Steroid-cyclohexane system

Physical Gelation of a Steroid-Cyclohexane System Kinetic Phenomenol( cal Approach... 

Figure 1. Phase diagram of the gelling steroid/cyclohexane system. The upper full line is the saturation curve (T) and the saturation temperature for the initial concentration. The dotted line indicates the separation between the two zones k and B. The lower full line indicates the transition to the cyclohexane plastic phase.(Reproduced with permission from Ref. 17. Copyright 1985 Academic Press.)...

P. Terech, Physical Gelation of a Steroid-Cyclohexane System Kinetic Phenomenological Approach, in Reversible Polymeric Gels and Related Systems (ed. P. Russo), American Chemical Society Symposium Series (ACS), 350, 115 (1987). 

Substituent groups on the steroid ring system can be either axial or equatorial. As with simple cyclohexanes (Section 4.7), equatorial substitution is generally more favorable than axial substitution for steric reasons. The hydroxyl group at C3 of cholesterol, for example, has the more stable equatorial orientation. Unlike what happens with simple cyclohexanes, however, steroids are rigid molecules whose geometry prevents cyclohexane ring-flips. 

Terech, R, Berthet, C. (1988). Magnetic orientational effects during the aggregation kinetics of a steroid/cyclohexane gelling system, J. Phys. Chem., 92 4269. 

Because the steroid ring system is rigid, functional groups bonded to ring atoms have well-defined positions. Substituents below the plane of the ring are designated as a those above the plane of the ring are P. We recall that down and up in substituted cyclohexane compounds are not synonymous with equatorial and axial. For the same reasons, this method of nomenclature for steroids does not indicate whether the substituent is equatorial or axial. 

It is worth noting that in this synthesis of Cecropia juvenile hormone a strategy which is the reverse of the one developed by W.S. Johnson [8] for the synthesis of steroids and other fused polycyclic systems bearing cyclohexane rings is used. This method involves a non-enzymatic cyclisation of a polyunsaturated intermediate with the appropriate stereochemistry (all-trans) (Scheme 13.3.6). Such cyclisation occurs with a really amazing stereoselectivity and several new chiral centres with the correct stereochemistry are created in one single step ... 

These considerations apply to all cycloalkane derivatives, including steroids. However, the chair form of a ring is inherently more stable than the boat form. Moreover, the fnsed-ring natnre of the system lends it a very considerable rigidity, and cis-trans isomerization wonld necessitate the breaking and formation of covalent bonds. Therefore, steroid snbstitnents maintain their conformation at room temperature, whereas cyclohexane substituents usually do not. Steroids are classified according to their substituents in addition to their occurrence. 

We will cover the conformations of cyclohexane in more detail than other cycloalkanes because cyclohexane ring systems are particularly common. Carbohydrates, steroids, plant products, pesticides, and many other important compounds contain cyclohexanelike rings whose conformations and stereochemistry are critically important to their reactivity. The abundance of cyclohexane rings in nature is probably due to both their stability and the selectivity offered by their predictable conformations. Nature probably forms more six-membered rings than all other ring sizes combined. 

Although the functionalization of unactivated C—H bonds by intramolecular nitrene insertion has been aj Iied to the synthesis of diterpene alkaloids and in the modification of steroids as described below, it has also been used to good effect in simpler systems. For example, 1-adamantyl azidoformate, readily prepared from 1-adamantanol, gives the oxazolidinone (17) on irradiation in cyclohexane by in-... 

The preference of a bulky substituent for the equatorial position in a cyclohexane ring often results in the establishment of a conformational equilibrium for the mobile mono-cyclic system, but the rigidity of a steroid molecule prevents conformational inversion. Thus the. epimeric 5a-cholestan-3j0-ol (i) and 5a-cholestan-3a-ol (2) have conformations in which the hydroxyl group is respectively held in the equatorial and... 

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