Oxazinium 3-Azapyrylium Salts

These cations (72) were first obtained by Wohl in 19011,218 from AH- 1,3-oxazine and trityl perchlorate in acetonitrile [Eq. (59)]. Later 

Schmidt et a/.219 cyclized /3-acylaminocarbonyl compounds with perchloric acid to obtain 73 [Eq. (60)], in ca. 60% yield. The ring is opened in alkali. 

Schmidt4 also described two possible 1,4-polar cycloadditions (Fig. 4, a and b) leading to 1,3-oxazinium salts [Eq. (61)] similar to 

Diagram of 1,4-polar cycloaddition to form 1,3-oxazinium salts. 

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There are three principal series of 1,3-oxazinium derivatives with different unsaturation namely, dihydro-1,3-oxazinium 1,1,3-oxazinium 2a,b, and aromatic 3-azapyrylium salts 3 (see Scheme 1). 

Many studies have been published in which new synthetic paths for 1,3-oxazinium and 3-azapyrylium salts as well as novel transformations of these compounds have been described. Such results permit one to consider the chemistry of 1,3-oxazinium cations to be a productive research field that deserves the attention of synthetic chemists. 

Among the 1,3-oxazinium derivatives, the 3-azapyrylium salts are the most interesting [they are also called 1 -1,3-oxazinium (72S333), 1,3-oxazin-1-ium (83MI1), 1,3-oxazinium (78AHC1 84MI1), and 1,3-oxazinylium salts (81BCJ2387)]. The absence of a universally accepted name for these compounds indicates the novelty of this field of heterocyclic cation chemistry. 

Perfluoroalkyl)dibenzothiophenium salts and their selenium and tellurium analogs are novel perfluoroalkylating agents. The synthesis and reactivity of these compounds are covered by T. Umemoto (Ibaraki, Japan). Finally, the first detailed survey of the chemistry of 1,3-oxazinium and 3-azapyrylium salts for over twenty years is provided by S. Lukyanov (Rostov-on-Don, Russia). 

The heterocyclic systems of 1,3-oxazinium ions are widely used in the organic synthesis . Thus, 5,6-dihydro-4/f-l,3-oxazines 40, being a cyclic derivative of 1,2-aminoalcohols, can serve as Cj-synthones for the preparation of aldehydes, ketones and carboxylic acids . The 47/-l,3-oxazines 47 can undergo oxidative dehydrogenation by treatment with the trityl salts or benzoquinone to form the 3-azapyrylium salts 48, which then give various jV-acylenamine derivatives 49-52 by reaction with water or with active methylene comjKJunds (equation 18) . 

Azapyrylium (1,3-oxazinium) salts are known,217 218 and their stability suggests the existence of aromatic delocalization. 

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