Decalin ring expansion

The driving force for ring expansion of 17-hydroxy-20-keto steroids probably comes from relief of strain accompanying conversion of trans-fused hydrindanes to tra/75-fused decalins. The greater susceptibility to rearrangement of 17/ -hydroxy-20-ketones as compared to the 17a-hydroxy-20-... 

You saw, at the beginning of the last section, a ring expansion reaction of a decalin. OTs... 

Now, the stoiy of this ring expansion is a little more complex than we led you to believe, because the starting material has three stereo genic centres ( ) and hence can exist as four diastereoisomers two fnms-decalins and two cis-decalins. What is more, the product has a double bond in a ten-membered ring will it be cis or trans7 (Both are possible—see Chapter 31.)... 

Intramolecular variants are the most common type of Diels-Alder reactions employed in total syntheses with carbocycles the most popular targets. Among the broad variety of structures in this class, bicyclic decalin structures, both cis and trans, are readily available on intramolecular Diels-Alder reactions. Aube demonstrated the use of a tandem intramolecular Diels-Alder/Schmidt reaction sequence in his synthesis of three stemona alkaloids. The first steps generates cw-decalin intermediate 124, which undergoes a ring expansion in the Schmidt reaction to furnish tricycle 125. ° ... 

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