Organic Chemistry

124 CHAPTER 29 Pericyclic Reactions

oxyluciferin is promoted to an excited state because suprafacial overlap can occur only

under photochemical conditions. When the electron in the excited state drops down to

the ground state, a photon of light is released. In this case, the luciferin molecule acts

as both the source of the unstable four-membered ring and as the dye molecule that

had to be added to the cold-light reaction.

A Biological Reaction Involving an Electrocyclic Reaction

and a Sigmatropic Rearrangement

7-Dehydrocholesterol, a steroid found in skin, is converted into vitamin by two

pericyclic reactions. The first is an electrocyclic reaction that opens one of the six-

membered rings in the starting material to form provitamin This reaction occurs

under photochemical conditions. The provitamin then undergoes a [1,7] sigmatropic

rearrangement to form vitamin The sigmatropic rearrangement takes place under

thermal conditions and is slower than the electrocyclic reaction, so vitamin contin-

ues to be synthesized for several days after exposure to sunlight. Vitamin is not the

active form of the vitamin. The active form requires two successive hydroxylations of

vitamin D 3 —the first occurs in the liver and the second in the kidneys.

D 3

D 3

D 3.

D 3.

D 3

luciferin

luciferase

oxyluciferin

HO S

N S

N

H

COH ATP

−

O

HO S

N S

N

H

COAMP

O

HO S

N S

N COAMP

O

OO

−

HO S

N S

N COAMP

O

O O

−

OO

HO S

N S

N

C OAMP

O

B

O

OO

HO S

N S

N

C

HO S O*

N S

N HO S O

N S

N

+ CO 2 + light

an unstable

four- membered ring

has an electron in

an excited state

CH 3

HO

H 3 C

H H

H 3 C

CH 3

CH 3

CH 3

h

an electrocyclic

reaction

7-dehydrocholesterol

HO

H 3 C

H

H 3 C

CH 3

CH 3

CH 3

provitamin D 3

a [1,7] sigmatropic

rearrangement

HO

H 2 C

H

H 3 C

CH (^3) CH 3
vitamin D 3