Page 897 - Advanced Organic Chemistry Part B - Reactions & Synthesis

of the unshared electrons on the ester oxygen. The exo TS is believed to be favored 873
by an electrostatic interaction between the oxygen and C(4).
SECTION 10.1
CH 3 Reactions and
CH 3 CH 3 H CH 3 Rearrangement
H AlCl 3 + SnCl 4 H H Involving Carbocation
H CO CH 3 Intermediates
2
O O
H O CHCO 2 CH 3 O OCH 3
Cl 3 Al Cl Sn
4
OH OH
CH 3
H CO CH 3 CO CH 3 CH 3
2
2
H H
HO CO CH 3 CH 3 CH 3
2
H HO CO 2 CH 3
syn anti
Despite the cyclic character of these TSs, both the bond distances and charge distri-
bution are characteristic of a high degree of charge separation, with the butenyl
fragment assuming the character of an allylic carbocation.
Visual models, additional information and exercises on the Carbonyl-Ene
Reaction can be found in the Digital Resource available at: Springer.com/carey-
sundberg.

Examples of catalyst control of stereoselectivity have been encountered in the
course of the use of the ene reaction to elaborate a side chain on the steroid nucleus.
The steroid 4 gave stereoisomeric products, depending on the catalysts and specific
aldehyde that were used. 33 This is attributed to the presence of a chelated structure in
the case of the SnCl catalyst.
4
CH 3

O CHCH OCH Ph O CHCH 2 OTBDMS
2
2
OCH3
4
SnCl 4 (CH ) AlCl H CH 3
3 2
H Al
O Ch 3
O OCH 2 Ph H Cl
H
Sn non–chelated OSiR 3
Cl 4 chelated TS
TS
CH 3 OH CH 3 OH
OCH Ph CH OTBDMS
2
2

33
K. Mikami, H. Kishino, and T.-P. Loh, J. Chem. Soc., Chem. Commun., 495 (1994).