Ring-Chain Isomerism

Ring-Chain Isomerism. Schonberg and Frese have suggested that there is an equilibrium, at least at elevated temperatures, between 2-oxo-l,3-dithioxo-l,3-diphenylpropane (37) and 3,5-diphenyl-l,2-dithiolylium-4-olate (38), the latter being the more stable isomer at room temperature. Kricheldorf found that, in nonpolar solvents, 2-thioxo-5-oxazolidones 

---

Some 4,5-disubstituted pyridazines exhibit ring-chain isomerism involving heterospiro compounds. For example, 5-(o-aminophenylcarbamoyl)pyridazine-4-carboxylic acid exists in a zwitterionic form in the solid state, but in a solution of DMSO it is almost exclusively 3, 4 -dihydro-3 -oxospiro[pyridazine-5(2//),2 (l //)-quinoxaline]-4-carboxylic acid (134). The equilibrium is strongly influenced by the nature of the solvent, the substituents on the pyridazine ring and the nucleophilicity of the group attached to the phenyl ring (Scheme 48) <80JCS(P2)1339). 

The ring-chain isomerism was studied in a series of 4-hydroxy-3,4-dihydro-1,2,4-triazines 87, which are models for cr -adducts at the 3 position of the... 

There are two important rhodium-catalyzed transformations that are broadly used in domino processes as the primary step. The first route is the formation of keto carbenoids by treatment of diazo keto compounds with Rh11 salts. This is then followed by the generation of a 1,3-dipole by an intramolecular cyclization of the keto carbenoid onto an oxygen atom of a neighboring keto group and an inter- or intramolecular 1,3-dipolar cycloaddition. A noteworthy point here is that the insertion can also take place onto carbonyl groups of aldehydes, esters, and amides. Moreover, cycloadditions of Rh-carbenes and ring chain isomerizations will also be discussed in this section. 

Cycloaddition of pyridine Ar-oxides (see Section 11.13.2) led to careful examination of thermodynamic aspects, though no experimental measurement was provided. Thermodynamic profile for the ring-chain isomerization of [l,2,3]triazolo[l,5- ]pyridines via a ring-opening pathway (Equation 2) was calculated. Based on this computational study, a multistep mechanism was proposed <20050BC3905>. 

M11 R. E. Valters, Ring-Chain Isomerism in Organic Chemistry (in... 

The imidoyl azide-tetrazole ring-chain isomerism of tetrazoles develops into a wide-ranging example of the Dimroth rearrangement in the thermal behavior of substituted 5-aminotetrazoles <84JHC627>. An example with 5-hydrazinotetrazoles is shown in Scheme 10 <88BSB543>. The reaction... 

The Tautomerism of Heterocycles J. Elguero etal., Adv. Heterocycl. Chetn., Suppl. 1,1976. Ring-Chain Isomeric Transformations of Hydroxy-, Amino-, and Mercapto-Derivatives of Carbonyl Compounds and Their Heteroanalogues R. Valters, Russ. Chem. Rev. Engl. Transl.), 1974, 43, 665-678. 

In addition to these rearrangements of more general importance, a number of isomerizations have been observed which apply only to specific cases. Some cycloolefines, like cyclooctatetraene and cycloheptatriene, are converted almost quantitatively to the alkylaromats styrene and toluene respectively 18). Of possible interest for preparative chemistry are ring-chain isomerizations which have been observed with certain nitrogen compounds 19). When pyrrole is subjected to a discharge it is largely converted to croton nitrile. 

P. S. Lobanov, Synthesis and Ring-Chain Isomerism of Aminoal-kyl and Aminoacyl Hydrazine Alkylidene Derivatives (in Russian). Thesis Cand. Chem. Sci. Diss., Leningrad State University, 1983. 

Rearrangement of the hydroperoxide of A1(9,-octahydroquinoline (22) was shown to proceed via the keto amide 23, which in a transannular reaction gave the 4-azaazulenone 24 (55JA6595). Ring-chain isomerizations have... 

A general review of ring chain isomerization is given in ref. [29]. 

A. A. Tolmachev, L. N. Babichenko, I. V. Komarov, S. V. Sereda, and A. K. Sheinkman, Ring-chain isomerism of l,3,3-trimethyl-2-formylmethyleneindoline (Fischer aldehyde) oxime and associated reactions, Chem. Heterocycl. Cpds., 1992, 148-153. 

A quantitative measure of electron distribution has been developed using C-NMR chemical shifts. These correlations also allow the prediction of C chemical shifts in substituted pyridazines (820MR192). C-NMR spectra of various pyridazines were recorded and analyzed. They were used to determine the most stable conformations of reduced pyridazines (87T2443) or the ring-chain isomerization of heterospiro compounds of type 122 (830MR42). Substituent effects of several 3(2//)-pyridazinones were correlated [83JCS(P1)1203], and nuclear relaxation rates of C and quadrupole relaxation of N have been measured for pyridazine (78MP997). 

Ring-chain isomerism can be observed in some imidazolidine derivatives. This is the specific case of equilibrium between an imine and an aminol. Substituent influence on this isomerization has been investigated by various spectroscopic methods. 

Ring-chain isomerism between l-[alkylidene(or arylidene)amino]-2-aminoethanes and imidazolidines has been investigated by NMR spectroscopic methods. Compounds 147 with R = H R = Me, Et, -Pr exist only in the cyclic form 147B, whereas with R = H R =Ph exist only in the linear form 147A. Both linear and cyclic forms are detectable in solution when R = Me R = Me, Et, -Pr, and R= H, R = /-Pr (Scheme 40) <1998OPP109>. 

The nitrone-oxaziridine ring-chain isomerization has attracted theoretical attention.The quantum yield for the reaction has been measured and the reaction has been examined in a rigid polymer matrix. ... 

---