Section 29.3 Electrocyclic Reactions 109thermal conditions forms the trans isomer under photochemical conditions, and the
compound that forms the trans isomer under thermal conditions forms the cis isomer
under photochemical conditions.
Under thermal conditions, (2E, 4 Z)-hexadiene cyclizes to cis-3,4-dimethylcyclobutene,
and (2E, 4 E)-hexadiene cyclizes to trans-3,4-dimethylcyclobutene.
As we saw with the octatrienes, the configuration of the product changes if the re-
actions are carried out under photochemical conditions: The trans isomer is obtained
from (2E,4Z)-hexadiene instead of the cis isomer; the cis isomer is obtained from
(2E,4E)-hexadiene instead of the trans isomer.
CH 3CH 3CH 3
CH 3CH 3CH 3CH 3CH 3(2E, 4 Z)-hexadiene trans-3,4-dimethylcyclobuteneHcis-3,4-dimethylcyclobuteneHH(2E, 4 E)-hexadieneHHHHhHhCH 3CH 3CH 3CH 3(2E, 4 E)-hexadiene trans-3,4-dimethylcyclobuteneH
H H∆ HH
CH 3HHCH 3HCH 3CH 3(^3) ∆
4
5 6
2
1
(2E, 4 Z)-hexadiene cis-3,4-dimethylcyclobutene
CH 3
(2E, 4 Z, 6 E)-octatriene
cis-5,6-dimethyl-1,3-cyclohexadiene
H
H
CH 3
CH 3
CH 3
(2E, 4 Z, 6 Z)-octatriene
trans-5,6-dimethyl-1,3-cyclohexadiene
CH 3
CH 3
h
h
CH 3
H
CH 3
H