3- Azapyrylium salts

Pyrylium salts such as 1704a and 3-azapyrylium salts 1704b react with P(SiMe3)3 1692 to give A -phosphorines 1706a [19] and 1706b [20], via 1705 (Scheme 11.4). 

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. 

The first 3-azapyrylium salts 48 and 50 were obtained from other heterocycles by oxidative dehydrogenation (Section III,C) and by 5-alkylation of 4-thiono-l,3-oxazine (49) (62CB937 72S333). Until recently only two... 

H, Aik), ions 51 are able to add the weakly nucleophilic nitriles to give N-acyliminium ions 14 and 52 (Section II,A,l)(89ZOR2416 91MI1). This catalysis was used in the development of new methods for the synthesis of 3-azapyrylium salts from ketones 53 and 54 (Scheme 7). 

It has been found that 3-azapyrylium salts 57 and 58 are obtained in reactions of pinacolone as well as vinyl chlorides 59 with N-acylnitrilium salt 56 (91ZOR2479 92ZOR2577). The acylation of vinyl chlorides 59 in benzonitrile also leads to salts 60 (91ZOR1986). The same salts 55 are formed as intermediates by acylation of acetylenes in the presence of nitriles (88ZOR1605). Vinyl chlorides and acetylenes are both direct derivatives of carbonyl compounds with the same oxidation level (72MI2 85KGS1443). 

Various 3-azapyrylium salts differently substituted are now easy to prepare due to the novel synthetic methods described above. 

Deprotonation of the a-position of a carbenium center is one of the most typical properties of carbocations. All the alkyl-substituted heterocyclic ions possess an appreciable CH-acidity [82AHC(S)1] the 3-azapyrylium salts are no exceptions to this rule. The formation of anhydro bases, i.e., methylene-1,3-oxazines (e.g., 69), from methyl-3-azapyrylium salts is well known (72S333). 

Later, a series of new transformations of alkyl-substituted 3-azapyrylium salts based on the CH-acidity of alkyl groups in positions 2, 4, and 6 of the... 

However, it is most surprising that alkyl-substituted 3-azapyrylium salts can undergo formylation and acylation at the side chain. These reactions are examples of the interaction of heterocyclic cations with cationoid electrophiles. Some of these processes are presented in Scheme 12 (90KGS134 91KGS265 94TH1). It is assumed that such transformations are possible due to the intramolecular deprotonation of the methyl group in 79 to form charged anhydro base 80 (see structure 39). The 4-acylmethyl-3-... 

This field has not been investigated extensively. Dehydration of 2-acylam-inobenzophenones 83 leads to benzo[J]-3-azapyrylium salts 84 previously... 

Diazapyrylium salts bearing various hetero groups (NR, OR, SR) are shown in Section V,B. Some amino-substituted 3-azapyrylium salts were... 

The restricted rotation about exocyclic partial double bonds in some 3-azapyrylium salts was investigated by temperature-dependent C- and H-NMR spectroscopy (87MRC688). 

The spectra of aromatic 3-azapyrylium salts 3 are very much like those for pyrylium salts [82AHC(S)1 88ZOR2232 91ZOR1986]. Benzoannelated... 

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). 

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