Organic Chemistry

024 CHAPTER 26 Lipids

Two German chemists,Heinrich
Otto Wieland (1877–1957)and
Adolf Windaus (1876–1959), each
received a Nobel Prize in chemistry
(Wieland in 1927 and Windaus
in 1928) for work that led to the
determination of the structure of
cholesterol.

Heinrich Wieland, the son of a
chemist, was a professor at the
University of Munich, where he
showed that bile acids were steroids
and determined their individual
structures. During World War II, he
remained in Germany but was openly
anti-Nazi.

Many steroids have methyl groups at the 10- and 13-positions. These are called

angular methyl groups. When steroids are drawn, both angular methyl groups are

shown to be above the plane of the steroid ring system. Substituents on the same side

of the steroid ring system as the angular methyl groups are designated

(indicated by a solid wedge). Those on the opposite side of the plane of the ring

system are (indicated by a hatched wedge).

PROBLEM 20

A at C-5 means that the A and B rings are _____ fused; an at C-5

means that they are _____ fused.

The most abundant member of the steroid family in animals is cholesterol, the pre-

cursor of all other steroids. Cholesterol is biosynthesized from squalene, a triterpene

(Section 26.6). Cholesterol is an important component of cell membranes

(Figure 26.1). Its ring structure makes it more rigid than other membrane lipids. Be-

cause cholesterol has eight asymmetric carbons, 256 stereoisomers are possible, but

only one exists in nature (Chapter 5, Problem 20).

The steroid hormones can be divided into five classes: glucocorticoids, mineralo-

corticoids, androgens, estrogens, and progestins. Glucocorticoids and mineralocorti-

coids are synthesized in the adrenal cortex and are collectively known as adrenal

cortical steroids. All adrenal cortical steroids have an oxygen at C-11.

Glucocorticoids, as their name suggests, are involved in glucose metabolism, as

well as in the metabolism of proteins and fatty acids. Cortisone is an example of a

glucocorticoid. Because of its anti-inflammatory effect, it is used clinically to treat

arthritis and other inflammatory conditions.

Mineralocorticoids cause increased reabsorption of and by the

kidneys, leading to an increase in blood pressure. Aldosterone is an example of a

mineralocorticoid.

Na+, Cl-, HCO 3 -

H H H H

H

HC

O

H H 3 C

HO

CH 3

CH 2 OH

O

H 3 C

CO

OH

CH 2 OH

CO

O

O

cortisone aldosterone

H 3 C

H 3 C

H

H H

HO

cholesterol

b-hydrogen a-hydrogen

A-substituents

B-substituents

Adolf Windausoriginally intended
to be a physician, but the experience
of working with Emil Fischer for a
year changed his mind. He
discovered that vitamin D was a
steroid, and he was the first to
recognize that vitamin contained
sulfur.

B 1

H H

H

R

H

R

H

CH 3 CH^3

A and B rings are trans fused

CH 3 CH^3

H

A and B rings are cis fused

CH 3 and H

are trans

CH 3 and H

are cis