Azapyrylium ions

Zimmer R., Reissig H. U. 1,2-Azapyrylium Ions Properties and Synthetic Applications J. Prakt. Chem. /Chem. -Ztg. 1995 337 521-528... 

The array of dienophiles amenable to these hetero Diels-Alder reactions is not limited to enol ethers and enamines since allylsilanes and simple alkenes have also been successfully employed [370, 371]. More recently, it has been shown that methoxy allenes such as 4-41 undergo formation of 6H-l,2-oxazines 4-43 upon cycloaddition to nitrosoalkenes such as 4-34 and subsequent tauto-merisation of the intermediate exo-methylene compound 4-42 (Fig. 4-9) [372, 373]. In these studies, 4-43 proved to be a versatile synthetical intermediate allowing oxidative demethylation or reductive removal of the methoxy group as well as nucleophilic substitutions after the generation of an azapyrylium ion [372 - 374]. Furthermore, ring contraction reactions of these oxazines leading to pyrroles [373] and y-lactames [375] are known. 

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). 

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). 

The same authors116 showed that 1,3-diketones 198 react easily with benzonitrile in the presence of a HC104-Ac20 mixture to form 3-azapyrylium perchlorates 199 (equation 63). This reaction did not occur without acetic anhydride, which is a catalyst since the acylium ions are not included in the end products. The application of the C-acylated ketones 198 leads to 3-azapyrylium salts 199 which are isomeric to those mentioned above (194), i.e. products 199 have an inverted orientation of heteroatoms comparatively to the three-carbon fragment of the cycle116. The azapyrylium salts 199 can be hydrolyzed to the enamides 200. 

It was observed136 that the / -oxochlorocarbenium ions 272 formed by acylation of vinyl chlorides 270 can undergo an intramolecular deprotonation to give the more stable hydroxycarbenium ions 274. In this case the carbonyl oxygen performs the function of an internal base (equation 75). The formation of 3-azapyrylium hexachloroantimonates 277 can be then explained by reaction of nitriles not with the more reactive chlorocarbenium ions 272, but with the more stable and less reactive hydroxycarbocations 274 (equation 75, path b). 

The structures of the 2-oxazolinium, 1,3,5-oxadiazinium and 3-azapyrylium salts obtained by means of electrophilic catalysis by acylium ions102,113 point to the fact that their formation proceeds not via Af,7V-bis-acyliminium ions 332 and 333, but via the TV-acyliminium ions 322 (i.e. those protonated at the nitrogen atom). In our opinion142, the acetoxy azaallenium ions 335 can be transformed to the Af-acyliminium cations 337 by a deacylation to the N-acylimines 336, followed by protonation of the latter (equation 91). Such a process is quite possible under the conditions used for... 

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) . 

In this respect the acylation of vinyl chlorides 270 in benzonitrile medium turned out to be a very interesting process. The action of acylium salts 271 on the vinyl chlorides is related to three problems, namely (a) the formation of the reactive chlorocarbenium ions 272, (b) the lengthening of the carbon chain as the first step of the future heterocycle construction and (c) the introduction of another functional group which shall have to subsequently ensure the cyclization, The chlorocarbenium ions 272 cannot rearrange, and therefore it is expected that the nitrile will add to ion 272 to form 3-azapyrylium salts of structure 273 (equation 74). However, it has been found unexpectedly that the 3-azapyrylium salts 277 obtained under these conditions have the inverted orientation of the heteroatoms in cycle, i.e. they are the structural isomers of the salts expected. The constitution of products 277 was confirmed by comparison with 3-azapyrylium hexachloroantimonates obtained by other methods. Moreover, they were characterized by IR and NMR spectroscopy as well as by mass-spectral study of ) -acylaminovinyl ketones 278, i.e. the products of opening of 3-azapyrylium cycles,... 

The corresponding -acylaminovinyl ketones are intermediates in the transformation of the 3-azapyrylium salts into 3-azathiapyrylium (l,3-thiazin-l-ium ) ions Polyunsaturated enamides are isolable in reactions of salts 48 with active methylene compounds (equation 18). The action of bases on 4-alkyl-substituted 3-azapyrylium salts 373 leads to 4-alkylidene-l,3-oxazines 374. These anhydro bases can be regarded as cyclic enamides. It has been shown that the treatment of 4-alkylidene-benzo-l,3-oxazines 375 with acids results in protonation of the nitrogen to give 376 rather than at the alkylidene group (equation 107). 

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