In the Claisen rearrangement

The use of the glucose chiral auxiliary by Lubineau et al. led to moderate asymmetric induction in the Claisen rearrangement (20% de) (Eq. 12.75).159 Since it could be removed easily, glucose functioned here as a chiral auxiliary. After separation of the diastereomers, enan-tiomerically pure substances could be obtained. 

The sequence with oxazole 73 also proves to be applicable to systems in which the double bond of the allylic alcohol fragment is incorporated into a heteroaromatic ring, such as in furfuryl alcohol (77a) and thiophene-2-methanol (77b).28 After hydrolysis of the oxazolone ring with water, V-benzoyl-3,3,3-trifluoro-2-(2-methyl-3-furyl)alanine (79a) and /V-benzoyl-3,3,3-trifluoro-2-(2-melhyl-3-thienyl)alanine (79b) are obtained. The 2-methylene helerocyclcs 78 originally formed in the Claisen rearrangement undergo rearomatizalion under the reaction conditions. 

Irradiation of aryl ethers leads to products analogous to those obtained in the Claisen rearrangement (185). The solvent employed strongly affects the yield of phenol but not that of the rearrangement product... 

Not only an aryl group—as in Figure 14.46—but also an alkenyl group can participate in the Claisen rearrangement of allyl ethers (Figure 14.47). Allyl enol ethers are the substrates in this case. Figure 14.47 shows how this kind of allyl alkenyl ether (D) can be prepared from an... 

The Claisen rearrangement was used in the asymmetric total synthesis of (+)-9(ll)-dehydroestrone methyl ether (5), a versatile intermediate in the synthesis of estrogens5 (Scheme 1. If). The key feature of the synthesis is the successful development of the asymmetric tandem Claisen-ene sequence. Thus, a solution of the cyclic enol ether 6 in toluene was heated in a sealed tube at 180 C for 60 hours to afford the product 9 in 76% isolated yield after deprotection of the silyl enol ether. The Claisen rearrangement of the allyl vinyl ether 6 occurred stereoselectively to give an intermediate (7), in which the 8,14-configuration was 90% syn. The stereoselectivity in the Claisen rearrangement can be explained... 

An important example is the reaction of prenyl bromide with phenols. This is simply carried out with K2CO3 in acetone as phenols are acidic enough (pKa 10) to be substantially deprotonated by carbonate. The product is essentially entirely from the S>f2 route, and is used in the Claisen rearrangement (Chapter 36). 

Alkene stereochemistry in the Claisen rearrangement comes from a chair-like transition state... 

In a follow-up paper, Jacobsen showed that the chiral guanidinium 80 provides catalytic power and enantioselectivity in the Claisen rearrangement systems such as Reaction 6.36. Here the reaction proceeded in 81 percent yield with an ee of 84 percent. The reaction is also diasteroselective for Reaction 6.37, the diastere-oselectivity increases from 11 1 without catalyst to 16 1 with 80. 

The rate enhancement observed with polar solvents corresponds well with a dipolar transition state in the Claisen rearrangement see Section 7.2.2.12. 

For an estimate of the steric effect of axial methyl groups in the Claisen rearrangement, see ref. 222. 

ACTIVATION PARAMETERS AND RATE DATA FOR THE INDIVIDUAL STEPS IN THE ClAISEN REARRANGEMENT AT 170 °C... 

RATE COEFFICIENTS AND ACTIVATION PARAMETERS FOR THE FORMATION OF 2-ALLYL-5-X-PHENOLS IN THE ClAISEN REARRANGEMENT ... 

Then, hydrolysis with TiC, acetic acid and water at room temperature led to the expected cyclopentanone (199) . The stereochemistry of the newly formed C-C bond in the six-membered ring is equatorial, which contrasts with the results obtained in the Claisen rearrangements ... 

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