By Henk C. van der Plas, Alan R. Katritzky

Degenerate ring changes of heterocycles are labeled as reactions during which a heterocyclic procedure is switched over into an analogous hetercyclic procedure. This monograph covers an authoritative, accomplished evaluate of a number of degenerate ring modifications in 5- and 6-membered heterocycles. It indicates how by means of 15N-labeled, 13C-labeled or selectively substituted compounds those degenerate ring trnasformations will be came upon and hwo many of the effects may be defined via the Addition Nucleophile, Ring beginning, Ring Closure [ANRORC] mechanism. one other major topci of the monograph is the occurance of degenerate ring modifications.

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Whereas for X ϭ SO2C6H5group the ANRORC process occurs for only a relative small percentage, for X ϭ SO2CH3 the ring-opening process is strongly favored. 27). However, it appeared that in the ␴-adduct 48 the SO2CH3 group is present in the deprotonated form. This behavior probably explains why 2-phenylsulfonyl-4-phenylpyrimidine reacts differently from 2-methylsulfonyl-4-phenylpyrimidine. 27). 6). (85T237). For that purpose the Swain nonresonance field constants F and resonance constants R were used (68JA4328).

All the results mentioned in this section indicate that the aminolysis of highly activated pyrimidines containing a leaving group at position C-2 or C-4(6), a reaction that is usually considered to take place via an addition–elimination process, can also occur via the less conventional SN(ANORC) mechanism. As an illustration: It is not unlikely that the aminolysis of 2-chloro-4,6-dicyanopyrimidine into 2-amino-4,6cyanopyrimidine with ammonia (77KGS821) may at least partly occur by participation of a SN(ANRORC) mechanism.

In the strong basic medium, this hydrogen is abstracted and a strongly delocalized anion is formed. Evidence for this anion formation is provided by 1H NMR spectroscopy (74RTC237; 75OMR86). It can be argued that in this anion most of the negative charge is localized in the N-1, C-2, N-3 region, making the nucleophilic attack at position 2 less favorable than addition at C-6, because of electron repulsion between the incoming amide ion and the highly charged region around position 2. 30. f. Aminodemethoxylation of Dimethoxypyrimidines It has been reported that 4,6-dimethoxypyrimidine and its 5-bromo derivative undergo an aminodemethoxylation to the corresponding 4-methoxy-6-aminopyrimidines on treatment with potassium amide/liquid ammonia (61JCS1298; 65JCS6659).

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