1.3- Oxazinium 4-olates

Reaction of chlorocarbonyl ketenes 126 with N-substituted cinnamic amides, resulted in mesoionic 1,3-oxazinium 4-olates 341 which can be subjected to ring-opening to afford ketenes 342 which in turn is submitted to an intramolecular criss-cross [2 - - 2] cycloaddition reaction via TS to give 3-aza-bicyclo[3.1.1]heptanetriones 343 in reasonable yields (Scheme 106) (2005JOC5859). 

The condensation of (chlorocarbonyl)phenylketene or (chlorocarbonyl) benzylketene with 1,3-dinucleophiles such as N-phenylthiocarbamates 520a and N-phenylcarbamates 520b resulted in the generation of relatively unstable mesoionic 1,3-thiazinium 4-olates I or 1,3-oxazinium 4-olates II which can be transformed into 3,5-diaryl l,3-thiazine-2,4,6-triones 521 and alkoxy-3,5-diphenyl-3H-l,3-oxazine-2,6-diones 522 (Scheme 172). In accordance with proposed mechanism, the nitrogen atom of 520 attacks the central carbon atom of the ketene group, with a low-lying LUMO,... 

The energy surface linking ketene (7, several conformers), the corresponding oxazinium olate (8), and related imidoylketenes, oxo-ketenimines, and their cyclization products has been calculated.19 Whereas (8) ring opens easily at room temperature, (7) is not directly observable, as its energy is ca 10 kcalmol-1 above (8). Many of the reactions in this manifold are pseudopericyclic in nature. 

Ring opening of mesoionic 1,3-oxaziniumolates (35) to acylketenes (36) occurs at room temperature, with a calculated activation barrier of <20 kcalmol-1. The acylketenes (36) are not directly observed because they have energies ca 10 kcal mol-1 above the oxazinium olates (35) and undergo intramolecular criss-cross 2 + 2-cycloadditions to recyclize to 3-azabicyclo[3.1.1]heptanetriones (37).59... 

In contrast to the oxazinium-olates, for example 46, the isoelectronic diazinium-olates 52, which cannot eliminate carbon dioxide, undergo the alternative cyclisation of the zwitterionic intermediate 53 resulting in formation of a [4+2] cycloadduct 54 (Scheme 14) (see also Section 2.1.2, Scheme 9) (81LA521). 

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