Claiscn rearrangement

The orfho-Claiscn rearrangement of phenyl vinyl ether 8 to 2-vinylphenol 9 (Scheme 4.5) is a very slow reaction, requiring 80 h of reflux in DMF to give a yield of only 34%. Using the CEM microwave system an 80% yield was obtained in 5 h at 196 °C and 200 kPa [6],... 

Krespan also found that allyl alcohol added to 1-mcthoxy-2-(trifluoromethyl)tetrafluoro-propcnc, derived from perfluoro(2-mclhylpropene). to give the rearranged ester 5. a presage of the Johnson Claiscn rearrangement. Reactions of allylic alcohols with vinyl ethers derived from perfluoro(2-methylpropene) were subsequently studied by Andreev and co-workers11 and are discussed below. 

The reaction of allyl alcohol with octafluorocyclopentene and the subsequent Claiscn rearrangement are complete within 2 hours at 25 C, giving the rearranged 2-allylheptafluorocy-clopenlanone (6).x... 

In the ortho ester Claiscn rearrangements of phenylsulfanyl-substituted allylic alcohols 11, the Z-isomer of product 12 is formed predominantly, but not always exclusively (Table 17).30 When triethyl orthopropionate (R2 = Me) is used, the diastcrcoselectivity is good to excellent with ( )-allylic alcohols 11, whereas the Z-isomer of 11a gives almost a 1 1 mixture. The stereochemical results were discussed on the basis of transition-state models.30... 

Johnson Claiscn rearrangement of propargylic alcohols 13 gives ethyl 3-(trifluoromethyl)alka-3,4-dienoates 14 in moderate yields.34... 

Tabic 21. Synthesis of 2-Fluoroalk-4-enoic Acids 16 by the Ireland-Claiscn Rearrangement of Allylic Fluoroacetates 15... 

The use of a silyl triflate and triethylamine to carry out the Ireland-Claiscn rearrangement was successfully employed by Nakai and co-workers60 in the early 1980s to convert allylic 3,3,3-trifluoropropionates 17 into 2-(trifluoromethyl)alk-4-enoic acids 18 (Table 22). The low dia-stereoselectivily in the rearrangement of 17b was attributed to the mixture of (E)- and (Z)-l-... 

Reacting an allylic alcohol with an ynavnine to generate an 0-allyl ketene (EA -acetal results in the Ficini-Claiscn rearrangement.1-53 Reaction of (E)- or (Z)-3a with (V,/V-diethyl-2-(phcnyl-sulfanyl)cthynamine results in low diastcreoselectivity in the formation of 6-(benzyloxy)-A ,A-diethyl-2-(phenylsulfanyl)-3-(trifiuoromethyl)hex-4-enamide (5).34 As is the case for the results in Table 23, the stereoselectivity and chemical yield are better for (E)-3a than for (Z)-3a. 

The Claiscn rearrangement of allyl vinyl ethers is usually an irreversible reaction due to the energetic benefit of forming aC-O double bond. However, in strained bicyclic systems the retro-Claisen rearrangement (3-oxa-Cope rearrangement) of y,<5-unsaturated aldehydes has been observed32. Sometimes equilibrium mixtures of vinyl ether and carbonyl compound were found. For example, the ratio of the valence tautomers, bicyclo[3.1.0]hex-2-ene-6-cWo-methanal to 2-oxabicyclo[3.2.l]octa-3,6-diene, is approximately 7 333. Nevertheless this reaction was used in the preparation of a key intermediate in a prostacyclin synthesis34. 

The aldol reaction -Claiscn rearrangement sequence has also been applied to the synthesis of 19 and 20. synthons having four stereogenic centers and being of potential use for the synthesis of polyether ionophores444. 

Table 24. Thia-Claiscn Rearrangement of Secondary S-Allyl Thioamides580...

The Claiscn rearrangement of chorismate 1, the last common intermediate in the biosynthesis of aromatic amino acids via the shikimate pathway, to prephenate 2 proceeds readily without catalysis263 and is greatly accelerated by a factor of 1.9 xlO6 by the enzyme chorismate mutase655-656. 

For chirality transfer via a furan ring transfer reaction of tetrahydrobenzofuranyl allenyl ethers and Claiscn rearrangement see refs 675 and 676. The synthesis of cyclopropanes via Claisen rearrangements of propargylic alcohol derivatives is reported in ref 677 and for acetylene Claisen rearrangements of thio derivatives see refs 678 and 679. 

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