Organic Chemistry

Section 8.8 The Diels–Alder Reaction: A 1,4-Addition Reaction 319

3-D Molecules:

s-trans Conformation of

Butadiene; s-cis Conformation

of Butadiene

In order to participate in a Diels–Alder reaction, the conjugated diene must be in an
s-cis conformation because in an s-trans conformation, the number 1 and number 4
carbons are too far apart to react with the dienophile. A conjugated diene that is locked
in an s-trans conformation cannot undergo a Diels–Alder reaction.

A conjugated diene that is locked in an s-cis conformation, such as 1,3-cyclopentadi-
ene, is highly reactive in a Diels–Alder reaction. When the diene is a cyclic compound,
the product of a Diels–Alder reaction is a bridged bicyclic compound—a compound
that contains two rings that share two nonadjacent carbons.

There are two possible configurations for bridged bicyclic compounds, because the
substituent (R) can point either toward the double bond (theendo configuration) or
away from the double bond (the exoconfiguration).

points toward

the double bond

points away from

R the double bond

R

endo exo

CH 2

CHCO 2 CH 3

+ no reaction

locked in an

s-trans conformation

s-cis conformation

H

H

H

mild

interference

H

H

H

s-trans conformation

H

H

H H

H H

1

4

1

4

++

1,3-cyclopentadiene CO 2 CH 3

bridged bicyclic compounds

H

CH 2

CHCO 2 CH 3

locked in an both rings share these carbons

s-cis conformation

1

4

3

2 1

5

2

3

4

5

81%

CO 2 CH 3

H

19%

++

1,3-cyclopentadiene

bridged bicyclic compounds

H

CH 2

CHC N N

C N

60%

C

H

40%

Tutorial:

Bicyclic compounds