Synthese et reaction de diacyl-1,4 alkyl-4 dihydropyridines

The action of zinc and acetic, propionic or butyric anhydride on C-4 substituted pyridines gives 1,4-diacyl 4-alkyl dihydropyridines, which can also be obtained by a free radical reaction. The 1-acetyl 4-methylene 5-methyl 4,5-dihydro azepine 24 has been prepared by solvolysis of tosylate 23 prepared from 1,4-diacetyl 4-methyl dihydropyridine. The 4-methyl quinuclidinone 28 was prepared from 1,4-diacetyl 4-methyl dihydropyridine 4 by hydrogenation, hydrolysis, bromination and cyclisation.     

Réactions rétrodiéniques—XI: Synthèse de composés polyinsaturés réactifs par double réaction de rétro-diels-alder

Double retro-Diels-Alder cleavage by flash thermolysis gave the new, unstable polyunsaturated ketones and hydrocarbons 1–5 for which NMR, IR and UV spectra at low temperature are reported. The Diels—Alder adducts of compounds 1–3 with various dienes, the 1,6 additions of several nucleophilic agents with 4, and the isomerisation of 5 in p-ethynylstyrene support the proposed structures.     

Préparation de thiocétones acycliques α-insaturées. dimérisation régiosélective en 4h-dithiine-1,3

α-Unsaturated acyclic thioketones 6 a–d were prepared by a retro-Diels-Alder reaction using flash thermolysis technique. They were detected at low temperature (blue coloured pyrolysate) but dimerize in solution as soon as the temperature rose to ?60° (case 6 a–c) and ?20° (case 6 d). The structure of 6 d was established by UV and NMR spectra taken at ?60°. Dimeric products were proved to be 4H - 1,3 dithiin by spectral data and by univocal synthesis of 7 c and 7 d from the corresponding monomeric ketone. 1–3 Dithiin 7 d was isomerized at 190° into the thermodynamic isomer (8). This observed orientation of dimerisation agrees with theorical results obtained from molecular frontier orbital calculations.     

Radiosensibilisation de la thymine en presence de tetramethyl-2,2,6,6 piperidone-4 oxyle-1. caracterisation des produits d'addition

The reaction of 2,2,6,6-tetramethyl-4-piperidone-1-oxyl with radiation-formed 5,6-dihydro-6-hydroxy-thymin-5-yl and 5,6-dihydro-5-hydroxy-thymin-6-yl radicals in oxygen-free aqueous solution generate respectively two major products 2 and 3. The chemical assignment of both covalent addition derivatives is based mainly on ¹³C NMR and mass spectrometry (chemical ionization and field desorption) analysis. The N-oxide 2, which was prepared in a specific manner, undergoes Cope elimination and radical rearrangement within the pyrimidine ring.     

Rearrangement photochimique de 4h-pyrannes et de dienones en 2h-pyrannes

The photoreactivity of the 4H-pyrans 1 was studied in detail by fluorescence and phosphorescence, by deactivation measurements for the singlet state and by the determination of the accessible quantum yields and rate constants. The photochemical isomerisation of the 4H-pyrans 1 into 2H-pyrans 2 occurs with concerted migration of the benzyl group, in a singlet state as a sigmatropic 1, 3-transposition. The 2H-pyrans 2 are also obtained by irradiation of the nonconjugated dienone isomers 3 of the 2H-pyrans 2.     

Selectivite de la reduction de cyclohexen-2 ones par NBu4BH4 dans divers solvants et par NaBH4 dans des conditions de transfert de phase

The 2-cyclohexenone 1 and isophorone 2 reductions with NBu₄BH₄ in aprotic solvents lead to a highly preferential 1–4 attack; i.e. 85% with 1 and 96% with 2 in THF. These regioselectivities are nearly the same as those observed with LiBH₄ in the presence of[2.1.1) cryptand confirming thus the cation influence. This method which is inexpensive and easy to work up, seems to constitute a general way to reduction of α-enones to saturate alcohols while other reagents such as K(sec Bu)₃BH are not able to reduce the carbon-carbon double bond of isophoron.Phase transfer catalysis conditions are not useful for selective reduction: large amounts of allylic alcohols are formed in liquid-liquid phase transfer conditions (60% in toluene-water); a good regioselectivity is only obtained when a cryptand is used as a transfer agent in solid-liquid conditions.     

Mecanisme de formation et de transformation des spirophosphoranes—IX: Reactions des spirophosphoranes a liaison PH avec les ethers vinyliques et les enamines

The reaction of 1,4,6,9-tetraoxa-5-phospha(V)spiro[4,4]nonane 1 with ethyl vinyl ether gives a spirophosphorane containing a PC bond, 5-(β-ethosyethyl)-1,4,6,9-tetra-oxa-5-phospha(V) spiro[4,4]nonane 2 (radical reaction), and a tricoordinated phosphorus compound, 2-(3,5-di-oxa-4-methylheptanoxyl)-1,3,2-dioxaphospholane 3 (ionic reaction). 2,2,3,3,7,7,9-Heptamethyl-1,4,6-trioxa-9-aza-5-phospha(V) spiro[4,4]nonane 6 gives exclusively a spirophosphorane containing a PC bond, 5(β-ethoxyethyl)-2,2,3,3,7,7,9-heptamethyl-1,4,6-trioxa-9-aza-5-phospha(V)spiro[4,4]nonane 7. The reaction of 1 with alcohol or ethyleneglycol and enamine yields a pentaoxyspirophosphorane and an amine by an oxidation-reduction condensation. Suggested mechanisms of these reactions are presented.     

Étude de réactions photochimiques—XXIV: Réactions intramoléculaires de type ionique des alcools allyliques et homo-allyliques: Une approche vers l'étude conformationnelle des cholestènes-4 et cholestènes-5 excités

The C₅C₆ double bond of triplet-excited homoallylic alcohols 1, 5, 8, 10 and 13 is deactivated by protonation. Three secondary intramolecular processes follow: (a) addition to yield the oxetans 2, 6, 9, 11, 14; (b) fragmentation (followed by photocycloaddition which gives the oxetans 3, 12, 15); (c) isomérisation (Δ⁵ → Δ⁴). The C₄C₅ double bond of excited allylic alcohols 4, 16, 18, 20 and 7 is deactivated in the same way but only one secondary process is observed: fragmentation (followed by photocycloaddition giving rise to the oxetans 3, 12, 17, 19 and 21). Reactions in both homoallylic and allylic series have the same carbonium ion as intermediate. The triplet-excited homoallylic series have a conformation different from the triplet excited allylic series. The particular reactivity of each series is assigned to the conformational difference.     

Reactivite des carbenes et carbenoides issus de diazomalonates mixtes de methyle et de benzyles substitues. Synthese de composes cycloheptatrieniques

The title diazomalonates, when treated with copper powder in octane, yield lactones of 1-hydroxymethyl 7-carboxy cycloheptatrienes. The yields of cyclisation product from such diazoesters may be increased either by a gem-dimethyl grouping or by an electron rich mesomeric group attached to the benzene ring. The methoxy-cycloheptatriene compound 3d is hydrolysed to the spirodienone 8. The carbenes formed from the diazomalonates 2a and b by photolysis or thermolysis undergo Wolff rearrangement and yield insertion products into CH bonds of solvent.     

Silver(I) Compounds with Perfluorinated Aromatic Thiols C6F5C6F4SH,C10F7SH,and C6F5SH as Precursors for Silver Nanoparticles

Silver(I) compounds with perfluorinated aromatic thiols (4-nonafluorodiphenylthiol C₆F₅C₆F₄SH (HL¹), 2-heptafluoronaphthalenethiol C₁₀F₇SH (HL²), and pentafluorothiophenol C₆F₅SH (HL³), namely AgL¹ (I), AgL² (II), AgL³ (III), were prepared. The thermal properties of compounds I–III and the composition of thermolysis products were studied. By powder X-ray diffraction and electron microscopy, it was shown that thermolysis of compounds I–III under argon and thermolysis of compound III in air yield Ag nanoparticles.     

Synthèse de N-Alkyl-arylamines par Thermolyse ou Photolyse d'Alkyl-3-diaryl-1,3-triazènes

Synthesis of N-Alkyl-arylamines by Thermolysis or Photolysis of 3-Alkyl-1,3-bis(p-chlorophenyl)triazenes The thermolysis of 3-alkyl-1,3-bis(p-chlorophenyl)triazenes in benzene or methanol leads to the formation of N-alkyl-p-chloroanilines ( 2 ) in 19–50% yield, N-alkyl-bis(p-chlorophenyl)amines ( 3 ) in 7.5–15.5% yield and 4,4′-dichlorobiphenyle ( 4 ) in 1–7% yield; besides with benzene as the substrate, 4-chlorobiphenyle ( 5 ) (12–20% yield) was also formed. The photolysis in methanol gives only the N-alkyl-p-chloroanilines ( 2 ) in 32–40% yield. In these two cases the results are consistent with a radical pair formation in a solvent cage, recombination (thermolysis) and/or diffusion (thermolysis and photolysis) with intermolecular abstraction of hydrogen. A free radical chain mechanism is involved in the photolytic process and the quantum yield is high.     

Stereoselective denitrohydrogenation reactions of 4-alkyl-5-glyco-4-nitrocyclohex-1-enes

The denitrohydrogenation of chiral 4-alkyl-5-glyco-4-nitrocyclohex-1-enes (1 and 3) with tri-n-butyltin hydride and azobisisobutironitrile proceeded in a completely stereoselective way. In each case, the hydrogen atom provided by tri-n-butyltin hydride adds to a free radical intermediate in a trans mode to the adjacent, sterically demanding, sugar side-chain. Attempts to perform denitroalkylation reactions of 1a, 3a or 8 with electron-deficient alkenes yielded only denitrohydrogenated products.     

Ein μ2-Benzylidenkomplex des Chroms mit ungepaarten Elektronen

The benzylidene-bridged dichromium complex [CpCr]₂(μ-Br)₂(μ-CHC₆H₅) (4) has been obtained by thermolysis of [CpCr(CH₂C₆H₅)]₂(μ-Br)₂. ¹H NMR spectroscopy shows that 4 is antiferromagnetic and a barrier of 64.0 kJ mol⁻¹ slows down the rotation of the phenyl group.     

Die photolytische α-spaltung von benzoinalkyläthern in sauerstoffhaltigem methanol

The primary step in the photolytical α-cleavage of benzoin alkyl ether in oxygen saturated methanol at room temperature is the formation of a benzoyl (1) and an α-alkoxybenzyl radical (2), which react via their peroxi radicals 3 und 4 to the final products yielding perbenzoic acid (5), alkyl benzoate (7), benzaldehyde (11), benzaldehyde dimethylacetal (10) and benzil to 100%. Product analysis of the final products of the photolysis of specifically deuterated benzoin methyl ethers shows that methyl benzoate, benzaldehyde and benzaldehyde dimethylacetal are formed via the α-methoxybenzyl radical (2) while the perbenzoic acid results from the benzoyl radical (1). Both the peroxi radicals 3 and 4 have an independent reaction pathway to the final products. Hydrogen abstraction of 3 and 4 from the solvent methanol give rise to hydroxy methyl radicals which yields formic acid, hydrogen peroxide, performic acid and formaldehyde via their hydroxy methyl peroxi radicals. The reaction pathway of the primary radicals is discussed.     

Selectivite de la reduction de l'androsten-4 dione-3,17 et de la progesterone par divers borohydrures: role de l'assistance electrophile par le cation ou un solvant hydroxyle

The reduction of Δ⁴-androsten-3, 17 dione 1 and of progesterone 2 by nBu₄NBH₄ is highly chemioselective: in THF only the a-enone moiety is reduced, the saturated C₁₇ or C₂₀ keto group being kept unchanged. When TMEDA is added, saturated alcohols are obtained, without any allylic alcohol when the reaction goes to completion. However this reduction is poorly stereoselective as 70:30 mixtures of A/B cis and trans ring junction compounds are obtained. In MeOH, the saturated keto group is more than 90% selectively reduced. However, the reduction of 1 and 2 by LiBH₄ and Zn(BH₄)₂ is poorly chemioselective. These results are interpreted in terms of competition between electrophilic assistance and steric effects.     

The Thermal Benzoxazinone-Benzoxazole Conversion,a Reexecution of a Mass Spectrometric Decay by Thermolysis

The base peak in the mass spectrum of several 1,4,2-benzoxazinone derivatives 1a–f (cf. Scheme 1 and the Table) suggests a clean carbon monoxide elimination leading to benzoxazole radical cations 2a–f . This benzoxazinone-benzoxazole conversion can be reproduced quantitatively by flash vacuum thermolysis. The isomeric 2,4,1-benzoxazinone 4 (cf. Scheme 2) eliminates carbon dioxide – the resulting biradical species 5 does not recombine to benzazete 10 but fragments into nitrile and benzyne (7).     

Radikalische reaktionswege bei thermisch induzierten umsetzungen von zirconocenkomplexen

The formation of products 2, 3 and 4 (a–l) from both the reaction of η²-benzophenone zirconocene 1 with alkyl halides and the thermolysis of the α-(Cp₂ZrCl)-substituted benzhydrylmethylether 9, the latter proceeding with anchimeric assistance of the metal, can be understood assuming a stepwise reaction path through radical intermediates. The proposed intermediate transition-metal benzophenone ketyl 12 exhibits a reaction pattern differing from analogous main-group-metal ketyls.     

A stereoselective total synthesis of Δ9-progesterone via an intramolecular cycloaddition reaction

A stereoselective total synthesis of 19-nor-Δ⁹⁽¹⁰⁾-progesterone (14) was achieved through des-A B-aromatic steroid (3) which was obtained by an intramolecular cycloaddition of the $\text{o}$-quinodimethane generated $\text{in situ}$ from the thermolysis of 3-isopropenyl-5-(4-metho-xybenzocyclobutenyl)pentan-2-one-2-ethylene ketal (2), and the compound (14) thus obtained was further converted into Δ⁹-progesterone (18).     

Reactivity of NBS towards N-allyl glycinyl derivatives

The reaction of precursors 3 with NBS in radical conditions does not yield any azetidine resulting from a 4-exo radical cyclisation with bromine atom transfer, or any brominated product at the glycinyl position. Instead, dibrominated adducts 4 and aminals 5 have been formed in good yields. We have investigated the mechanism of this puzzling transformation and shown that the homolytic abstraction of a hydrogen atom by a Br^• radical does not take place at the capto-dative position but at the allylic one. The resulting α-bromo derivative 8 would then evolve into an iminium intermediate with concomitant production of bromine and of a succinimidyl anion, which explains the formation of products 4 and 5. To cite this article: C. Koerber et al., C.R. Chimie 6 (2003).     

Mecanisme de la reaction de Horner-Emmons—II: Effet de solvant sur la stereoselectivite de la reaction d'aldehydes aromatiques et de phosphononitriles

The ratio of Z and E cinnamonitriles obtained from aromatic aldehydes and phosphonitriles 1a and 1b mainly depends upon solvent and upon the phosphonate nature. If it has two H α to CN (1b), the solvent affects the equilibrium of the intermediate oxyanions 5b and 6b. The aldolization step reversibility and stereoselectivity does not intervene unless the aldehyde is orthosubstituted. If the phosphonate has only one H α to CN, kinetically controlled stereoselectivity of the whole process depends upon the solvent effect upon reversibility factor and aldolization selectivity: in DME and THF the total stereoselectivity is low, while in HMPT, the E isomer predominates; under equilibrating conditions (HNa-HMPT) the Z isomer predominates.