Steroid skeleton stereospecific

The steroid side chain (at Ca )) also deserves mention. The two-carbon chain in the pregnane series possesses a certain conformational mobility, but interactions with the rest of the molecule make this markedly less than in a simple aliphatic compound. Physical measurements have demonstrated distinct preferences for particular conformations in various C(20)" substituted pregnanes [37,38], and these conformations are significant in interpreting reactions of these systems. Reduction of a 20-0x0 group, for example, occurs with a fair degree of stereospecificity as a result of its asymmetric environment when considered in relation to the neighbouring parts of the steroid skeleton (p. 139). 

Notice that although there are four asymmetric centers in the molecule, one isomer is formed in 58% yield. This points to a concerted cyclization step in which the stereochemistry incorporated in the C==C double bonds is transferred to the developing asymmetric centers in the product. (This phenomenon is analogous to the well-known stereospecific formation of the steroid skeleton by cationic cyclizations of similar olefinic systems with defined stereochemistry at the C—C double bonds.)... 

Poor yields were observed initiating the reaction by epoxide ring opening107. A steroidal skeleton was prepared using allylsilane for 5-exo cyclization in the termination step. Both induction and termination are not stereospecific and a mixture of diastereomers is isolated in modest yield93. 

This example aside, the utility of the electrohydrocyclization reaction in the assembly of natural products remains essentially untapped. The reason for this observation is not entirely clear. Consider, for example, the rapid and efficient construction of the perhydrophenanthrene skeleton as exemplified by the conversion illustrated below [18]. The reaction is stereospecific, leading to the trans-anti-trans ring fused adduct 13 in a 65 to 72% yield. It takes little thought to imagine the application of this powerful transformation to the synthesis of steroids. Yet, this opportunity has not b n realized. 

Different new stereospecific cyclization reactions leading to the tetracyclic skeleton of steroids have been reported. 

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