Lewis-acid catalyzed rearrangement

Thermal or Lewis acid catalyzed rearrangement of N alkylanilines to o (p-) akylated anilines... 

Schleyer s Lewis acid-catalyzed rearrangement method, which is based on diamondoid thermodynamic stability during carbocation rearrangements, has had little or no success in synthesizing diamondoids beyond triamantane. In recent years, outstanding successes have been achieved in the synthesis of adamantane and other lower molecular weight diamondoids [42 9]. Some new methods have been developed and the yield has been increased to 60%. 

Predict the structure and stereochemistry of the Lewis acid-catalyzed rearrangement of the following epoxides. 

Lewis acid-catalyzed rearrangement of phenol esters and lactams to 2- or 4-ketophenols. Also known as the Fries-Finck rearrangement. 

A synthesis of a dimethyl derivative of 501 has also recently been announced by Hirao, et al. Following conversion of 513 to its dimesylate, Lewis acid-catalyzed rearrangement gave dienedione 514 as the major product. Heating of the disemi-carbazone of 514 with powdered KOH furnished the diolefin which was transformed into 515 by ozonolysis. When the ditosylhydrazone of 515 was heated with potassium... 

Some unusual reactions have been described for 2-(4-chlorophenyl)-2-(3,3-dimethylallyl)-4-phenyl-5(277)-oxazolone 84. This compound undergoes a Lewis acid-catalyzed rearrangement to give a tetrahydrofuropyrrole 85. On the other hand, depending on the reaction conditions, thermolysis of 84 produces the azabicyclohexene 86 or a substituted 2,3-dihydropyridine 87 together with the caged compound 88 formed by dimerization of the 2,3-dihydropyridine and the azabicyclohexene (Scheme 7.21). " ... 

Interestingly, Lewis acid catalyzed rearrangement of 6-methyltricyclo[8.2.0.0, 6]dodecan-9-one gave the spiroannulated triquinane ketone (l/ , 5/ )-l-methylspiro[bicyclo[3.3.0]octane-2,lf-cyclopentane]-6-one as the major product (70%).85... 

Campholenic Aldehyde Manufacture. Campholenic aldehyde is readily obtained by the Lewis-acid-catalyzed rearrangement of a-pinene oxide. It has become an important intermediate for the synthesis of a wide range of sandalwood fragrance compounds. Epoxidation of (+)- Ct-pinene (8) also gives the (+)-o -a-pinene epoxide [1686-14-2] (80) and rearrangement with zinc bromide is highly stereospecific and gives (-)-campholenic aldehyde... 

For an example of a Lewis-acid catalyzed rearrangement, see Cameron Slimson Aust. J. Chem. 1977,30, 923. "Barry Beale Carr Hei Reid/. Chem. Soc., Chem. Commun. 1973, 177. 

Similarly, Lewis acid-catalyzed rearrangement of epoxy alcohols 84 (Scheme 37) gives in most cases the exo-cyclized tetrahydropyrans 85 as the main products (80-97%) however, with La(OTf)3 as the catalyst, the main products (80-90%) are endo-cyclized oxepanes 86 <1998TL393>. 

A. Lewis Acid Catalyzed Rearrangements 1. General Aspects... 

The most useful and general approach for the preparation of diamonoid molecules arises from Lewis acid catalyzed rearrangements of polycyclic hydrocarbons. The ubiquitiy of these rearrangements coupled with the availability of several highly selective direct substition reactions which may be subsequently applied to the rearranged hydrocarbons (see Section V.A. 1,2), makes a wide variety of diamonoid substrates readily available. The need for direct, multistep syntheses arises only when special substitution patterns not available via the substitution approach are required (see Section V.A. 3). 

The principle of the Lewis acid catalyzed rearrangements of hydrocarbons is well documented 4,81. Lewis acids react with a promotor deliberately added or present as an impurity in the reaction mixture to form carbonium ions which initiate intermolecular hydride transfers involving the hydrocarbon. These hydride transfers appear to be fairly unselective processes. While the expected tertiary > secondary > primary selectivity order is observed, the differences are significantly reduced relative to typical carbonium ion reactions. Possibly this is due to a hydride transfer mechanism which involves a pentaco-ordinate carbon transition state in which charge development on carbon would be minimized 38dh... 

Reversible, random carbonium ion formation is not required to explain the rearrangements of both 2 and 37 to adamantane in the highly acidic media. Sequential 1,2 alkyl shifts coupled with the well documented 5316,2- and 3,2-hydride shifts of the norbomyl system permit a rearrangement pathway analogous to that discussed earlier as the most likely route for the Lewis acid catalyzed rearrangement of 2 to adamantane. 

In addition to the preparations of ethanoadamantane via Lewis acid catalyzed rearrangement of various polycyclic hydrocarbons described above (Section II. A.1), a ring closure reaction of a substituted adamantane has also been developed. Treatment of 2-adamantyl diazoketone with copper results in the intramolecular carbene insertion illustrated in Eq. (48) 14°1. 

Scheme 3. Preparation and Lewis Acid-catalyzed Rearrangement of Quinol Monoketals...

Unlike Lewis-acid-catalyzed rearrangements of organic compounds, ionic intermediates are not important in the mechanism. No evidence has been found to support the existence of silylenium ions, RgSi, and careful studies have ruled out carbocations as well. 

This experiment, which combines well anticipated and totally unexpected results, was designed to compare the thermal versus the Lewis acid-catalyzed rearrangement of a doubly activated cyclopropane. This twofold activation was understood in terms of the electron donating effect of the p-anisyl substituent at C-2 that is complementary to the familiar electron withdrawal of the 1,1 -diacyl substitution, which closely resembles Problems 15 and 23. This combination, called captodative cyclopropanes, considerably facilitates some of the various ring unraveling pathways open to the trimethylene ring. 

Unlike an A, A -disubstituted imidazoline-2-thione, the reaction of imidazolidine-2-thione 665 itself with 2-phenyloxirane in the presence of BF3 leads to the formation of l,3-bis[(i4)-2-phenylethen-l-yl]im>dazolidine-2-thione 666 (Scheme 161) <2005HCA3253>. This unusual mode of reaction is presumably due to a Lewis-acid-catalyzed rearrangement of oxirane to aldehyde followed by an ene-like reaction with the iminothiol. 

The Durst group has taken advantage of these competing reactions (halohydrin intermediate, Lewis acid-catalyzed rearrangement, and regioselectivity issues) to form... 

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