Paul Margaretha

Electrochemical reduction of 2-chloro-2-cyclohexenones

Abstract The electrochemical reduction of the 2-chloro-2-cyclohexenones 2a–2c (mercury cathode, CH 3 CN,Bu 4 NBF 4 ) was studied by means of cyclic voltammetry, dc polarography, coulometry and chemical product analysis. The results were compared to those obtained with the analogous 2-cyclohexenones 1a–1c. Due to the chloro-substituent on C —2 the half wave potentials E 1/2 for the first electron transfer step become less cathodic and the overall mechanism of reduction of a chloro-substituent on C —6 is modified. In addition, strong inhibition effects were observed in the cyclic voltammograms of compound 2 and it was found that other 2-halogeno-2-cycloalkenones exhibit a similar behaviour.

Solid State Photochemistry of Isocoumarins and Isothiocoumarins

Abstract 1H-2-Benzothiopyran-1-ones (isothiocoumarins) and 1H-2-benzopyran-1-ones (isocoumarins) behave dissimilarly on solid state irradiation at room temperature as the former photocyclodimerize efficiently while the latter only undergo slow photodecomposition. Both 4H-naphtho[2,1-c]pyran-4-one and thiopyran-4-one turn out to be photostable under these same reaction conditions.

Organic sulfuranyl radicals

Sulfuranyl radicals are intermediates in a variety of addition/elimination reactions of sulfides, disulfanes, sulfoxylates and S-neterocycles as well as in the reduction of triorgano- and heteroorgano sulfonium salts. Their lifetimes — depending on their substutution pattern — range from < 1 μs to being thermally stabee in solution at room temperature with no reduction in concentration over severll months.

The photochemical behaviour of cyclic oxa-enones

Abstract Photodimerization of the title compounds 1 and 2 , as well as photoaddition to polarized olefins, proceed in a regiospecific manner. Product analysis by NMR is facilitated because of the simple spectra of the oxa-enones. A reaction pathway for the photoaddition of 1 to 2-butene is suggested.

(1:1)-Kondensationsprodukte von Aldehyden mit Dimedon

ZusammenfassungWährend die Kondensation von Dimedon mit Aldehyden ausschließlich zuMichaeladditionsprodukten (Molverhältnis 2:1) führt, erhält man aus dem Natriumsalz des Dimedons und Aldehyden die entsprechenden Äthylene (1:1-Produkte); diese sind starke organischeLewissäuren.AbstractMichael reaction-type condensations of aldehydes with dimedone generally lead to reaction products which are made up from 2 moles of aldehyde and 1 mole of dimedone. However, if the sodium salt of dimedone is used, reaction products corresponding to a molar ratio 1:1 are obtained. These substituted ethylenes are strong organicLewis acids.

Structure determination of photocyclodimers of 2-cycloalkenones via enantioselective gas chromatography and GC/MS analysis

Abstract Enantioselective gas chromatography combined with GC/MS analysis allows the direct assignment of constitution and configuration of the photocyclodimers of cyclic enones 1a – 1h .

Studies on the photochemical reactions of .alpha.,.beta.-acetylenic ketones with tetramethylethylene

Photolyse de melanges de pentyne-3one-2 et de tetramethylethylene dans C 6 H 6 conduisant aux cis- et trans-ethylidene-4 methylene-5 tetramethyl-2,2,3,3 tetrahydrofuranne se transposant en vinyldihydrofuranne et en oxetanne. Mecanisme: mise en jeu de carbenes

Photocycloadditions to 1-thiocoumarin

Abstract Irradiation (λ = 350 nm) of 1-thiocoumarin (1b) in benzene or acetonitrile produces one photodimer (head-to-head, anti) selectively. In the presence of 2,3-dimethylbut-2-ene, a 4:1 mixture of cis- and trans-fused [2+2] cycloadducts is obtained. Cycloaddition to 2-methylpropene proceeds regioselectively, the product being established by a proton nuclear magnetic resonance (nuclear Overhauser effect) experiment.

Photocycloaddition of Cyclohex-2-enones to 2-Methylbut-1-en-3-yne

Irradiation (350 nm) of 2-alkynylcyclohex-2-enones 1 in benzene in the presence of an excess of 2-methylbut-1-en-3-yne (2) affords in each case a mixture of a cis-fused 3,4,4a,5,6,8a-hexahydronaphthalen-1(2H)-one 3 and a bicyclo[4.2.0]octan-2-one 4 (Scheme 2), the former being formed as main product via 1,6-cyclization of the common biradical intermediate. The (parent) cyclohex-2-enone and other alkylcyclohex-2-enones 7 also give naphthalenones 8, albeit in lower yields, the major products being bicyclo[4.2.0]octan-2-ones (Scheme 4). No product derived from such a 1,6-cyclization is observed in the irradiation of 3-alkynylcyclohex-2-enone 9 in the presence of 2 (Scheme 4). Irradiation of the 2-cyano-substituted cyclohexenone 12 under these conditions again affords only traces of naphthalenone 13, the main product now being the substituted bicyclo[4.2.0]oct-7-ene 16 (Scheme 5), resulting from [2+2] cycloaddition of the acetylenic C−C bond of 2 to excited 12.

Photochemistry of 6-trifluoromethyl-4,4-dimethyl-2-cyclohexenone

Abstract The photochemical behaviour of the title compound, newly synthesized in four steps from 4,4-dimethylcyclohexanone, is compared to that of 4,4-dimethyl- and 4,4,6-trimethyl-2-cyclohexenone. All three enones undergo rearrangement to bicyclo[3.1.0]hexan-2-ones and 3-isopropyl-2-cyclopentenones in benzene, t.butanol and acentonitrile, but only the CF 3 -enone is also reduced to the saturated ketone in the latter two solvents.