or, respectively, trifluoroethylation)28 in the corresponding alcohols, by using the same procedure as above.…”

Tandem Oxidative Acetalization−Intramolecular Diels−Alder Reactions of 2-Methoxyphenols. Simple Synthesis of Bicyclo[2.2.2]octenone Derivatives

“…Unambiguous determination of the structures of products of these intramolecular Diels−Alder reactions makes clear the fact that the relative stereochemistry around the alkene double bond is transmitted to the product. Although there exist precedents for the formation of regioisomers in the intramolecular cycloadditions of certain 2,4-cyclohexadienones that are tethered at C 6 to alkenes by two- and three-carbon spacers, 23d,e in the present cases only single products were produced and the products of type A were never observed.

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“…It is pertinent to mention that there are quite a few precedents for the intramolecular Diels−Alder reactions of 2,4-cyclohexadienones tethered to alkene moieties with all-carbon or heteroatom containing spacers, prepared by means of lengthy sequences of reactions. − Notable and shortest among them is the Yates approach, which provides tricyclic lactones in low yields via Wessely oxidation of phenols with lead tetraacetate in the presence of substituted acrylic acids followed by cycloaddition of thus formed acrylates upon heating . In the present case also, the tethering has been achieved in a similar manner by an oxidation reaction but through an acetal.…”