Cornforth - type Rearrangement
According to semiempirical calculations3
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.