Ring transformation reaction

Ring Transformation Reactions Initiated by Nucleophilic Attack. 284... 

In both these ring transformation reactions, the ANRORC troika process takes place [99AHC(74)8]. Usually, however, the action of a nucleophile on 1,2,4-... 

X-Ray elucidation provided a final proof for the structure elucidation of 8-(4-chlorophenyl)-8-hydroxy-5-methyl-8/f-[l,4]thiazino[3,4-H[l,2,4]oxadiazol-3-one 49, which was obtained as a product of ring-transformation reaction <1997J(P2)2407>. This analysis revealed that the oxadiazole ring is planar, whereas the thiazine ring is in a distorted half-chair conformation with a displacement asymmetry parameter AC2 (S—G(3)) = 0.031. The structure analysis represents the first X-ray elucidation of a [l,4]thiazine ring fused to a [l,2,4]triazole moiety. 

In the case of a ring-transformation reaction (discussed in detail in Section 11.19.7) <2005T7489>, negative enthropy of activation has been determined and this finding allowed to conclude that the isomerization follows a pericyclic pathway. 

All synthetic works belonging to this section relate, with one exception, to the preparation of tetrazolo[l,5-A -[l,2,4]triazines. Although the final outcome of this outstanding work as published by Chupakhin et al. <2005IZV713> is also formation of this ring system, the reaction pathway actually involves a ring transformation reaction. The interesting result is summarized in Scheme 10. 

The occurrence of degenerate ring transformation reactions in the hy-drazinodehalogenation of halogeno-l,2,4,5-tetrazines (see Section II,D,3.) raised the question as to whether corresponding reactions would also occur with the less reactive halogenopyridazines. Extensive investigations on hy-... 

Hydrazine-induced degenerate ring transformation reactions are recorded (80JHC1733) with 3-thiooxo- or 3-thiomethyl-4-aryl-5-oxo-4,5-... 

In only a few cases are ring transformation reactions found in which a in-termolecular CCC transfragment reaction occurs. An example of this rearrangement has been observed in the reaction of 3,5-dinitropyridin-4(l//)-one (99) with diethyl sodio-3-oxopentanedioate. It provided in good yield 3,5-di(ethoxycarbonyl)pyridin-4-(lH)-one (102) and 1,3-dinitroacetone as second product (78H108). Its structure was proven by the formation of l-phenyl-3-nitro-4-hydroxy-5-phenylazopyrazole (103), obtained on heating of the diphenylhydrazone of 1,3-dinitroacetone (Scheme III.53). 

In Sections III,A,4 and III,B,l,f, examples of ring transformation reactions have been mentioned. Here we shall discuss in detail the interesting case of isomerism between an eight-membered ring with an 8-7r-electron system (thus, in principle, antiaromatic) and a 10-n pentalene derivative (briefly mentioned in Section III,B,4,a). Scheme 14 shows... 

Other processes, limited to heteroaromatic systems, include the Sn(ANRORC) reaction (Scheme 4),17,18 and ring transformation reactions.18,19 Reactions which proceed via a-adduct intermediates, but do not lead to substitution on the aromatic nuclei, such as the Sn(AEAE) reaction,20 or give nonaromatic products, are not included. Also not covered are processes involving aryl-metal intermediates, such as most copper-catalyzed aromatic substitutions. 

Within the following subsections the stability of the bicyclic ring system, plus the hydrogenation, reduction, dehydrogenation, oxidation, and quater-nization of the compounds are reviewed. This is followed by discussion of substitution reactions affecting the pyrido [ 1,2-a]pyrimidine ring, transformations of the side chains, and finally ring transformation reactions. 

As part of their study on aza-transfer reactions, Tisler and co-work-ers289 290 reacted 2-hydrazino-4-oxo-4//-pyrido[l,2-u]pyrimidine with various diazo derivatives and obtained 2-azido- and 2-amino-4-oxo-4/f-pyrido[l,2-u]pyrimidines. From 2-azido-4-oxo-4/f-pyrido[l,2-u]pyrimi-dines various derivatives of 2-(l,2,3-triazol-l-yl)-4-oxo-4//-pyrido[1.2- ]-pyrimidine (221) were prepared by reaction with acetylene,287 1,3-dioxo compounds,166 ordiethylamine.291 For further ring transformation reactions of the 2-azido-4-oxo-4//-pyrido[l,2- ]pyrimidine (102), see Section lll.C.lO. 

Base-catalyzed condensation of cyanothioacetamide with ethyl / -(ethoxy-methylene)acetoacetate furnished (127), which was quantitatively 5-methylated to yield (128). Ester reduction and alcohol protection generated (129), which was oxidized to the corresponding sulphone with mcpba. Guanidine cycli-zation to (130) and deprotection then afforded (131), identical in all respects with material prepared by the previously described ring-transformation reaction. 

With hydroxide anions (or with water at pH >5) pyrylium salts form pseudobases 16 (2-hydroxy-2//-pyrans) that tautomerize to acyclic 3-penten-l,5-diones 17 via a thermally allowed electrocyclic reaction. Most ring transformation reactions of pyrylium salts involve a similar ring-opening. 

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