A-Acylenamines

The thermal transformation of 2,3-dihydroisoxazoles 142 leads to a-acylenamines 143 the proposed mechanism again involves azomethine ylides (01T4349). 

Among the examples of [l,3]-sigmatropic rearrangements is the photoinduced [1,33-acyl shift of A-acylenamines, which has been investigated extensively and reviewed in 197868 xhus, the irradiation of /V-acyl enamines, known as enamides, provides a facile synthesis of enaminones. 

Cyclic derivatives having the A -acylenamines or N-acylimines structure can be prepared starting from the cyano esters as well as from the enol forms of / -ketonitriles. 

Synthesis of A/-acylenamines from carbonyl compounds and nitriles 1463... 

Alkyl aryl ketones 191 react with benzonitrile in the presence of a perchloric acid/acetic anhydride mixture to yield the 3-azapyrylium salts 194113. The structure of 194 indicates the intermediate formation of A-acylenamines 192 which are acylated at the carbon to 193, the precursor to 194 (equation 61). 

A detailed investigation of the photolysis mechanism of enamines has been conducted by Hoffmann and Eicken The rearrangement proceeds through radical processes. When A -acylenamine 73 was irradiated at the wavelength corresponding to the n n -transition of the amide at approximately 200 nm, the amide bond was cleaved to the radical pair. This radical pair could either recombine and revert back to the reactant or undergo a [l,3]-acyl shift to give the imine 74. In turn 74 underwent rapid tautomerization to the enaminone 75, which was in photochemical equilibrium with its isomer 75 (Scheme 6). 

As can be seen from equations 2-5, the formation of A -acylenamines proceeds from derivatives of carboxylic acids (having an oxidation level equal to three ), namely anhydrides or add halides RCOX or amides RCONH2, on the one hand, and from carbonyl compounds or their derivatives (oxidation level equal to two ), namely enamines 3, acetylenes 5, or vinyl ethers 6, on the other. Indeed, the acetylenes 5, being the dehydration products of enols, may be regarded as having a masked carbonyl functionwhile the enamines 3 rank with vinyl ethers, vinyl esters and enols, as the heteroanalogs of these carbonyl derivatives (see Section IV). 

As mentioned above, the bis-amides 9a are used as precursors in the synthesis of A -acylenamines. The conditions used for this reaction depend on the purpose of the synthesis and are described in numerous works, and reviewed in a series of articles . The acid catalyst is usually concentrated sulfuric acid, but a successful application of 85% phosphoric, chlorosulfonic, methanesulfonic and formic acids was reported, and reactions were also conducted in the presence of anhydrous hydrogen chloride. The solvents used are usually glacial acetic acid and chlorinated hydrocarbons. It is believed that the water necessary for the formation of the bis-amides under anhydrous conditions is obtained from conversion of the carboxylic acid to the anhydride, or even the sulfuric acid . 

The A -acylenamines and their precursors bis-amides 9 are used as amidoalkylation reagents . The main purpose of the latter is to produce the A-acyliminium ions 2 which act as highly reactive electrophilic reagents . Consequently, the application of N-acyliminium ions 2 considerably extends the preparative possibilities for functionalization of aromatic, heterocyclic and unsaturated compounds. 

Salts 91 having alkyl substituents in position 2 react easily with triethyl orthoformate to give the -ethoxyvinyl derivatives 92. C deprotonation of 92 by reaction with amines leads to curious cyclic A-acylenamines 93-95 (equation 33). 

The yields of the pyrimidines 147 and the pyridines 151 are not high and vary greatly, depending on the structure of the starting materials. The latter can also determine the nature of the product. Thus, a mixture of 2,4-dimethyl-6-phenylpyrimidine, 2,4-dimethyl-4,6-diphenyl-4//-l,3-oxazine and 1-chloro-l-phenylethene was obtained by refluxing a 1 3 3 ratio of the acetophenone-benzonitrile-phosphorus oxychloride mixture, followed by treatment of the crude product with sodium carbonate solution .It has been shown that addition of an additional ketone molecule to the iminium intermediate is an alternative to the formation of the A -acylenamines and this route is catalyzed by aluminum chloride. Auricchio and coworkers believe also that the reaction of ketones with nitriles catalyzed with both protic and Lewis acids must be considered as a Ritter reaction (see, however, Section V.C). The cationic intermediate 152 thus formed can undergo either a proton shift giving enamide 153 or addition of another ketone molecule... 

The formation of A-acylenamine 131 by the reaction of A-acetylbenzonitrilium hexachloroantimonate 340 with isobutyrophenone 129 is explained by a similar hydrogen transfer(equation 93). 

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