Fragmentation of diastereoisomers
The decalin shown below can undergo ring expansion. However, this ring expansion is fairly complex because the starting material has three stereogenic centres (*), and hence can exist as four diastereoisomers: two trans-decalins and two cis-decalins; the alkoxide can exist in both the axial and equatorial position. The product is a ten-membered ring which contains a double bond. This double bond can be either cis or trans.
One of the four diastereoisomers cannot fragment, whereas the other three can. Two of them give a trans double bond and the other a cis one. Click the diastereoisomers shown below to see which product is formed: (Can you guess which diastereoisomer does not undergo ring expansion and why?)
NOTE: The mesylate leaving group was used in place of tosylate to enable the calculations to run more smoothly