Larger Cycloalkanes 4-5 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. Strain energy of 14 kcal mol -1.

Polycyclic Alkanes 4-5 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.