Page 530 - Advanced Organic Chemistry Part B - Reactions & Synthesis

504 Entry 3 involves another sultam auxiliary. The chirality of the product is consistent
with approach of the diene from the re face of a conformation in which the carbonyl
CHAPTER 6 oxygen is syn to the sulfonyl group.
Concerted
Cycloadditions, CH 3 CH
Unimolecular 3
Rearrangements, and
Thermal Eliminations N
S O
O O Al Et
Et

Entry 4 shows a carbohydrate-derived auxiliary with SnCl as the Lewis acid. This
4
dienophile also gives good enantioselectivity using TiCl as the Lewis acid. Entry 5 is a
4
proline-derived oxazolidinone auxiliary used in conjunction with ZrCl . The observed
4
diastereoselectivity is consistent with a chelated TS having an s-cis conformation at
the carbonyl group.
CH H
CH 3 3
CH 3 H CH
Ph Ph 3
Ph Ph
O
N N O
Zr
O
O O O
Entry 6 uses a chiral auxiliary derived from pyroglutamic acid. Entry 7 is an example
of the use of pantolactone as a chiral auxiliary to form a prostaglandin precursor.
The alkenyl oxonium ion dienophiles generated from dioxolanes can be made
diastereoselective by use of chiral diols. For example, acetals derived from anti-
pentane-2,4-diol react under the influence of TiCl /Ti i-OPr with stereoselectivity
4 4
ranging from 3:1 to 15:1.
CH 3
CH 3 CH 3
CCH CH
CH 2 2
O CH 2 O H
CH 3
O R TiCl or O
4
3
3 3
CH 3 (CH ) SiO SCF 3 CH 3 R
Ref. 92
Dioxolanes derived from syn-1,2-diphenylethane-1,2-diol react with dienes such as
cyclopentadiene and isoprene, but in most cases the diastereoselectivity is low.
Ph OH
CH 3 O
Ph Ph
) SiO SCF
O (CH 3 3 3 3 O CH CCH CH Ph O CH 3
OC H + 2 2
2 5
Ph O CH CH 2 Ph O 82% yield,
55:45 dr
Ref. 93
92 T. Sammakia and M. A. Berliner, J. Org. Chem., 59, 6890 (1994).
93
A. Haudrechy, W. Picoul, and Y. Langlois, Tetrahedron: Asymmetry, 8, 129 (1997).