4-5
Larger Cycloalkanes
Rings larger that cyclohexane have more strain.
•Bond angle distortion
•Partial eclipsing of hydrogens
•Transannular steric repulsions
Medium sized rings adopt several conformations that are very
close in energies, such as cyclodecane:
Strain energy of 14 kcal mol-1.
Large-sized cycloalkanes such as cyclotetradecane are able to
adopt staggered and all-anti conformations similar to straight
chain alkanes and are essentially strain free. Attachment of
substituents, however, usually introduces some strain.
4-5
Polycyclic Alkanes
Polycyclic alkanes may contain fused or bridged rings.
The fused system, decalin, can be compared to the disubstituted
molecule, 1,2-diethylcyclohexane.
Decalin is an example of a fused bicyclic
ring system.
The shared carbon atoms are called
ring-fusion carbons.
Groups attached to the ring-fusion
carbons are called ring-fusion
substituents.
A second example of ring fusion, norborane, can be compared to
the compound, cis-1,3-dimethylcyclopentane:
Norborane is an example of a bridged bicyclic ring system.
Two non-adjacent carbon atoms belong to both rings and are
called bridgehead carbon atoms.
Bicyclic ring systems can be either cis- or trans-fused:
Examples of terpenes include:
• Chrysanthemic acid
Natural insecticide
• Grandisol
Boll weevil sex attractant
• Menthol
Peppermint oil
• Camphor
Camphor Tree
• -Cadinene
Juniper and ceder trees
• Taxol
Pacific yew tree,
Anti-tumor drug
Steroids are tetracyclic natural products with powerful
physiological activities.
Steroids frequently function as hormones, or regulators of
biological activities.
Synthetic steroids are used in the treatment of cancer, arthritis,
allergies, and in birth control.
Steroids consist of 3 fused cyclohexane rings fused to a
cyclopentane ring. The ring junctions are usually trans.
The rings are labeled A,B,C,D.
Methyl groups at C10 and C13 and
oxygen at C3 and C17 are
common.
Due to the trans ring fusion, an all
chair conformation is assumed
with the ring junction hydrogens
and methyl groups in axial
positions.
Groups attached above the plane of the steroid ring structure are
termed  while those below are termed .
Axial methyl groups are referred to as angular methyls because
they sharply protrude from the framework.
Three common steroids are:
Cholesterol is present in almost all human and animal tissue. It can
precipitate in the arteries, causing arteriosclerosis and heart disease. It
is a precursor for bile acids and steroid hormones.
Cholic acid is a bile acid involved in emulsification, digestion and
absorption of fats.
Cortisone is involved in regulating electrolyte and water balance in the
body, as well as carbohydrate and protein metabolism.
Sex hormones are divided into three types: Male sex hormones
(androgens), female sex hormones (estrogens), and pregnancy
hormones (progestins).
Examples of each are
•Testosterone
Produced by testes. Responsible for masculine characteristics.
•Estradiol
Responsible for secondary female characteristics and participates
in regulation of menstrual cycle.
•Progesterone:
Responsible for preparing the uterus for the implantation of an
egg.