Facteurs stériques influençant la transposition de Wagner-Meerwein des carbocations de type pinanyle

Steric factors influencing Wagner-Meerwein rearrangement of pinanyl carboniumions. The course of the Wagner-Meerwein rearrangement of ions obtained by protonation of substituted α- or β-pinenes is strongly dependent on the configuration of the substituents. The situation is complicated by the possibility of double bond isomerisation (α-→β-pinenes) before rearrangement. Simple rules for predicting the products are given.     

Intermediaires dans le rearrangement de dimroth d'imidazo[1,2-a]pyridines

Imidazo[1,2-a]pyridines activated by a nitro group at position 6 or 8 undergo the Dimorth rearrangement in aqueous basic media. Intermediates of the rearrangement have been detected and their structures spectroscopically investigated.     

Réarrangement de thiénobenzothiépinones en thiénoisothiazolinone et benzoisothiazolinone

Under the conditions of the Schmidt reaction the thienobenzothiepinones 1 , 2 and 13 involve a rearrangement respectively in N-benzythieno[2,3-d]isothiazolin-3-one ( 5 ) and benzoisthiazolin-3-one ( 6 ). A mechanism for this rearrangement is proposed.     

Pyrrolothienopyrazines. IV. Synthese de derives de 1′amino-5 pyrrolo[1,2-a]thieno[2,3-e]pyrazine

5-Aminopyrrolo[1,2-a]thieno[2-3-3]pyrazine derivatives were obtained by intramolecular cyclisation of N³(1-pyrrolyl)-2-thienylurea derivaties. Synthesis of these latter compounds was achieved from 2-(1-pyrrolyl)-3-thenoylazide via a curtius rearrangement. The H nmr spectra are discussed.     

Utilisation du 13C en Spectrométrie de Masse: Etude de la Fragmentation de Disaccharides

Twenty-nine α,β-(1 → 3) and (1 → 4) or β-(1 → 6) disaccharides, in the glucose series were analysed by electron impact mass spectrometry. In addition to the known mechanism, a few ¹³C labelled compounds led us to suggest that the ion c was formed from the reducing unit B and the C-1′ of the non-reducing residue A, while A was the starting point for rearrangement and fragmentation processes.     

Fragmentations Principales de Mono-, Di-, Triet Tetra-azaindolizines Sous L'Impact Electronique

The principal fragmentations of 41 azaindolizines having an imidazo[1,2-a]pyridine, imidazo[1,5-a]pyrimidine, imidazo[1,5-b]-1,2,4-triazine, triazolo[2,3-b], -[4,3-b]-, -[3,4-c]- and -[3,2-c]-1,2,4-triazine structure are described. It is shown that each structure has its own characteristic fragmentation pathways except where a Dimroth rearrangement into an isomeric product can be postulated. The degradation scheme is correlated with the number and the position of the nitrogen atoms of the azaindolizines.     

Diénamines hétérocycliques. III. Réexamen de la réaction de la base de Fischer avec le tétracyanoéthylène

Heterocyclic dienamines III. A re-examination of the reaction of Fischer's base on tetracyanoethylene Depending on the order of addition, Fischer's base 5 (1,3,3-trimethyl-2-methylidene-indoline) reacts 1:1 with tetracyanoethylene to give either the tricyanovinylation product 6 or the spiro compound 7 . A skeletal rearrangement of a zwitterionic intermediate can explain the formation of the spiro compound. The latter undergoes a thermal isomerization yielding by ring expansion the tetrahydroquinoléine 8 . On reaction with LiAlH₄ or CH₃ONa 7 and 8 lead both to triazatetracycles. All structures are assigned on the basis of spectral data.     

Synthése,stéréochimie,et réactivité thermique de (N-méthylidenealkylamino)-2 aziridines

1-Alkyl-2-(N-methylidenealkylamino)aziridines are obtained by the reaction of primary amines with either α-chloroacraldehyde or α-chlorocrotonaldehyde. Structural assignments are made by nmr spectroscopy. The thermal rearrangement of 1-alkyl-2-(N-methylidenealkylamino)-3-methylaziridines to pyrroles is described.     

Contraction de cycle a partir de la tosyloxy-8 bicyclo[4.2.0]octanone-7 cis

Cis 8-endo tosyloxybicyclo[4.2.0]oct-3-en-7-one treated with H₄AlLi undergoes a stereospecific ring contraction to give endo 7-formyl bicyclo[4.1.0]hept-3-ene and endo 7-hydroxymethyl bicyclo[4.1.0]hept-3-ène. On the contrary, the same compound treated with MeONa in ether or methanol (conditions of the Favorski rearrangement) gives, beside solvolysis products when CH₃OH is solvent, the two epimeric esters 7-endo and 7-exo carbomethoxybicyclo[4.1.0]hept-3-ène. Several pathways are postulated for the rearrangement.     

Spectres de masse des composés organiques. 3e Communication.. Perte de méthyle dans la fragmentation des alcanes

The reaction mechanism for the loss of methyl and methane from the parent ion is investigated for n-alcanes. By using different deuterated species, mainly hexanes, it is shown that these two reactions proceed by two paths, one being the simple split of a C-C-bond, whereas the other consists in an internal rearrangement followed by a loss of a C-atom from within the chain. This process is becoming the main process for this class of fragmentation as the hydrocarbon chain is getting longer. A small internal isotope effect has also been found.     

Un nouveau type de sucres azotés,les gem-azo-alcools. Communication préliminaire

Treatment of keto-sugar p-nitrophenylhydrazones with lead tetraacetate led to the corresponding gem-azo-acetates. The reaction is highly stereoselective, only one of the two possible epimers at the new asymmetric carbon being formed in measurable quantity. Catalytic de-acylation of these gem-azo-acetates yielded, quantitatively, representatives of a new class of nitrogen-containing sugars: the gem-azo-alcohols. When treated with potassium t-butylate, the gem-azoacetates underwent a rearrangement with ring expansion leading to N-aminolactams.     

Ylides du soufre dérivés de sucres Communication préliminaire

S-Methylation of 6-S-benzyl-6-deoxy-1,2-O-isopropylidene-3-O-methyl-α-D-xylo-6-thiohexofuranos-5-ulose ( 1 ) gave the expected sulfonium salt 2 which on alcaline treatment yielded the stable sulfur ylide 3 . This compound constitutes an useful synthetic intermediate in carbohydrate chemistry. On heating in 1,2-dimethoxyethane, it underwent a Stevens rearrangement which led to an extension of the carbon chain of the sugar and, reacted with Michael acceptors, it gave cyclopropanation reactions.     

Mechanochemically Initiated Achmatowicz Rearrangement

The Achmatowicz rearrangement converts furfuryl alcohols, obtainable from renewable carbohydrates, into 6-hydroxy-2H-pyrane-3(6H)-ones, which are versatile intermediates for organic synthesis. We describe here the first examples of a solvent-free mechanochemical Achmatowicz rearrangement. Furfuryl alcohols were prepared from furfurals using mechanochemically initiated reductions and Reformatsky reactions. Mechanochemical reaction conditions for the Achmatowicz rearrangement of the obtained furfuryl alcohols were optimized and applied to a series of derivatives, yielding the corresponding rearrangement products in yields of 39 to 95%.     

Migration du groupe éthoxycarbonyle dans la transposition de Wagner-Meerwein

Ethoxycarbonyl group migration in the Wagner-Meerwein rearrangement . 13 β-Hydroxy-esters have been treated with P₂O₅ in benzene at 80°. Olefin-forming dehydration, when possible, was found to be the main reaction. When this is excluded, tertiary or benzylic hydroxy-esters react in a manner most easily explained by migration of the COOEt group, thus avoiding the formation of α-carbonyl-carbenium ions. On the other hand, in primary hydroxy-esters (incapable of direct olefin formation), phenyl and methyl groups migrate in preference to COOEt, indicating in this case a concerted reaction.     

Studies in the claisen rearrangement. Part XII . Synthesis of benzofuro[3,2-c] [l]-6a,lla-dihydro-lla-methylbenzopyrans from 1,4-diaryloxy-2-butynes

Successful Claisen rearrangement of a number of 1,4-diaryloxy-2-butynes is reported. The products of such a rearrangement are the benzofuro[3,2-c]benzopyrans. This novel rearrangement offers a facile synthetic route to tetracyclic derivatives resembling the naturally occurring pterocarpans.     

An unexpected γ-hydrogen rearrangement in the mass spectra of di-ortho -substituted alkylbenzenes

In contradiction of long-accepted mass spectrometric dogma, the site-specific γ-hydrogen rearrangement is observed in alkylbenzenes in which both ortho positions are blocked with methyl substituents. Mass spectrometric studies of a series of five trimethyl- and three tetramethylisopentylbenzenes have shown that this rearrangement is only suppressed to a significant degree in those compounds in which all three positions ortho and para to the isopentyl group are blocked. Deuterium labelling has confirmed the γ-site-specific origin of the migrating H atom while metastable studies have been used to investigate the mechanism of the rearrangement process.     

Transposition des cycles satures en spectrometrie de masse II–Ethylene acetals derives de cyclopentanones et de cyclohexanones

The detailed study of the mass spectra of cyclopentanone and cyclohexanone acetals produces new evidences for the rearrangement of the corresponding molecular ions. The rearrangement was quantitatively studied with the help of a simple model. The influence of different structural features is discussed.     

A Fused Benzocyclooctene Ring System via an Aromatic Cope Rearrangement: Thermal Reactions of trans‐1‐Aryl‐2‐ethenylcyclobutanecarbonitriles

The results of an aromatic Cope rearrangement of a trans‐1‐aryl‐2‐ethenylcyclobutanecarbonitrile are reported (Scheme). The use of this rearrangement for the construction of the fused benzocyclooctene ring system and a preliminary study of the electronic requirements to favor such a transformation are also described.     

Etude d'un Couple de Diastereoisomeres en Spectrometrie de Masse: Les 2,3-Diphenyl-butanes Thréo et Erythro

The mass spectra of erythro and threo 2,3-diphenylbutanes are assigned to one rearrangement and one simple cleavage. After calculation of these spectra, it is suggested that their differences are issued essentially from the enthalpy difference of the molecular ions and not from the enthalpy difference between the rearrangement transition states. It is shown that in the gaseous state the threo 2,3-diphenylbutane is more stable than the erythro isomer.     

Obtention de nouveaux squelettes diterpéniques au cours de l'isomérisation des époxydes-7,8 isopimarates de méthyle par l'éthérate de trifluorure de bore

The action of borontrifluoride etherate on methyl isopimarate 7,8 epoxides has been studied. A backbone rearrangement and the obtention of compounds with new skeletons are reported.