Ring-chain rearrangement

Interestingly, Pham-Tran eta/, have pointed out the remarkably contrasting behavior of phosphirane and silirane in their ring-chain rearrangements <2001J(P2)766>. 

Ring-chain rearrangement of azoles Furazans from isoxazoles... 

The presumed mechanism of the oxidative ring enlargement is shown for 52 (R = Ph). The first step is the oxidation of the isoxazolidine to the unstable N-oxide 58, which rearranges to the unstable dipolar 59 this yields the methylene nitrone 60 by tautomerization. 60 gives oxazine 53, through ring-chain tautomerization (79JOC1819). 

The zwitterion may either grow in chain-length or split off smaller rings or rearrange forming one large ring ... 

Ring-chain equilibria of 1,2,3-triazoles and benzotriazoles with the isomeric a-diazo imines result in the well-known Dimroth rearrangement (see Section 4.01.4.1.1). 

The principal methods for forming the heterocyclic ring of benzofuroxans involve oxidation of o-quinone dioximes, thermolysis of o-nitroaryl azides, and oxidation of o-nitroanilines (Scheme 25). Ring chain tautomerism (Section 4.05.5.2.1) for the A -oxides of asymmetrically substituted benzofuroxans is more facile than for monocyclic analogues and mixtures of isomers may result. Benzofuroxans are also formed by Boulton-Katritzky rearrangement of 7-nitro-2,l-benzisoxazoles and 4(7)-nitrobenzofuroxans (Section 4.05.5.2.5). 

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 oxazolidine system proved a good protecting group with which to mask the ethanolamine moiety in the a-formylation and a-benzoylation of 558, and it could also be used as an aldehyde donor in the rearrangement, based on the ring-chain tautomeric character of 559, under acidic conditions to yield 3-(2-hydroxyethyl)-substituted 1,3-oxazin-4-ones 560 (Scheme 106) <1996JOC3358>. 

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. 

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... 

Valence and prototropic tautomeric reactions are among the most important mechanisms that govern transformations of a broad variety of photochromic organic systems.1,2 Until recently, no examples of photochromic compounds have been reported whose photochromic behavior was due to a combination of these two tautomeric reactions. Such a combination, which is characteristic of ring-chain tautomerism3 has been implemented in the photochromic and thermochromic rearrangements of a novel type of heterocyclic photochromes, derivatives of 2,3-dihydro-2-spiro-4 -(2, 6 -di-iert-butylcyclohexadien-2, 5 -one)perimidine la and its analogs.4 The occurrence of a proton transfer step is in accord with the fact that the AvV -dimethyl derivative of la exhibits no photochromic properties. 

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