By Henk C. van der Plas, Alan R. Katritzky
Degenerate ring modifications of heterocycles are labeled as reactions during which a heterocyclic process is switched over into a similar hetercyclic method. This monograph covers an authoritative, entire review of a bunch of degenerate ring variations in 5- and 6-membered heterocycles. It exhibits how by way of 15N-labeled, 13C-labeled or selectively substituted compounds those degenerate ring trnasformations will be stumbled on and hwo many of the effects should be defined via the Addition Nucleophile, Ring commencing, Ring Closure [ANRORC] mechanism. one other major topci of the monograph is the occurance of degenerate ring changes.
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Additional info for Advances in Heterocyclic Chemistry, Vol. 74
There have been discussions whether the amide addition at C-6 is charge-controlled or orbital-controlled. Charge density calculations in 4phenylpyrimidine (MNDO method) predict that the addition of the amide ion would preferably take place at position 2 (95UP1); this, however, does not agree with the experimental results. Therefore, the conclusion seems justiﬁed that the addition is not charge-controlled. Frontier orbital calculations, using the SCF-PPP method, show that the frontier orbital densities in the LUMO of pyrimidine are zero at C-2 and C-5, making these positions SN(ANRORC) REACTIONS IN AZINES 31 quite unreactive for nucleophilic attack (77UP1).
On recyclization of 67a, the ring-labeled 2-amino-4-phenyl-1,2-dihydropyrimidine (67) is formed, which is air-oxidized into ring-labeled 2-amino-4phenylpyrimidine (61). It is evident that on ring closure of (67b) the aminolabeled 62 is formed. 33), experiments were carried out with shorter reaction times, making it possible to retrieve the starting material. It was found that this isolated starting material indeed contains 15N-labeling in the ring. 33. In conclusion, the Chichibabin amination of 4-phenylpyrimidine into 2-amino4-phenylpyrimidine can thus be catagorized as a degenerate ring transformation.
If cyclization would have taken place before loss of the hydrogen bromide, then the 1 : 1 C-6 amino adduct 55 would have been formed, which is in equilibrium with the starting material, and a decrease of the 15N content in the retrieved starting material should have been found. As indicated previously, this is not observed. 28). 7) (78JHC1121). As mentioned previously, only in the amination reaction of 4-piperidino5-bromopyrimidine was an SNH tele amination observed. Besides the 6amino-4-piperidinopyrimidine, a small amount of 2-amino-4-piperidinopyrimidine was isolated.