Lewis acids catalysed Fries rearrangement of isopropylcresol esters

Summary In the course of the Fries rearrangement, aluminium chloride frequently induces migration or elimination of alkyl groups. The results obtained with titanium tetrachloride for the synthesis of vicinalo-hydroxyketones are compared with those obtained with aluminium chloride for some aliphatic and aromatic esters of isopropylcresols. In order to understand the migration and elimination processes occurring, the stabilities of theo-hydroxyketones are studied in the presence of aluminium chloride at different temperatures. Furthermore, all-vicinalo-hydroxyketones were prepared by the Fries rearrangement of 6-tert-butyl-p-thymol with titanium tetrachloride.

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Lewis-Säure-katalysierte Fries-Umlagerung von Isopropylkresolestern

Zusammenfassung Im Verlauf der Fries-Umlagerung induziert Aluminiumchlorid des öfteren eine Wanderung oder Eliminierung von Alkylgruppen. Die Resultate mit Titantetrachlorid bei der Synthese von vicinaleno-Hydroxyketonen werden mit denen mit Aluminiumchlorid für einige aliphatische und aromatische Ester des Isopropylkresols verglichen. Um zu einem Verständnis der auftretenden Wanderungs-und Eliminierungsprozesse zu gelangen, wurden die Stabilitäten vono-Hydroxyketonen bei verschiedenen Temperaturen in der Gegenwart von Aluminiumchlorid untersucht. Außerdem wurden all-vicinaleo-Hydroxyketone mittels Fries-Umlagerung von 6-tert-Butyl-p-thymol mit Titantetrachlorid hergestellt.

     

The acid catalysed rearrangement of a diterpenoid epoxide

A study has been made of the principal rearrangement products resulting from formic acid treatment of 19-hydroxy-ent-beyerene epoxide (3). In concentrated solutions 3 has been found to undergo a deep seated rearrangement to the allylic alcohol (14). A mechanism for the formation of 14 is proposed involving a novel 1,4-hydride shift in the bicyclo[3:2:1]octane C/D ring system following cleavage of the C₁₅O bond. Supporting evidence has been obtained from a study of the specifically labelled epoxide (4), the deuterium in 14 appearing exclusively at C₁₂. Four products (7 and 11–13) emanating from the known beyerane → kaurane interconversion have been identified.     

Rapid and facile Lewis acid catalysed Boc protection of amines

Efficient Boc protection of amines was carried out using (Boc)₂O in the presence of a catalytic amount of ZrCl₄ (10 mol %) in acetonitrile at room temperature. The reaction times are very short and the yields are generally high.     

Lewis acid mediated claisen-type rearrangement of aryl dienyl ethers

In the presence of BF₃·.OEt₂, aryl dienyl ethers were rearranged under mild conditions to afford dienyl phenols in good yields.     

Controlled accessibility Lewis acid catalysed thermal reactions of regenerated cellulosic fibres

A combination of techniques have been used to characterise lyocell regenerated cellulose fibre subjected to low-moisture thermal-catalytic reactions with zinc chloride Lewis acid. Application from non-swelling ethanol reduces catalyst accessibility, but at high temperatures migration takes place through the internal fibre morphology. The extent of chain scission is reduced at lower temperatures, leading to a higher leveling-off degree of polymerisation (LODP). In contrast, application of zinc chloride from water results in a lower LODP, due to the more even distribution of catalyst. The weights of extractable polymer material increase according to two separate rate constants, following established semicrystalline models. A higher Arrhenius activation energy for chain scission is seen for zinc chloride application from ethanol, which may be due to the physical mobilisation of the cellulose polymer at high temperature, associated with a cellulose T_g. This may also aid recrystallisation. Cellulose dehydration endotherms and pyrolysis exotherms are shifted to lower temperature for application of zinc chloride from ethanol compared to water, which may be the result of a higher local concentration of catalyst and a faster reaction onset.     

Tandem Lewis acid catalyzed coupling/semipinacol rearrangement of allylic alcohol

Construction of the polyaryl quaternary unit through a ZnBr2 catalyzed tandem coupling/semipinacol rearrangement participated by allylic cations was reported for the first time.     

Lewis acid catalyzed [1,3]-sigmatropic rearrangement of vinyl aziridines

This paper details the copper-catalyzed ring expansion of vinyl aziridines to 3-pyrrolines. Broad substrate scope (24 examples) using tosyl- and phthalimide-protected vinyl aziridine substrates is observed. Cu(hfacac)2 was determined to be superior to all other catalysts tested.     

Molecular rearrangement of epoxide derived from sesquiterpenes by Lewis acid catalysis

The molecular rearrangement under mild conditions of two epoxides derived from the sesquiterpenic himachalenes has been studied using BF₃·Et₂O as the Lewis acid catalyst. Both reactions are efficient and selective affording in each case one new enantiomerically pure ketone isolated in good yield. Their structures have been fully characterized by spectroscopic methods. Mechanisms for the BF₃-catalyzed reactions are proposed.     

Defunctionalisation catalysed by boron Lewis acids

Selective defunctionalisation of organic molecules to valuable intermediates is a fundamentally important transformation in organic synthesis. Despite the advances made in efficient and selective defunctionalisation using transition-metal catalysis, the cost, toxicity, and non-renewable properties limit its application in industrial manufacturing processes. In this regard, boron Lewis acid catalysis has emerged as a powerful tool for the cleavage of carbon–heteroatom bonds. The ground-breaking finding is that the strong boron Lewis acid B(C₆F₅)₃ can activate Si–H bonds through η¹ coordination, and this Lewis adduct is a key intermediate that enables various reduction processes. This system can be tuned by variation of the electronic and structural properties of the borane catalyst, and together with different hydride sources high chemoselectivity can be achieved. This Perspective provides a comprehensive summary of various defunctionalisation reactions such as deoxygenation, decarbonylation, desulfurisation, deamination, and dehalogenation, all of which catalysed by boron Lewis acids.

The combination of boron Lewis acid catalysts and hydride sources enables the cleavage of various carbon–heteroatom bonds.     

Acid catalysed novel rearrangement of bicyclic dienone

The dienone (4) on heating with methanolic hydrochloric acid underwent ring cleavage yielding the aromatic ketal (6) which on acid hydrolysis produced the aldehyde (7). Similar treatment of the dienone (4) with acetic anhydride and p-toluenesulfonic acid furnished the ester (8) which on alkaline hydrolysis afforded the dimeric product (10). The structure of the products (6), (7), (8) and (10) are defined on the basis of the spectral evidence.     

Enantiospecific synthesis of tricyclo[5.2.1.0]decanes via acid catalysed rearrangement of isotwistanes

An acid catalysed rearrangement was employed for the enantiospecific conversion of isotwistanol to tricyclo[5.2.1.0^4,8]decanes, which provided support for the proposed biosynthesis of allopupukeananes from pupukeananes. The strategy has been further extended to the enantiospecific synthesis of a homobrexane.     

Lewis acid mediated [2,3]-sigmatropic rearrangement of allylic alpha-amino amides

Boron trifluoride and BBr(3) mediated [2,3]-sigmatropic rearrangements of allylic alpha-amino amides have been developed affording secondary amines in good yields. (E)-Crotyl and (E)-cinnamyl alpha-amino amides 2b and 2c exhibit excellent syn-diastereoselectivity upon rearrangement with either Lewis acid. The allylic amine 2a forms upon treatment with BF(3) or BBr(3) a five-membered heterocylic complex in which a single halide anion has been displaced by the carbonyl oxygen atom. The structures of the Lewis acid-amine complexes were elucidated using NMR spectroscopy. A plausible reaction mechanism, based on DFT calculations, is presented. Thus, BF(3)- or BBr(3)-complexed allylic amines 2 are shown to preferentially proceed, after deprotonation, via an endo transition state.     

Lewis acid mediated [2,3]-sigmatropic rearrangement of allylic ammonium ylides

An investigation of the Lewis acid mediated [2,3]-sigmatropic rearrangement of allylic ammonium ylides has been conducted by employing various bases and boron Lewis acids. Using BBr₃ together with the phosphazene base 6, various allylic amines could be rearranged in good yields with low to excellent diastereoselectivity.     

Chiral Lewis acid catalysis in a visible light-triggered cycloaddition/rearrangement cascade

Cascade (domino) reactions facilitate the formation of complex molecules from simple starting materials in a single operation. It was found that 1-naphthaldehyde derivatives can be converted to enantioenriched (82–96% ee) polycyclic benzoisochromenes via a cascade of ortho photocycloaddition and ensuing acid-catalysed rearrangement reactions. The cascade was initiated by irradiation with visible light (λ = 457 nm) and catalysed by a chiral AlBr₃-activated 1,3,2-oxazaborolidine (14 examples, 65–93% yield). The absolute configuration of the products was elucidated by single crystal X-ray crystallography. Mechanistic experiments suggest that the ortho photocycloaddition occurs on the triplet hypersurface and that the chiral catalyst induces in this step the observed enantioselectivity.

Chiral Lewis acid catalysis enables the formation of enantioenriched polycyclic benzoisochromenes 2 from simple 1-naphthaldehydes 1 in an efficient cascade process triggered by a visible light-initiated ortho photocycloaddition.     

Lewis acid-catalyzed rearrangement of multi-substituted arylvinylidenecyclopropanes

An interesting rearrangement of arylvinylidenecyclopropanes having three substituents at the 1- and 2-positions of the corresponding cyclopropane catalyzed by Lewis acids to give 6aH-benzo[c]fluorine derivatives via a double intramolecular Friedel-Crafts reaction or to give an indene derivative via an intramolecular Friedel-Crafts reaction is described.     

Trimethylsilylcyanation of heterocyclic imines catalysed by Lewis acids

The addition of Me₃SiCN to Schiff bases 1a–8a, synthesized by the reaction of furan and thiophene aldehydes with 3‐ and 4‐aminobenzotrifluorides, has been studied in the presence of various Lewis acids. A series of the corresponding trifluoromethyl derivatives of heterocyclic α‐aminonitriles 1–8 was synthesized in 38–80% isolated yields. It was found that 4A molecular sieves (MS) accelerate the addition and increase the yields of the products. The investigated catalysts are ranked by their activity in the following order: AlBr₃ + 4AMS > AlBr₃ > AlCl₃ > Ti[O(iPr)]₄. A single crystal of N‐(5‐methyl‐2‐thienylcyanomethyl)‐3‐trifluoromethylaniline was obtained and studied by X‐ray diffraction. The results showed that it was the crystal of the (R) isomer of this compound. Copyright © 2000 John Wiley & Sons, Ltd.