Epimerization steroid 11-hydroxyl group

In the CyS/Cy6 case a reversible addition process has been suggested by Suginome and Nickon to explain epimerization of hydroxyl group when photolyzing nitrite esters of steroidal or terpenic compounds such as 175 which gives 176 (Scheme 79). ... 

The facile and selective oxidation of both primary and secondary hydroxy groups with certain nucleotides led Pfitzner and Moffatt (48) to explore the scope of the carbodiimide-methyl sulfoxide reagent with steroid and alkaloid alcohols. Relatively minor differences were apparent in the rate of oxidation of epimeric pairs of 3- and 17- hydroxy steroids whereas the equatorial lLx-hydroxyl group in several steroids was readily oxidized under conditions where the axial epimer was unreactive [cf. chromic oxide oxidation (51)]. 

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... 

B. l-Hydroxy-3-amino-steroids.— The presence of a hydroxy-group reinforces the importance of the ions a at the expense of the ions b. This effect is specially important in the case of the primary amines. Isomers with axial hydroxyl undergo a rearrangement before fragmentation, leading to the formation of characteristic (M — 43) ions. This stereoselectivity is also found in other epimeric steroidal alcohols. 

The 26-hydroxyl group is further oxidized to ecdysonoic acid (119) and 20-hy-droxyecdysonoic acid (120) [205] which have been identified in developing eggs of the desert locust, S. gregaria (Fig. 23). The occurrence of both compounds in Spodoptera littoralis pupae was also established. These oxidations are apparently inactivation steps. 26-Hydroxylation, 3-epimerization, 3-acetylation, and conjugation diminish the interaction with the steroid receptor and thus cem be viewed as inactivation. 

Firstly, steroid acceptor 6 was prepared by introduction of a la hydroxyl group to diosgenin followed by epimerization under an oxidation-reduction sequence.(Scheme I)... 

To ascertain the configuration of the 6-hydroxyl group in phenolic steroids produced by incubation with rat liver, it was necessary to establish the configuration of the synthetic epimeric 6-hydroxy compounds. This was achieved by aromatizing 6a-hydroxy-4-androstene-3,17-dione and... 

Admittedly only the axial and epimeric pairs can be reasonably easily separated from each other further subdivision is difficult and more is said about this in connection with the urinary 17-ketosteroids. Even the 3j5,6a-isomer (epiandrosterone) cannot be separated from the important J ,3iS-compound (dehydroepiandrosterone) using normal TLC. As mentioned in the preceding section, complex formation on silver nitrate-containing layers helps in this instance. Many synthetic Cig -steroids contain a 17a-alkyl as well as the 17j8-hydroxyl group the former reduces the polarity in the order — CH3, — CgHg, — CH=CH2 equals — CsCH, — C3H7. 

Epimerization of steroid 11-hydroxyl groups via keto groups—Protection of keto groups as cyclic ketals s. 11, 53 Stereospecific reduction... 

---