Intramolecular Bronsted acids

The first hydration step was promoted by Bronsted acids containing weakly or noncoordinating anions. In the second step, an intramolecular hydrogen transfer in the secondary alcohol was catalyzed by ruthenium(III) salts with chelating bipyridyl-type ligands. The possible complexation of the latter with the diene did not inhibit its catalytic activity in the allylic rearrangements, under acid-catalyzed hydration conditions. 

A radical-cation initiated intramolecular cycloaddition of 3 to 4 has been reported to occur using tris(4-bromophenyl)amminium hexachloroantimonate (TBAH).22 A number of Bronsted acids including trifluoroacetic acid can also effect this reaction. 

The reaction is based upon the two components condensation between an aldehyde or ketone 6 (or their synthetic equivalents) and alcohol 95, which contains an allylsilane (or vinylsilane) moiety. The IMSC reaction is mediated by Lewis or Bronsted acids, which activate the carbonyl group of 6 towards nucleophilic attack. After addition of alcohol 95 on the activated carbonyl, the oxonium cation 96 is formed, which is intramolecularly captured by the pendant allylsilane function, leading to oxygen-containing rings 97 (Scheme 13.38). This process typically requires a stoichiometric (or more) amount of Lewis acid. 

Diazoacetates are commonly used for the formation of aziridines from imines under Lewis or Bronsted acidic conditions - a process known as the aza-Darzens reaction. A useful twist on the reaction is achieved if the 1,2-addition intermediate undergoes deprotonation of the a proton prior to intramolecular aziridine formation with N2 extrusion. Such an interrupted aza-Darzens reaction accomplishes a... 

On a molar basis, most organic compounds contain similar amounts of hydrogen and carbon, and processes involving transfer of hydrogen between covalently bound sites rank in importance in organic chemistry second only to those involving the carbon-carbon bond itself. Most commonly, hydrogen is transferred as a proton between atoms with available electron pairs (l), i.e. Bronsted acid/base reactions. The alternative closed shell process, hydride transfer or shift, involves motion of a proton with a pair of electrons between electron deficient sites (2). These processes have four and two electrons respectively to distribute over the three atomic centres in their transition structures. It is the latter process, particularly when the heavy atoms are both first row elements, which is the subject of this review. The terms transfer and shift are used here only to differentiate intermolecu-lar and intramolecular reactions. 

The application of the intramolecular Nicholas reaction by C. Mukai et al. made it possible to develop a novel procedure for the construction of oxocane derivatives. Interestingly, several Lewis and Bronsted acids gave rise to complex mixtures. However, the use of mesyl chloride/triethylamine in refluxing DCM afforded the desired oxocane as the sole product. 

Apart from acting as effective Lewis base catalysts, the quinuclidine structure of cinchona alkaloids can also participate in the other cycloaddition reaction by a different catalytic mechanism. Calter et al. described an interesting asymmetric interrupted Feist-Benary reaction between ethyl bromopyruvates and cyclohexadione. They proposed that the protonated cinchona alkaloid would perform as a Bronsted acid to form hydrogen-bonding interaction with a-ketoester moiety, rendering it more electrophilic toward attack by either the enol or enolate of cydohexandione. Then intramolecular alkylation would afford the formal [3 + 2] cycloadduct (Scheme 10.12) [16]. 

Bi(III) is an efficient catalyst for the formation of tetrahydropyran-4-ols from homoallylic alcohols by the Prins reaction application to styrenes leads to 1,3-dioxanes <05SC1177> cr i-2,6-Disubstituted tetrahydropyrans are selectively formed in a Bi-mediated intramolecular oxa-conjugate addition of a,P-unsaturated ketones 10 the actual catalyst is considered to be the Bronsted acid derived from the Bi salt <05TL5625>. cat. BiXa... 

The initial reaction is a Diels-Alder addition to give a mixture of the two possible adducts (PI5.3) and (PI5.4). It can be seen that the former is perfectly set up for a very interesting double intramolecular cyclisation when treated with a Bronsted acid (Figure SI5). 

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