Regioselective Claisen rearrangement

The naturally occurring phenalenone atrovenetin (238), produced by the fungus Penicillium atrovenetum, has been synthesised by a route which employs the novel, regioselective Claisen rearrangement-cyclisation (234)—> (235), together with the... 

Several naturally occurring 6-substituted and 3,6-disubstituted coumarins have been obtained by regioselective Claisen rearrangements and tandem Claisen-Cope rearrangements (94JCS(P1)3095,3101). 

As in the latter case, in which pre-existing diene and dienophile parts are located on the side chains of the parent aromatic ring, Kraus et al. reported a high yield and regioselective Claisen rearrangement followed by the intramolecular Diels-Alder reaction leading to anthracyclines 101 from the hydroquinone mono allyl ether derivative 100 [78]. 

In a different vein, the biosynthetic route taking as a model [8,9], one of the most common methods to prepare furocoumarin derivatives, involves the initial O-allylation of a hydroxycoumarin followed by the regioselective Claisen rearrangement of the resulting allyl ether to generate the corresponding hydroxyallylcoumarin which is subsequently cyclized and oxidized (Scheme 19). These two steps can be performed in any order, and several protocols have been developed so far [13]. 

Bruder, M., Smith, S.J., Blake. A.J., and Moody, C.(. (2009) Synthesis of ( )-hkonide B (smenochromene D) using a regioselective Claisen rearrangement, separation of the enantiomers and stereochemical assignment Otg. Biomol. Chem., 7, 2127-2134. 

A thio-Claisen rearrangement174 was used for the regioselective synthesis of thiopyrano[2,3-b]pyran-2-ones and thieno[2,3-b]pyran-2-ones (Eq. 12.76). A convenient method for the aromatic amino-Claisen rearrangement of N-(l,l-disubstituted-allyl)anilines led to the 2-allylanilines being produced cleanly and in high yield by using a catalytic amount of p-toluenesulfonic acid in acetonitrile/water (Eq. 12.77).175... 

A review of Claisen rearrangements in aqueous solution has appeared. The synthesis of natural products utilizing tandem Diels-Alder additions with sigmatropic rearrangement processes has been reviewed, and a brief review of the regioselective synthesis of coumarins, quinolones and thiocoumarins with 3,4-fused pyran or furan ring systems by the Claisen rearrangement has been presented. ... 

Unusual regioselectivity has been observed180 during the reductive coupling of alkynes and allenes by hydrozirconation and zinca-Claisen rearrangement. 

Acceleration of Claisen rearrangements.2 The Claisen rearrangement of an allyl vinyl ether is markedly accelerated by a stabilized a-sulfonyl carbanion at the 2-position. Thus 1 and 2 rearrange to the y,d-unsaturated ketone 3 in the presence of potassium hydride and 18-crown-6 at moderate temperatures. Rates can be further enhanced by addition of HMPT. Substitution of methyl groups on either the allyl or vinyl units does not affect the regioselectivity but can accelerate the rate of rearrangement. 

Rapid Claisen rearrangement of allyl phenyl ether and meta-substituted derivatives has been reported to occur with poor regioselectivity at 250 °C in dicationic ionic liquids, especially (24).23 Claisen rearrangement of resorcinol allyl ethers (25) was found to have poor regioselectivity under thermal or microwave conditions, but it was further found that selectivity for the 6-substituted product (26) over (27) could be improved to 13 1 using boron trichloride and an appropriately protected ether (25 ... 

Synthesis.—Reisch, Mester, and co-workers have made important contributions this year by synthesizing the alkaloids furacridone (34) and ( )-rutacridone (37) for the first time. Regioselective etherification of 1,3-dihydroxy-jV-methylacridone (32 R = H) gave the acetal (33), which furnished furacridone (34) as the major product of acid-catalysed cyclization (Scheme 4). Claisen rearrangements of the 3-allyloxy-acridone (32 R = CH2CH=CH2) and the propargyl derivative (32 R = CH2C=CH) were also studied.18... 

The arylsulfonyl carbanion accelerated Claisen rearrangement is completely regioselective and has also been found to be highly diastereoselective (Scheme 2). The stereochemical course of the reaction conforms to the familiar chair-like transition state model usually invoked for the classic thermal process. Recently, high degrees of asymmetric induction have been observed in tlie rearrangements of chiral cyclic phosphoramidate stabilized carbanion derivatives. ... 

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-... 

The enone (130), the pesticide Coumaphos, undergoes regioselective dimerization on irradiation at wavelengths > 313 nm. The product was identified as the dimer (131). Irradiation of the furocoumarin, Imperatorin (132), in methanol affords a low yield of the (2t2)-dimer (133). This product is accompanied by the dealkylated compound (134) and the rearrangement product (135). This last compound is formed by a photo-Claisen rearrangement. The influence of solvent was studied and the dimer was obtained only in methanol. The other two products were formed in a variety of polar and non polar solvents. In any solvent the dealkylated compound (135. was the main product. ... 

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