Cornforth - type Rearrangement

According to semiempirical calculations the Cornforth rearrangement4 of e.g. 4-acyl- oxazoles occurs via ring opening and recyclisation of  a carbonyl-stabilised zwitterionic intermediate. For the present system this mechanism implies formation of an a-oxo acylkation.





Such an intermediate is expected to be unstable; in fact, the calculation for the ring opening of the furandion lacton ring lead to a transition state for elimination of CO to give a-oxo ketenes rather than cyclisation to the pyrroledione 2. The activation energy for this elimination is ~ 30 kcal mol-1 . Thermal elimination of CO from furan-2,3-diones  is a useful method for the production of a-oxo ketenes. 5 
 


 

Figure 1. Transition state for lacton ring opening (leading to elimination of CO rather than rearrangement).
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Rearrangement  1 -> 2 via a Cornforth - type mechanism appears thus highly unlikely.