Page 587 - Advanced Organic Chemistry Part B - Reactions & Synthesis

acetals gives ,
-unsaturated amides (Section 6.4.2.4). In all cases, the reactions occur 561
with 1,3-transposition of the allylic group.
SECTION 6.4
O R′ O R′ [3,3]-Sigmatropic
XO XO O R′ R N R 2 N O R′
2
Rearrangements
R R R R
R′′ R′′ R′′ R′′
X = alkyl, silyl

6.4.2.1. Claisen Rearrangements of Allyl Vinyl Ethers. The [3,3]-sigmatropic
rearrangement of allyl vinyl ethers leads to ,
-enones and is known as the Claisen
rearrangement. 219 The reaction is mechanistically analogous to the Cope rearrangement

and occurs at temperatures above 150 C. As the product is a carbonyl compound, the
equilibrium is usually favorable. The reaction introduces an -acyl alkyl group at the
-carbon of the allylic alcohol, with 1,3-transposition of the allylic double bond.
R
R O CH 2
R OH + ZOCH = CHR′ R′
R′ CH O
The reactants can be made from allylic alcohols by mercuric ion-catalyzed exchange
with ethyl vinyl ether. 220 The allyl vinyl ether need not be isolated and is often
prepared under conditions that lead to its rearrangement. The simplest of all Claisen
rearrangements, the conversion of allyl vinyl ether to 4-pentenal, typifies this process.

O
CH 2 CHCH OH
2
+ Hg(OAc) 2 [CH 2 CHCH OCH CH ] CH 2 CHCH 2 CH 2 CH
2
2
Δ
CH 2 CHOCH CH 3 96%
2
Ref. 221
Acid-catalyzed exchange can also be used to prepare the vinyl ethers.
H +
RCH CHCH OH + CH CH OCH CH 2 RCH CHCH OCH CH 2
2
3
2
2
Ref. 222
Vinyl ethers can also be generated by thermal elimination reactions. For example,
base-catalyzed conjugate addition of allyl alcohols to phenyl vinyl sulfone generates 2-

(phenylsulfinyl)ethyl ethers that can undergo elimination at 200 C. 223 The sigmatropic
219
F. E. Ziegler, Chem. Rev., 88, 1423 (1988); A. M. M. Castro, Chem. Rev., 104, 2939 (2004).
220 W. H. Watanabe and L. E. Conlon, J. Am. Chem. Soc., 79, 2828 (1957); D. B. Tulshian, R. Tsang, and
B. Fraser-Reid, J. Org. Chem., 49, 2347 (1984).
221
S. E. Wilson, Tetrahedron Lett., 4651 (1975).
222 G. Saucy and R. Marbet, Helv. Chim. Acta, 50, 2091 (1967); R. Marbet and G. Saucy, Helv. Chim.
Acta, 50, 2095 (1967).
223
T. Mandai, S. Matsumoto, M. Kohama, M. Kawada, J. Tsuji, S. Saito, and T. Moriwake, J. Org. Chem.,
55, 5671 (1990); T. Mandai, M. Ueda, S. Hagesawa, M. Kawada, J. Tsuji, and S. Saito, Tetrahedron
Lett., 31, 4041 (1990).