Microwave irradiation Claisen rearrangements

Propargyl naphthyl ethers can be rearranged to to naphthofurans in shorter reaction times when exposed to microwave irradiation (Scheme 89) <1996JCM338>. These alkynes first undergo Claisen rearrangement to form an allene intermediate which in turn spontaneously cycloisomerizes to the benzofuran product. 

Rearrangements. Under microwave irradiation the Claisen rearrangement of N-allyl-anilines is effectively catalyzed by BF3 OEt2. Other Lewis acids are much inferior catalysts under the same conditions. ... 

A few solvent-free examples of Claisen rearrangement reactions under the action of microwave irradiation have been described. One involves the double Claisen rearrangement of bis(4-allyloxyphenyl)sulfone into bis(3-allyl-4-hydroxy-phenyl)sulfone (Scheme 8.43) [111]. Similarly, 3 -allyl-2 -hydroxyacetophenone has been obtained in quantitative yield from 2 -allyloxyacetophenone by Bennett et al. [112]. 

Thermal rearrangement of the simple allyl ether was successful in preliminary studies (Eq. 3.1.43) [56]. Unfortunately, repeated attempts to induce the Claisen rearrangement of 66 resulted in nearly complete recovery of the starting material (Eq. 3.1.44, Table 3.1.5, entry 1). With these results at hand, the microwave irradiation was attempted to induce the desired rearrangement. As a result, a small increase in yield was observed when 66 was reacted under the conditions given in Table 3.1.5, entry 2. The microwave irradiation of a solution of 68 after only 15 min resulted in a 71% yield of the desired allene 69 as a single diastereoisomer. After extensive optimization studies, it was shown that microwave irradiation with 15 consecutive 60-s pulses allowed the formation of the 69 in 88% yield. 

Tab. 3.1.5 Claisen rearrangement studies using microwave irradiation (Eq. 3.1.44).

Acceleration of the aromatic Claisen rearrangement under microwave irradiation conditions [46] and the photo-Claisen rearrangement has also been reported [47]. 

Claisen rearrangement of Merrifield resin derivatized with O-allyl phenolic ethers into ortho-allylic phenols under microwave irradiation was performed by Kumar et al. [ 135]. The reactions were carried out in a household microwave oven, in which samples were irradiated in an open Erlenmeyer flask for 4-6 min (600 W) to afford products in 84-92% yield. In comparison, under thermal conditions, similar yields were obtained after 10-16 h at 140 °C (Fig. 39). 

Nicolaou has recently reported the use of microwave irradiation for the Claisen rearrangement of 77. The rearrangement product then underwent a [2 + 2] cycloaddition and loss of Boc protecting groups to produce the bicyclo[3.2.0]heptane derivative 78. ... 

When repeated attempts failed to accomplish the Claisen rearrangement of 79 to allenyl ketone 80, Ley and co-workers relied on microwave irradiation to address this challenging bond construction. When 79 was irradiated with fifteen consecutive sixty second microwave pulses, a reaction proceeded to give a single diastereomer of the allene with excellent yield. ... 

Similar processes have been designed and utilized extensively by Barriault and coworkers for efforts in natural product synthesis. An impressive example of this strategy was utilized for the synthesis of the neo-clerodane skeleton of teucrolivin A. Thermal rearrangement of 92 under microwave irradiation results in a tandem sequence featuring an oxy-Cope rearrangement, followed by the Claisen rearrangement and subsequent ene reaction to give 93. 

It has been shown that microwave irradiation speeds up the benzil-benzilic acid rearrangement and increases the yield of product. Spiro ketones of the type (223) have been found to be susceptible to nucleophilic-induced retro-Claisen condensations. These have led to molecular rearrangements that destroy spiro connectivity see Scheme 55. The rearrangements of radical anions derived from aliphatic ketones have been studied using electrochemical techniques. A model system has been unveiled in which nucleophilic catalysis of amide isomerization is characterized for... 

Bermner and Organ (2008) reported [3,3]-aza-Claisen rearrangement under microwave irradiation to obtain good yield of pyrroles within 30 min as compared to one pot, domino aldehyde/amine condensation and imine-allene cyclization while Aza-Claisen rearrangement of the unexplored. A -thiophenojg acetyl-a-vinyl piperidine substrate and the oxone-induced transannulation via microwave accelerated, chiral transfer using the achiral phenylsulfide auxiliary has been described by Sim etal. (2012). 

Aza-Claisen rearrangement of allylic imidates and thiocyanates in presence of o-xylene under microwave irradiation resulted in corresponding amides and isothiocyanates (Gonda et al., 2007). The reaction rate was enhanced 8-30 and 24-80 times, respectively. 

Microwave irradiation strongly enhances the rate of Claisen rearrangement. This helps in solving the problem of the long-term heating under conventional conditions. The yield of rearranged produets in classical method is about 85% in 6 h where as it was inereased to 92% in 6 min by using N,N-dimethylformamide as solvent under mierowave irradiation. 

It was observed that the rate of catalytic enantioselective Claisen rearrangement was drastically increased imder microwave irradiation (Nushiro et al., 2013) without any loss of the enantioselectivity. It was revealed that enantioselectivity decreased... 

Claisen rearrangement of 4-methyl-7-[4-(substituted-but-2-ynyloxy]-coumarins under the microwave irradiation was reported by Valizadeh and Shockravi (2006). A good yield of 4 -substituted methyl-4,5 -dimethylangelicins was obtained in short reaction times. 

Srikrishna and Nagaraju (1992) observed the three-step ortho ester Claisen rearrangement of allyl and propynyl alcohols in dry condition under microwave irradiation. The reaction was over in 10-15 min. 

The Claisen rearrangement of aldehydes into substituted-2-oxohex-5-enoic acids in a tandem three step, one pot method, water, K COj and under microwave irradiation (50 W) was reported (Quesada and Taylor, 2005). The reaction was competed in 10 min and good to excellent yield (55-93%) was obtained. 

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