Pivot vs. Slither Mechanism

Zimmermanhas provided strong circumstantial evidence that zwitterionic intermediates can be involved in formation of cyclopropyl ketones from dienones. His approach was to generate the dipolar species via ground state chemistry and show that these rearrange to the photoproduct  

The rearrangement from (30) to (31) can occur via a slither or pivot mechanism  

Since the rearrangement afforded endo product from endo zwitterion and exo product from exo zwitterion, the rearrangement follows the slither mechanism. 

In light of these results, the study of Rodgers and Hart is particularly interesting. They found that as one increases substitution at C-3 and C-S, there is a greater stereospecificity in cyclopropyl ketone formation  

This preferred ground state conformation, coupled with the slither mechanism, would account for the observed stereospecificity [formation of (34c)]. The spirodienone (35) undergoes photorearrangement to form 

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