Facile ring isomerization

Activation of Some Condensed Tannins via Facile Ring Isomerizations Potential Adhesive Applications... 

However, the immediate prospect of tannin activation via facile ring isomerization and without introduction of solubilizing sulfonic groups, is the reduction of resorcinol requirements in those cold-set mimosa-resorcinol-formaldehyde... 

Steynberg, J.R, D.A. Young, J.F.W. Burger, D. Ferreira, and D.G. Roux Phloba-tannins via Facile Ring Isomerization of Profisetinidin and Prorobinetinidin Condensed Tannin Units. J. Chem. Soc., Chem. Commun., 1013 (1986). 

Steynberg J P, Young D A, Burger J F W, Ferreira D, Roux D G 1986 Phlobatannins via facile ring isomerizations of profisetinidin and prorobinetinin. J Chem Soc Chem Commun 1013-1014... 

The lack of stereospecificity in equation (72) (as compared to equations (70) and (71)) is readily accommodated by the long lifetime of the triplet ion pair produced by the sensitized irradiation method. The latter allows the facile ring opening of DBC+ cation radical followed by isomerization of the resulting xylylene cation radical, i.e.,... 

The chemical effect of a gem-difluoro group on cyclopropane is to facilitate the thermal cleavage of the distal ring bond. This is illustrated by the facile endojexo isomerization of tricyclic cTO-difluorocyclopropane 1, which occurs at 60 C.4... 

A particularly facile ring opening by thermal isomerization has been discovered in the alkaloid field... 

The trimethylsilyl ethers of the 7,7-dichloronorcaren-l-ols (200) and (203) have been reported to undergo facile ring expansion to the corresponding a-chlorocycloheptadienones (201) and (204) on hydrolysis with acidic methanol.However, the isomeric, A -norcarenol system (202) was found to be... 

The parent compounds undergo facile hydrolysis to aminoaldehydes subsequent to the covalent hydration and reversible ring-opening as described above for pyrido[4,3-d]pjrrimidines (Section IV, B). 2-(3-Pyridyl)pyTido[2,3-d]pyrimidine undergoes hydrolysis to yield 2-aminonicotinaldehyde and nicotinamide when treated with N—HCl under reflux for 3 hours. This mechanism also probably involves a covalent hydrate. 2-Methylpyrido[4,3-d]pyrimidin-4(3H)-one, although much more stable than the parent compound, is readily hydrolyzed with dilute acid, whereas the isomeric compounds from the other three systems are stable under such conditions. 

These results indicate that, during thermolyses of fructose-containing saccharides, di-D-fructose dianhydrides are formed readily, but subsequent isomerization is extremely slow—even in the presence of added acid. However, under these conditions, the protonating power of any acid is moot. At the high temperatures used, residual water would be driven off rapidly, unless the reaction vessel is pressurized therefore, reaction occurs in the anhydrous melt. It is presumably protonation of one of the ring oxygen atoms in the dianhydrides that constitutes the first step in isomerization, followed by scission of a C-O bond to yield one of the oxocarbenium ion intermediates postulated in Refs. 31 and 80. Such ions have also been postulated as intermediates in the isomerization of spiroketals to a more-stable product. This latter isomerization can be extremely facile 104 dilute aqueous acid,120 or non-aqueous Lewis-acid conditions121 have been used to effect such transformations. 

For cycloheptatriene and a series of its derivatives various thermal unimolecular processes, namely conformational ring inversions, valence tautomerism, [1,5]-hydrogen and [l,5]-carbon shifts, are known. An example of such multiple transformations was described65 which can provide a facile approach to new polycyclic structures by a one-step effective synthesis (yields up to 83%) of the two unique ketones 156 and 157. The thermolysis of the neat ether 151 at 200 °C for 24 h gives initially the isomeric allyl vinyl... 

As to the cation-radical version of this isomerization, there are testimonies on the transition of the norcaradiene carcass into the cycloheptatriene skeleton. Calculations at the B3LYP level shows that cycloheptatriene cation-radical is more stable than norcaradiene cation-radical by ca. 29 kJ mol (Norberg et al. 2006). Hydrocarbon ion-radicals with strained ring structures have a tendency to undergo facile rearrangement to enforce the unpaired electron delocalization and release their strain energy. 

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