Page 959 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

for synthetic purposes is the Wittig rearrangement, in which a strong base converts 943
allylic ethers to -allyl alkoxides. 329
SECTION 10.6
– Sigmatropic
O CHZ HO CHZ
base H + Rearrangements
–
ZCH OCH CH CHR ZC HOCH CH CHR CH 2 CHCHR CH 2 CHCHR
2
2
2
As the deprotonation at the ’-carbon must compete with deprotonation of the -
carbon in the allyl group, most examples involve a conjugated or electron-withdrawing
substituent Z that can facilitate deprotonation. 330 In addition to direct deprotonation,
there are other means of generating the anions of allyl ethers. 331 332
The stereochemistry of the Wittig rearrangement can be predicted in terms of a
2
cyclic TS in which the -substituent R prefers an equatorial orientation. 333
H H H
R 2 R 2 R 2
: –
O
O O
Z Z Z
transition structure
A consistent feature of the stereochemistry is a preference for E-stereochemistry at
the newly formed double bond, but the reaction can also show stereoselectivity at the
newly formed single bond. This stereoselectivity has been carefully studied for the
case where the substituent Z is an acetylenic group.
H H OH
R 2 R 2
CH 2
CH 3 3 R
–
O OH R
CH 3
R R
E-isomer anti- isomer
H H OH
R 2 R 2
R 2
-
O OH R
CH 3 CH 3
CH 3
R Z-isomer R syn-isomer
The preferred stereochemistry arises from the TS that minimizes interaction between
2
the alkynyl and R substituents. This stereoselectivity is exhibited in the rearrangement
of 37 to 38.
329
J. Kallmarten, in Stereoselective Synthesis, Houben Weyl Methods in Organic Chemistry,
R. W. Hoffmann, J. Mulzer, and E. Schaumann, eds., G. Thieme Verlag, Stuttgart, 1996, pp. 3810;
T. Nakai and K. Mikami, Org. Reactions, 46, 105 (1994).
330 For reviews of [2,3]-sigmatropic rearrangement of allyl ethers, see T. Nakai and K. Mikami, Chem.
Rev., 86, 885 (1986).
331
W. C. Still and A. Mitra, J. Am. Chem. Soc., 100, 1927 (1978).
332 K. Hioki, K. Kono, S. Tani, and M. Kunishima, Tetrahedron Lett., 39, 5229 (1998).
333
R. W. Hoffmann, Angew. Chem. Int. Ed. Engl., 18, 563 (1979); K. Mikami, Y. Kimura, N. Kishi, and
T. Nakai, J. Org. Chem., 48, 279 (1983); K. Mikami, K. Azuma, and T. Nakai, Tetrahedron, 40, 2303
(1984); Y.-D. Wu, K. N. Houk, and J. A. Marshall, J. Org. Chem., 55, 1421 (1990).