It could be proven that N,O- and N,S-Ketene acetals with a benzoyl group on the nitrogen are much more stable than the ketene acetals with a benzyl group. These compounds show aza-Claisen reactivity. Due to the high temperatures and long reaction times needed the yields with N,O-ketene acetals are poor and side reactions are observed. Catalysis difficult. N,S-ketene acetals are much more stable than the N,O-ketene acetals. But here the temperatures needed for the rearrangement are even higher. At these temperatures isomerization of the initially formed product has been observed. Catalysis could not be achieved. But the overall yield of the aza-Claisen rearrangement is much higher, especially in the case of ketene acetals with crotyl or cyclohexenyl-groups.
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