Ring expansion systems

However, it is worth noting that the equilibrium in real ring-expansion systems, without linear chains, should rather give higher equilibrium concentrations of macrocycles than in systems in which linear chains are formed. It stems from the mass balance not including the absent linear polymer fraction (see a short discussion in the next section), but it has not yet been verified experimentally. 

Thus, p in systems without linear polymer and initial monomer concentration larger than the critical value is larger than 1 and consequently the total concentration of macrocycles in equilibrium ring-expansion systems without reaction routes leading to linear polymer, is higher than in systems of ring-chain equilibria. 

The thiepin system 31 is formed quantitatively by ring expansion of the diazoacetate derivative 30 via carbene formation catalyzed by 7r-allylpalladium chloride and its intramolecular insertion[31], The 4-diazomethyl-4//-pyrane 32 is expanded to the oxepine 33 in quantitative yield with the same catalyst[32]. 

Synthesis From Other Ring Systems. These syntheses are further classified based on the number of atoms in the starting ring. Ring expansion of dichlorocyclopropane carbaldimine (53), where R = H and R = ryl, on pyrolysis gives 2-arylpyridines. Thermal rearrangement to substituted pyridines occurs in the presence of tungsten(VI) oxide. In most instances the nonchlorinated product is the primary product obtained (63). 

Ring expansion of five- to six-membered rings such as oxazole —> pyridine derivatives via a Diels-Alder reaction is a well-established procedure. However, the conversion of a six-membered heterocycle into a five-membered ring system has not been exploited to any great extent, and those systems that have been studied usually involve a cationic species. 

The structures of chalcogen-nitrogen compounds are frequently unpredictable. For example, the reactions of heterocyclic systems often result in substantial reorganization of their structural frameworks, e.g. ring expansion or contraction. The formation of acyclic products from ring systems (or vice versa) is also observed. 

ArNSNSAr from a mixture of ArNSNAr and Ar SNSNSAr. The limited data available indicate the occurrence of transformations analogous to the ring expansion or ring contraction processes observed for S-N ring systems. 

Cyclic hydroxamic acids and V-hydroxyimides are sufficiently acidic to be (9-methylated with diazomethane, although caution is necessary because complex secondary reactions may occur. N-Hydroxyisatin (105) reacted with diazomethane in acetone to give the products of ring expansion and further methylation (131, R = H or CH3). The benzalphthalimidine system (132) could not be methylated satisfactorily with diazomethane, but the V-methoxy compound was readil3 obtained by alkylation with methyl iodide and potassium carbonate in acetone. In the pyridine series, 1-benzyl-oxy and l-allyloxy-2-pyridones were formed by thermal isomeriza-tion of the corresponding 2-alkyloxypyridine V-oxides at 100°. 

Very few reactions of carbenes with heterocyclic systems containing more than one hetero atom have been studied. They are confined to variants of the Reimer-Tiemann formylation of thiazoles, pyra-zoles, iminazoles, and indolizines/ and ring expansion does not appear to have been observed. 

Subsequently, it was shown that ring expansion of these bicyclic systems is followed by a series of proton shifts involving the isomeric 2- and 5-methyleneazepines. The former were detected spectroscopically ( H NMR) whereas, in some cases, the latter were isolated.127 For example, 4,6-dimethyl-2-methylene-3-phenyl-3-azabicyclo[4.1,0]hept-4-ene-l, 5-dicarbonitrile (28), on treatment with hydrochloric acid in chloroform, yields the 4-methyleneazepine 29, which on prolonged heating with acid is converted into the 1//-azepine 30. 

Azepines are much less common than the 1H- and 3//-isomers, and one of the few general synthetic approaches to this system is by base-catalyzed ring expansion of 4-(chloromethyl)-l, 4-dihydropyridines. e. g. I.29 Curiously, the ring expansion is also effected by potassium cyanate in refluxing ethanol. 

The azirine 1 reacts with thiobenzoyl isocyanate to give the Diels-Alder adduct 2. which at 80CC undergoes ring expansion to the thiadiazepinone 3, the only known representative of this ring system.327... 

For the synthesis of the complex natural product, the terminus six-membered ketone 55 had to be transformed into an oxepane ring. For this necessary transformation, the authors were attracted by the single-carbon homologation of a pyr-anone (a sort of ring-expansion) because, in prindple, it could be used in an iterative sense at any stage of the 6-endo cydization in their poly-TH P-based synthetic approach for the synthesis of trans-fused 6,7,6 (THP-oxepane-THP) and 6,7,7 (THP-oxepane-oxepane) ring systems [28]. Treatment of ketone 55 with TMSCHN2... 

The mechanisms of these ring expansions are not certain. Both concerted" and diradical" pathways have been proposed, and it is possible that both pathways operate, in different systems. 

Scheme 3.68. Cyclization/ring expansion for the construction of bridged polycyclic systems.

Regiospecific ring expansion has been observed on irradiation of the 3-substituted 1-iminopyridinium ylids 92 to give the l//-l,2-diazepines 93.78 Various novel ring systems have been prepared in an analogous fashion. Thus, for example, the previously unknown 1H-1,3-benzodiazepines 94 were obtained by irradiation in dichloromethane of the isoquinoline /V-imides 9579 a possible pathway is outlined in Scheme 2. The novel ring systems, If/-1,3- and 3//-2,3-thieno[2,3-d]diazepines, have similarly been prepared by irradiation of 7-methylthieno[2,3-c]pyridine /V-imides,80 and... 

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