N ‐Vinylketenimine

N ‐Vinylketenimines. The title compounds 5 are generated in situ from N ‐vinyl‐iminotriphenylphosphoranes 1 and ketenes 2 in a Wittig‐type reaction. The phosphoranes are useful intermediates for the preparation of numerous heterocycles as illustrated by the formation of 1,3‐oxazines 8 , pyridinones 9 , pyrimidinones 10 and 1,3‐thiazines 11 . Water adds readily to 5 to yield vinylacetamides 7 , whereas thermal rearrangement gives rise to the isoquinoline 4 or the spiro system 6 , respectively. Under very mild conditions, crystalline precursors (type 3 ) of the N ‐vinylketenimines, which still contain the triphenylphosphonio group, have been isolated. These react like the N ‐vinylketenimines generated in situ, giving however less byproducts.