1.3- Oxazinium salts

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

The earliest syntheses of 5,6-dihydro-4//-1,3-oxazinium salts 1 were two-component cyclization reactions three-carbon 1,3-bifunctional fragments... 

It should be noted that the 1,3-bifunctional starting compounds 4-7 are quite difficult to obtain. Moreover, the direction of acid-catalyzed cycliza-tion of 3-acylaminopropanols depends considerably on their structure. Thus, N-acylaminoalcohols 8 can be cyclized to give either 5,6-dihydro-4//-1,3-oxazinium salts 10 or 2-oxazoline derivatives 12 as well as mixtures thereof (82MI1, 82MI2 see also 78AHC1). 

Some work devoted to the synthesis of 1,3-oxazinium salts has been published. These salts are assumed to be intermediates in the acylation of olefins in the presence of nitriles (84ZOR1357 87ZOR1546). This process corresponds to the interaction between )3-chloroketones 32 and nitriles. 

As mentioned above, the polar 1,4-cycloaddition of N-acyliminium ions 14 and 18 to olefins gives 5,6-dihydro-4H-l,3-oxazinium salts 16 (Section fI,A,l), or forms the 1,3-oxazinium salts with acetylenes (Section III, A). The same ions 89 and 91 add nitriles to furnish 1,3,5-oxadiazinium salts 90 and 92, little investigated until now (65CB334 88ZOR230). Salts 92 are unstable and can dissociate at elevated temperatures with nitrile liberation. Therefore, they may serve as original reservoirs for active jV-acyliminium cations. 

The resin-bound 1,3-oxazinium salt 116, obtained by oxidation of 4//-l,3-oxazines 115 with 2,3-dicyano-5,6-dichloro-p-benzoquinone (DDQ), behaved as /3-diketone equivalents and formed pyrazoles 117 through a functionalizing release process on treatment with hydrazines (Scheme 18). When the hydrazines were substituted (R = Me, Ph), the oxazinium salts reacted selectively to afford one regioisomer 117 <2004JC0846>. 

However, 2,4,6-trisubstituted pyrylium salts with certain active methyl and methylene compounds undergo ring fission and subsequent cyclization to benzenoid products. 2,4,6-Triphenylpyrylium ion (261 Z = O) in this way forms 2,4,6-triphenylnitrobenzene (299) with nitromethane and the substituted benzoic acid (300) with malonic acid, the latter reaction involving a decarboxylation. In reactions of this type, 1,3-oxazinium salts react with active hydrogen compounds to give pyridines (Scheme 25). 

Oxazin-4-ones are obtained by cycloadditions between isocyanates and ketenes (Scheme 37). Routes to 1,3-oxazinium salts consist of 1,4-cycloadditions either between a,(3-unsaturated (3-chlorocarbonyl compounds and nitriles or between /V-acylimidoyl chlorides and alkynes. Tin(IV) chloride is an effective catalyst for both reactions (c/. Scheme 38). 

Methyl-1,3-oxazinium salts may be deprotonated to methylene derivatives (70) by bases such as triethylamine (Scheme 23). 

Oxazin-4-ones and -thiazin-4-ones are well represented in the chemical literature. Thiazin-4-ones can be synthesized from 1,3-oxazinium salts by the action of hydrogen sulfide and potassium carbonate (81H(15)85l) and oxazin-4-ones are obtained by cycloadditions between isocyanates and ketenes (Scheme 73), or alkynes (Scheme 74), or between nitriles and acylketenes (Scheme 75). Similarly diketene is often used and affords oxazin-4-ones by its reactions with imidates and cyanamides (Scheme 76) (80H(14)1333>. 

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

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

The properties of 1,3-oxazinium salts are similar to those of pyrylium salts.4194,219,255-259 1,3-Oxazinium salts are intermediates for the synthesis of derivatives of pyrazole, isoxazole, pyridine, pyrimidine, quinoline, isoquinoline, chromene, benzopyrylium salts, butadiene, etc.4... 

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