Homework # 4 Solutions
This is a [5,5]-sigmatropic shift, which means 10 electrons are involved. 10 electrons satisfies the Huckel condition (4n+2). Therefore, the reaction proceeds with suprafacial,suprafacial stereochemistry.
The first reaction is a [3,3]-sigmatropic rearrangement. This requires disrotatory motion, that will lead to the cyclooctadiene product. This is an allowed reaction.
The second reaction is 2+2 cycloreversion. If this trans isomer were to undergo the same reaction as the cis isomer above, it would have to go conrotatory, which give rise to one trans double bond (see below), which would be quite strained. Therefore, the trans isomer does not proceed via a Cope rearrangement, but does undergo the cycloreversion at elevated temperatures.
The reaction on the left involves 10 electrons, which satisfies the Huckel rule. The reaction is therefore suprafacial,suprafacial and leads to the correct product. On the other hand, the reaction on the right involves 8 electrons, which satisfies the Mobius rule. The reaction must then go by a suprafacial,antarafacial path which must place a trans double bond into the ring system. This would be too strained and therefore the reaction does not occur.
Endo is preferred over exo. Drawn below are the four possible isomers, the highlighted isomer is the endo product with S stereochemistry.