rn^n© k2co3 rVNv
Scheme 4.15 Proposed mechanism for the synthesis of N-methyl oxazolinones from oximes.
reactions, respectively) in Scheme 4.15. This reaction is also the first ever reported example of a sigmatropic rearrangement involving DMC. We have gathered further evidence for this mechanism from the comparison of the reactivity of C5-C7 cyclic oximes, which is resumed in Scheme 4.16.
As can be seen, cyclohexanone- and cycloheptanone-oximes give the cyclic rearranged products, while cyclopentanone-oxime yields a complex reaction mixture with no traces of the corresponding oxazolinone. Since sigmatropic rearrangements proceed through a highly ordered transition state,36 the required geometry is perhaps not achieved when a more strained C=C bond is involved, as occurs in the rigid cyclopentene ring (Scheme 4.15). Also, the reaction of allylimines with DMC give rearranged products. As an example, N-cyclohexylideneallylimine undergoes N-methylation by DMC followed by an aza-Claisen rearrangement (Scheme 4.17).27
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