Steroids, ring juncture stereochemistry

Most steroids share a common tetracyclic ring system, but are adorned differently in terms of oxidation state at various carbons. The three targets we will consider are only the tip of the iceberg. Fused ring systems with different ring juncture stereochemistry (cis vs trans), different sidechains, and different oxidation patterns are common. A few examples are shown here (see problems for some questions about fiisidic and cholic acids). 

The RDA reaction is often observed from steroid molecular ions, and it can be very indicative of steroidal stmcture. [107,110,113,114] The extent of the RDA reaction depends on whether the central ring junction is cis or trans. The mass spectra of A -steroidal olefins, for example, showed a marked dependence upon the stereochemistry of the A/B ring juncture, in accordance with orbital symmetry rules for a thermal concerted process. In the trans isomer the RDA is much reduced as compared to the cis isomer. The effect was shown to increase at 12 eV, and as typical for a rearrangement, the RDA reaction became more pronounced, whereas simple cleavages almost vanished. This represented the first example of such apparent symmetry control in olefinic hydrocarbons. [114]. 

The stercochcmisto of the H ut CS is always indicated in the name. The stercoehetitistry of the other H atoms is not indicated unless it dilTers from Sor-eholestane. Changing the stereochemistry of any of the ring juncture or backbone carbons (.shown in Fig. 2. >l with a heavy line on 5a-choles-tane) greatly changes the shape of the. steroid, as seen in the examples of 5a,8cr-andFostane and S/3-androstane (Fig. 2. -2). 

Altering the stereochemistry at any of the ring-juncture with a heavy-dark line (see general steroid formula s Fig. 23.1) changes immensely the prevailing shape of the steroid , as may be observed in the above cited examples of S -androstane and 5a, 8a-androstane. 

---