Fundamentals of Medicinal Chemistry

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9 C D

A B

(a)

(Trans A/B ring junction ) (Cis A/B ring junction )

(b)

Figure 1.23 (a) The ring and number system of steroids. (b) The conformations of the ring

junctions found in steroids with saturated rings

unsaturated and in some compounds ring A is a benzene ring. Six membered

saturated rings usually have a chair conformation whilst the five mem-

bered saturated rings have an envelope configuration. For steroids with satur-

ated A and B rings with chair conformations the substituents at the A/B ring

junction can betransorcisbut those of the B/C and C/D ring junction are

usuallytrans(Figure 1.23(b)). Bonds that lie in the plane or project above the

plane of the rings are known asb-bonds (solid lines) whilst bonds that are

directed below the plane of the rings are designated asa-bonds (dotted lines).

The traditional number system used for steroids is also extended to include their

side chains. Many steroids are biologically important. Cholesterol, for example,

is an important component of mammalian cell membranes (Appendix 3), whilst

ergosterol (Figure 1.24) occurs in the cell membranes of fungi. Steroids such as

testosterone and oestradiol act as hormones.

OH

H

HO

H

H H

OH H 3 C

O

H

H

H H

H 3 C

HO

H

CH 3

CH 3

H H

CH 3

H 3 C

H 3 C

H 3 C

H H

H

HO

H

H H

H 3 C

H 3 C

CH 3

CH 3

H 3 C

Cholesterol Ergosterol Testosterone Oestradiol

Figure 1.24 Examples of some important naturally occuring steroids

1.5.5 Terpenes

Terpenes are compounds whose carbon skeletons can be artifically divided into

isoprene units (Figure 1.25), although there are some exceptions to this rule.

22 BIOLOGICAL MOLECULES