Page 614 - Advanced Organic Chemistry Part B - Reactions & Synthesis

588 which the substituent Z is an alkynyl group. The E-isomer leads to anti product and
the Z-isomer to the syn product.
CHAPTER 6
Concerted H H OH
Cycloadditions, H H
Unimolecular
Rearrangements, and CH 3 – O CH 3 OH
Thermal Eliminations H H R CH 3
anti-isomer
R
R E-isomer
H H H OH
H
H – O H OH
H C R
CH 3 3 CH 3
R syn-isomer
R
Z-isomer
The preferred TS minimizes interaction between the Z and allylic substituents. This
stereoselectivity is illustrated in the rearrangement of 18 to 19.

H H H H CH OSiR 3
2
H
CH 3 CH 3
R SiOCH 2 CH 3
3
O R SiOCH 2 CH
3
H CH 3 OH CH 3 OH 3
19
18
Ref. 294
There are other means of generating the anions of allyl ethers. One of the most
useful for synthetic purposes involves a lithium-tin exchange on stannylmethyl ethers
(see Section 7.1.2.4). 295

R
R O SnR 3 RLi R O Li
OLi
CH 2
Another means involves reduction of allylic acetals of aromatic aldehydes by SmI . 296
2
R
3 equiv SmI 2
ArCH(OCH CH CHR) 2 ArCHOCH CH CHR ArCHCHCH CH 2
2
2
–
OH
[2,3]-Sigmatropic rearrangements of anions of N-allyl amines have also been
observed and are known as aza-Wittig rearrangements. 297 The reaction requires anion
stabilizing substituents and is favored by N-benzyl and by silyl or sulfenyl substituents

294
M. M. Midland and J. Gabriel, J. Org. Chem., 50, 1143 (1985).
295 W. C. Still and A. Mitra, J. Am. Chem. Soc., 100, 1927 (1978).
296 H. Hioki, K. Kono, S. Tani, and M. Kunishima, Tetrahedron Lett., 39, 5229 (1998).
297
C. Vogel, Synlett, 497 (1997).