Claisen rearrangements, aromatic

Scheme 10.35 Aromatic Claisen rearrangements of catechol mono allylic ethers with sulfur-containing boron catalyst.

This reaction also is concerted and proceeds via a six-membered transition state, but here the species (59), corresponding to the ene-one intermediate (53a) in the aromatic Claisen rearrangement, is in fact the end-product. This is so because there is in (59) no energetic driving force, comparable to re-aromatisation in (53a— 52a), to promote its enolisation. 

Gamer AL, Koide K (2008) Oxidation state-specific fluorescent method for palladium(II) and platinum(IV) based on the catalyzed aromatic Claisen rearrangement. J Am Chem Soc 130 16472-16473... 

Concerning the aromatic Claisen rearrangement certain salient observations are summarized in the following equations [233]. 

An aromatic Claisen rearrangement has been used as a key step in a total synthesis of racemic heliannuols C and E.18 A formal synthesis of (-)-perhydrohistrionicotoxin has used Claisen rearrangement of an amino acid ester enolate as the key step, in which almost total chirality transfer was observed from (S, )-oct-3-en-2-ol in the sense predicted by a chair-shaped transition state with chelation control of enolate geometry.19 Treatment of 1-(cyclohex-l-enyl)-6-methoxy-2-propargylindanol derivatives with base... 

The experimental KIEs were determined for the aliphatic Claisen rearrangement in p-cymene at 120°C and for the aromatic Claisen rearrangement either neat at 170°C or in diphenyl ether at 220°C. Changes in 2H, 13C or 170 composition were determined for unreacted substrates. For carbon analysis of allyl vinyl ether the C5 carbon was used as an internal standard. The C4 atom and rneta aryl protons were used as references in analysis of allyl phenyl ether. The 170 analysis was based on a new methodology. The results are summarized in Table 1, along with predicted isotope effects calculated for experimental temperatures by means of different computational methods. The absolute values of predicted isotope effects for C4 and C5 atoms varied with theoretical level and all isotope effects were rescaled to get reference effects equal to 1.000. 

The Claisen rearrangement has attracted much attention as an attractive tool for the construction of new carbon-carbon bonds. Taguchi et al. reported the enantioselective and regioselective aromatic Claisen rearrangement of catechol mono allylic ether derivatives by means of Corey s chiral boron reagent (Eq. 70) [53a,54]. The mechanism of enantioselectivity is that a rigid five-membered cyclic intermediate is formed by reaction of catechol mono allylic ethers with the chiral boron reagent and this is fol-... 

Vinyl allyl ethers undergo concerted, 6-center, non-aromatic Claisen rearrangements to unsaturated aldehydes or ketones, viz. 

Aromatic Claisen rearrangements in 2,3-unsaturated sugar systems are useful for the stereo-controlled synthesis of aryl-branched sugars [87] (O Scheme 20). The a-anomer 97 is much less reactive in comparison to the /3-anomer 99. This thermal rearrangement is carried out by refluxing in iV,iV-diethylaniline. The efficiency of the reaction is almost independent of the nature of the p-substituent in the phenyl group. 

Rearrangements. By applying Corey s diazaborolidine catalyst to aromatic Claisen rearrangement, a chiral side chain is introduced into the ortho position of catechols. Highly enantioselective rearrangement of 0-allylimidates 111 containing a methoxybinaphthyl substituent has been witnessed. ... 

Europium tris(6,6,7,7,8,8,8-heptafluoro-2 -dimethyloctane-3 -dionate). Eu(fod)3 Aromatic Claisen rearrangement Excellent chirality transfer accompanies the rearrangement of allyl aryl ethers. 

B. Allyl Aryl Ethers. Aromatic Claisen Rearrangement. 460... 

The overwhelming majority of literature devoted to isomerizations of allyl aryl ethers is connected with the aromatic Claisen rearrangement and is summarized in detail in many reviews . Although the [3,3]-sigmatropic isomerization of phenol ethers to the corresponding C-alkylated derivatives has enjoyed widespread application in organic synthesis for over seventy years, it continues to be a very important reaction for the construction of a carbon-carbon bond. This section presents only recent reports. 

In general, the aromatic Claisen rearrangement can be illustrated by equation 68. The initial step in the thermal Claisen rearrangement of an aUyl aryl ether leads to an ortho-dienone which usually enolizes rapidly to form the stable product, an ort/io-allylphenol (so-called orf/zo-Claisen rearrangement, 147 148 149). However, if the rearrange-... 

While the aliphatic Claisen rearrangement has proven to be a major synthetic tool for controlling the stereochemistry in a C—C bond formation, the aromatic Claisen rearrangement has not been exploited as an asymmetric aryl alkylation protocol. A facile... 

A highly enantioselective and regioselective aromatic Claisen rearrangement was carried out using the reaction of catechol monoallyl ethers 192 with the chiral boron reagent 193. This reaction occurs without the formation of either the para-rearrangement or the abnormal Claisen rearrangement products (equation 89) . 

The aromatic Claisen rearrangement was employed in the synthesis of building blocks for various macrocyclic compounds, such as pendant-capped porphyrins , multidentate macrocycles containing 1,3,4-oxadiazole, imine and phenol subunits, as well as to prepare longithorone B, a sixteen-membered farnesylated para-benzoquinone . 

---