Intramolecular Buchner reaction

Intramolecular Buchner reaction. Rh(OAc), is more efficient than CuCl as the catalyst for cyclization of the a-diazo ketone 1, derived from 3-phenylpropionic acid, to bicyclo 5.3.0]decatrienone (2). This product isomerizes in the presence of triethylamine to the more stable trienone 4. A useful isomerization to 3-tetralone (3) occurs in the presence of trifluoroacetic acid. 

Duddeck, H., Ferguson, G., Kaitner, B., Kennedy, M., McKervey, M. A., Maguire, A. R. The intramolecular Buchner reaction of aryl diazoketones. Synthesis and x-ray crystai structures of some poiyfunctionai hydroazuiene iactones. J. Chem. Soc., Perkin Trans. 11990, 1055-1063. 

Reaction of a phenyl ring with diazoacetic esters to give cyclohepta-2,4,6-trienecarboxylic acid esters. Intramolecular Buchner reaction is more useful in synthesis. Cf. Pfau-Platter azulene synthesis. 

Example 3, An intramolecular Buchner reaction within the Grubbs catalyst ... 

The thermal or photochemical Buchner reactions produce complex mixtures of cycloheptatrienyl esters, and the daunting complexity of the product mixtures was reduced or even eliminated with the advent of transition-metal catalysts, at first copper-based, then in the early 1980s rhodium(II) catalysts, which were developed by the Belgium group led by Noels and Hubert. The rhodium(II)-catalyzed cyclopropanations of aromatics, especially intramolecular cyclopropanations, have enjoyed a certain popularity due to their high regioselectivity and stereoselectivity. Since the intramolecular Buchner reaction is much more widely used in organic synthesis than the intermolecular version, the former is the focus of this review. 

The intramolecular Buchner reaction of aryl diazoketones has been carried out using both copper(I) and rhodium(II) catalysts. For example, 1-diazo-4-phenylbutan-2-one 27a cyclizes in bromobenzene with copper(I) chloride catalysis, furnishing 3,4-dihydroazulen-l(2//)-one 30 in 50% yield after purification by chromatography over alumina. Trienone 30 is not the primary cyclization product, and the less conjugated isomeric trienone 29a is first produced, but contact with alumina causes isomerization to 30. The yield of this cyclization is further improved when rhodium(II) acetate is used as the catalyst instead of copper(I) chloride. Thus a catalytic amount of rhodium(II) acetate brings about the nearly quantitative conversion of 27a to 29a within minutes in hot dichloromethane. Compound 29a isomerizes to 30 on treatment with triethylamine, and rearranges to 2-tetralone 31a when exposed to silica gel or acid. 

The effect of ortho- and weto-substitution in the above-mentioned intramolecular Buchner reactions has been examined. When the 2-methoxy-substituted diazoketone 32 is subjected to rhodium(II) acetate catalysis, a single cycloheptatrienone 34 is obtained in 94% yield.This result is consistent with the outcome of the rhodium(II) trifluoroacetate-catalyzed intermolecular reaction of ethyl diazoacetate with anisole, which yields no product arising from addition of the ketocarbenoid on the most hindered site of the anisole. Dihydroazulenone 34 rearranges to tetralone 36 under acidic conditions, and isomerizes to the conjugated ketone 35 under basic conditions. It is interesting that the catalyzed decomposition of the para-methoxy derivative 37 provides exclusively 6-methoxy-2-tetralone 40 with no trace of the putative trienone 39. ... 

The first direct chemical evidence for the formation of the norcaradiene system in the intramolecular Buchner reaction was obtained in the rhodium(II)-catalyzed decomposition of l-diazo-4-(2-naphthyl)butan-2-one 44. This reaction provides the tetracyclic norcaradiene 45 and tricyclic ketone 52 in 71% and 8% yield, respectively. When a catalytic amount of trifluoroacetic acid is added to 45, tricyclic ketone 51 is formed. It is surprising that compound 45 is recovered quantitatively after treatment with triethylamine in dichloromethane under reflux. The formation of 52 is explained in terms of an attack of the carbenoid carbon of 44 on the 2,3-double bond of the naphthalene nucleus followed by double bond migration in the tricyclic nonconjugated ketone 49. 

There are several examples of catalyzed aromatic cycloadditions leading to heterocyclic systems. The rhodium(II) acetate-catalyzed intramolecular Buchner reactions of iV-benzyldiazoacetamides 64a/b afford azabicyclo[5.3.0]decatrienes 66a/b in excellent yields. In contrast, the N-methyl derivative 64c gives 66c in moderate yield. Use of rhodium(II) perfluorobutyrate (Rh2(pfb)4) in place of rhodium(II) acetate increases the yield to 54%. Unlike its carbon counterpart, dihydroazulenone 29a (vide supra), 66a is insensitive to either trifluoroacetic acid or boron trifluoride etherate, even in excess, and the unrearranged reactant is recovered intact even after prolonged treatment at room temperature. 

Arenes suffer dearomatization via cyclopropanation upon reaction with a-diazocarbonyl compounds (Btlchner reaction) [76]. Initially formed norcaradiene products are usually present in equilibrium with cycloheptatrienes formed via electrocyclic cyclopropane ring opening. The reaction is dramatically promoted by transition metal catalysts (usually Cu(I) or Rh(II) complexes) that give metal-stabilized carbenoids upon reaction with diazo compounds. Inter- and intramolecular manifolds are known, and asymmetric variants employing substrate control and chiral transition metal catalysts have been developed [77]. Effective chiral catalysts for intramolecular Buchner reactions include Rh Cmandelate), rhodium carboxamidates, and Cu(I)-bis(oxazolines). While enantioselectivities as high as 95% have been reported, more modest levels of asymmetric induction are typically observed. 

Substrate-controlled stereoselective dearomatizations provide cycloheptatriene derivatives in high diastereomeric excess, and the reaction has been used to prepare 7-membered ring systems found in several natural products. Scheme 15.27a illustrates the Rh(II)-catalyzed conversion of diazo derivative 72 to polycyclic cycloheptatriene 73, which was subsequently converted to har-ringtonolide [78]. Note that the initial cycloheptatriene product of the Buchner reaction is converted to a more stable isomer by the action of DBU. In some instances, intramolecular Buchner reactions afford norcaradiene products that are not in equilibrium with the corresponding cycloheptatrienes. These examples arise as a consequence of conformational constraints inherent in the substrates. Cu-catalyzed Buchner reactions have been anployed to access derivatives of stable norcaradiene fragments found in several natural products (e.g., gibberellin GA j and (-r)-salvileucalin B, Scheme 15.27b and c, respectively) [79]. 

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