Page 615 - Advanced Organic Chemistry Part B - Reactions & Synthesis

on the allyl group. 298 The trimethylsilyl substituents can also influence the stereose- 589
lectivity of the reaction. The steric interactions between the benzyl group and allyl
SECTION 6.5
substituent govern the stereoselectivity and it is markedly improved in the trimethylsilyl
derivatives. 299 [2,3]-Sigmatropic
Rearrangements
X Ph
BuLi X
t-BocNCH C CHR THF–HMPA CH 2 NH-t-Boc
2
–40°C
CH Ph R
2
R R
X X
R X anti:syn
CH 3 H 3:2 Ph
H H 1:1 Ph
C 2 N N
5
) CH 4:3
(CH 3 2 H
CH 3 Si(CH ) <1:20 Boc Boc
3 3
C H 5 Si(CH ) 1:18
3 3
2
(CH ) CH Si(CH ) 1:11
3 3
3 2
anti syn
Some examples of synthetic application of the anionic Wittig rearrangement are
given in Scheme 6.19. The reaction in Entry 1 provided a 93:7 ratio favoring the syn
isomer, as expected for the preferred endo TS. Entry 2 is an example that employs the
lithium-stannane exchange to generate the anion. The reaction in Entry 3 accomplishes
a ring contraction. Under normal conditions, it is selective for the trans stereoisomer,
as would be expected from steric factors in the TS. In the presence of HMPA, the cis
isomer dominates, but the reason for the change is not known.
H CH 3 H CH CH
CH
3
3 3
CH 2 CH
O OH O OH 2
H H H H H H
sterically preferred TS trans cis

In Entry 4 the silyl group appears to introduce a controlling steric factor, leading to
the observed stereoisomer. The unsubstituted terminal alkyne, which reacts through
the dianion, gives the alternate isomer.

X
X
H H
CH 3 CH3
CH 3 CH3
H H
O O
favored for X = (CH ) Si favored for X = Li +
3 3

298 J. C. Anderson, S. C. Smith, and M. E. Swarbrick, J. Chem. Soc., Perkin Trans. 1, 1517 (1997).
299
J. C. Anderson, D. C. Siddons, S. C. Smith, and M. E. Swarbrick, J. Org. Chem., 61, 4820 (1996).