Steroid conformations

Figure 27.11 Steroid conformations. The three six-membered rings have chair conformations but are unable to undergo ring-flips. The A and B rings can be either cis-fused or trans fused.

Chiroptical Properties O.R.D. and C.D.). A full account of the proposed Reverse Octant Rule for chiral olefins has appeared, and is supported by data for 68 unsaturated steroids. Most endocyclic olefinic steroids conform qualitatively to the rule which, for the lowest energy transition n — n ), reverses the signs of the more familiar carbonyl octant rule. The application of the rule is illustrated (Figure 1) for an oestr-5-ene (6) and a cholest-5-ene (7). The former... 

The structure and stereochemistry of chiloscyphone (74) have been determined and o.r.d. and c.d. spectra are consistent with a non-steroidal conformation. " From Taiwania cryptomerioides, three new muurolane-type sesquiterpenoids have been isolated" viz., (75), (76), and (77). In accordance with the absolute... 

As far as the C(4) stereochemistry is concerned, chamaecynone (388) with a non-steroid conformation and an a-methyl group at C(4) showed stronger activity than isochamaecynone (389), which has steroid conformation and bears a j8-methyl group at C(4). 

The crystal and molecular structure study of cis-l-oxa-7,7,10-trimethyldecahne-2,5-dione has shown that the a priori most stable steroid conformation (10a) exists in the solid. Perhaps surprisingly the a priori less stable form (10b) is observed (n.m.r.) in solution. A direct-methods study of cis- and trans-9,10-his(trimethylsilyl)-9,10-dihydroanthracene has been made. In the former the central ring is a deformed boat, in the latter a deformed chair. 

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