Three-membered ring systems

General. - Thermally induced ring-enlargement of vinyl three-membered heterocycles has been reviewed.  

Department of Chemistry Biochemistry, Ohio University, Athens, OH, USA bergmeis ohio.edu 

Division of Pharmaceutical Sciences, Duquesne University, Pittsburgh, PA, USA lapinskyd duq.edu 

This review covers the chemical literature on epoxides and aziridines for the year 2007. As in previous years, this review is not comprehensive but rather covers a selection of synthetically useful and interesting reactions. Three-membered ring systems, epoxides and aziridines in particular, are excellent synthetic intermediates. This is largely due to their ability to be converted into other functional groups such as diols, diamines, and amino alcohols to name but a few. The chapter has been divided into two sections, one covering epoxides and the other covering aziridines. Each of these sections has been further divided into two additional sections, one on the synthesis of the heterocycle and one on the reactions of the heterocycle. There is some overlap between methods for the synthesis of epoxides and aziridines and any overlap has been noted in the text. 

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Figure 7-2. Strain energies [kj/mol] of three-membered ring systems.

Although it may be experimentally impossible to distinguish and quantify the effect of each of the above factors separately within given three-membered ring systems, a comparison with the properties of higher ring systems (i.e., 5-14) may provide an estimate of the contribution of the sulfone or the sulfoxide function to these properties. 

C. The Sulfone and Sulfoxide Functionality in Three-Membered Ring Systems Activating and Directive Effects... 

The question arises whether there are any unique characteristics associated with the acidity of a-hydrogens when the sulfone or the sulfoxide group is incorporated within a three-membered ring system. 

To summarize under favorable conditions the acidity of a-hydrogens facilitates the generation of a-sulfoxy and a-sulfonyl carbanions in thiirane and thiirene oxides and dioxides as in acyclic sulfoxides and sulfones. However, the particular structural constraints of three-membered ring systems may lead not only to different chemical consequences following the formation of the carbanions, but may also provide alternative pathways not available in the case of the acyclic counterparts for hydrogen abstraction in the reaction of bases. 

A direct attack of nucleophiles on the sulfur atom of the sulfone or sulfoxide group in acyclic or large-ring sulfones and sulfoxides is rather rare, or unknown, excluding metal hydride reductions and/or reductive deoxygenations. The situation is completely different in the three-membered ring systems. 

An initial attack of a lithium reagent on the sulfur atom of 16, leading to alkenes, has been discussed in the previous section. The similarity in the chemical consequences of the electrophilicity of both the sulfone and sulfoxide functional groups in strained three-membered ring systems is thus established. 

Actually, thiirane dioxides (17) have so far never been isolated in these reactions cis- and trans-olefins were the main products, and all attempts to obtain the three-membered ring system and prevent the loss of SO 2 failed. Hence, the method can be used only for the in situ formation of intermediates. 

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. 

The 1,3-dipolar reagent can in some cases be generated by the in situ opening of a suitable three-membered ring system. For example, aziridines can add to activated double bonds to give pyrrolidines, for example," ... 

The chemical shifts of the H-methyl groups in thiiranes 31a, 31b and 31c were found to be = 1.59,1.44 and 1.45, respectively. The chemical shifts of the -anti-methyl hydrogens (i.e. those of R ) where found to be (5 = 1.25,1.23 and 1.27 in 32a-c compared with <5= 1.74 and 1.64 for syn-R -hydrogens in 32a and 31c, respectively . The consistency of the deshielding effect in accordance with the position of the -hydrogens in ring sulfoxides is thus apparent. These observations validate the applicability of the S—O anisotropy rule to the three-membered ring system. 

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