Three-membered Heterocyclic Systems

Nakasuji, K. Nishino, I. Murata, H. Ogoshi, and Z. Yoshida, Angew, Chem. Internat. Edn., 1977, 

Ab initio calculations for the C2H2S system indicate that thiiren (35) is the least stable species in the series (31)-(35). However, clear evidence for the presence of 

Destro, T. Pilati, and M. Simonetta, J.C.S. Chem. Comm., 1977,576 G. Capozzi, V. Luccliini, G. Modena, and P. Scrimin, Tetrahedron Letters, 1977,911. 

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Table 5 Ranges of NMR Data of Three-membered Heterocyclic Systems...

Table 6 Ranges of NMR Data for Three-membered Heterocyclic Systems with Exocyclic Unsaturation...

These polymers are based on the three-membered heterocyclic system either as the epoxy or oxirane ring (1.3). 

For the antiaromatic three-membered heterocycles, experimental data are available only for thiirenes (and there is some doubt about the true antiaromaticity of thiirenes). Bond lengths have been calculated, however, for these antiaromatic 47r-systems (80PAC1623). In comparison with the corresponding saturated heterocycles, the C—X bond lengths are increased by 0.05 to 0.17 A and the C—C bond length is decreased by 0.2 A. 

The Chemistry of Heterocyclic Compounds has been published since 1950 under the initial editorship of Arnold Weissberger, and later, until his death in 1984, under the joint editorship of Arnold Weissberger and Edward C. Taylor. In 1997, Peter Wipf joined Prof. Taylor as editor. This series attempts to make the extraordinarily complex and diverse held of heterocyclic chemistry as organized and readily accessible as possible. Each volume has traditionally dealt with syntheses, reactions, properties, structure, physical chemistry, and utility of compounds belonging to a specihc ring system or class (e.g., pyridines, thiophenes, pyrimidines, three-membered ring systems). This series has become the basic reference collection for information on heterocyclic compounds. 

As mentioned in CHEC-II(1996), three main routes have been reported for the formation of furazan rings (1) the dehydrative cyclisation of 1,2-dioxims (2) the deoxygenation of furoxans and (3) the Boulton-Katritzky rearrangement of other five-membered heterocyclic systems <1996CHEC-II(4)229>. In this section the recent publications on the synthesis of furazans published after 1996 are discussed. 

Figure 1-3. Solution stable three-membered heterocyclic ring systems.

The study of three-membered heterocycles, both as reagents and synthetic targets, has continued to flourish. More than ever, these systems represent an attractive combination of predictable, controllable, and versatile reactivity that has been extensively utilized by the synthetic community. As in earlier volumes, this review seeks not to be comprehensive, but rather to provide an overview of the past year s literature from the synthetic chemist s perspective, focusing on preparatively useful protocols or novel methodology, and highlighting advances in selectivity and ease of use. Organization of the chapter is similar to that seen in previous years. 

As with the other procedures for the preparation of six-membered heterocyclic systems which proceed via formation of only one ring bond there are relatively few methods which involve formation of a ring bond y to the heteroatom and which can best be classified as [6 + 0] processes rather than [4 + 2], [3 + 3], etc, processes. Of those which can be so represented, however, a number are important processes which are widely used for the synthesis of saturated, partially saturated and aromatic six-membered heterocyclic systems and their benzo derivatives. Mechanistically, the nucleophile —> electrophile approach is by far the most common, but in contrast to the reactions discussed in the previous three sections, radical cyclizations are of considerable utility here. 

Synthetic methods for the preparation of six-membered heterocyclic systems which proceed via the formation of three or four bonds are virtually restricted in application to the monocyclic heterocycles and have been most widely applied to pyridine and pyrimidine derivatives. In principle, reactions which proceed with the formation of three ring bonds can be sub-classified into three groups, namely, those involving [4 + 1 + 1] atom fragments, [3 + 2 + 1] atom fragments and [2 + 2 + 2] atom fragments. 

The basicities of saturated heterocycles are similar to those of analogous open chain systems, with the exception of three-membered heterocycles, in which the basicity is markedly reduced. Table 1 gives pvalues for the equilibria between free and monoprotonated heterocycles. As the ring size increases, the protonated species become more stable and the pKa values approach those of the open chain analogues. Increasing basicity (thiirane < oxirane < aziridine) prevails in gas phase proton affinities (Table 2) (80JA5151). 

The basicities of saturated heterocycles are similar to those of analogous open-chain systems, with the exception of three-membered heterocycles, in which the basicity is markedly reduced. Table 1 gives pK3 values for the equilibria... 

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