Five-membered ring systems heteroatoms

The synthesis of these five-membered ring systems has classically been discussed in terms of the ring system formed. In recent years many synthetic procedures have been classified in terms of the bonds being formed and the position and nature of the heteroatoms involved. Both methods have their advantages, and also their drawbacks. 

Five-membered ring systems can be obtained from hetero-l,3-dienes on reaction with oxiranes and thiiranes. To avoid competition from a possible 1,4-addition, the nucleophilic attack of the terminal heteroatom of the diene has to be sterically or electronically hindered by incorporation of the heteroatom into... 

The second chapter concerns the tautomerism of five-membered ring systems with a single heteroatom concentrating mainly on pyrroles, furans, and thiophenes. It is authored by Professor W. Friedrichsen and Dr. T. Traulsen (University of Kiel, Germany) together with Drs. J. Elguero and A. R. Katritzky. 

As a t rpical example. Scheme 3 shows the possible formation of five-membered ring systems with two heteroatoms in the aceto-in-ammonium sulfide model system (Barone, R., Universite d Aix-Marseille III, unpublished results, 1986). Similar predictions can be made for the formation of substituted pyrrolidones and for additional five- and six-membered rings. 

Formation of five-membered ring systems (1,2-addition) can compete with formation of the seven-membered heterocycles (1,4-addition). If the first step of the reaction sequence, namely the nucleophilic attack of the terminal heteroatom of the diene, is hindered by steric or electronic effects, the five-membered ring product is formed exclusively. 

The chapter is grouped according to the main heterocycle types in order of increasing complexity, commencing with five-membered ring systems containing one heteroatom... 

Five-membered ring systems with more than two heteroatoms 3.4.1. Triazoles... 

As first examples of unsaturated five-membered ring systems with one additional heteroatom adjacent to phosphorus,... 

Four- and five-membered ring systems will preferentially span the axial-equatorial sites in a trigonal bipyramid, even at the expense of forcing more electronegative substituents to occupy the electronically less favourable sites. As a result of -interaction, the axial-equatorial placement is relatively more favourable for ring systems with heteroatoms substituted at phosphorus than for those with P-C bonds. 

A very powerful and elegant methodology intensely developed in recent years for the construction of five-membered ring systems, e.g. of cyclopentanoid natural products, is based on the in situ formation of highly reactive Cj intermediates from a variety of synthetic precursors. These intermediates 1 can then serve as three-earbon, 2n-electron (Y = Hj) or 4 -electron (Y = CH, O) components in cycloaddition reactions, specifically of the [3-1-2] type, with various carbon-carbon - or carbon-heteroatom multiple bonds as 2rt-components. 

Fully unsaturated five-membered ring systems with one other heteroatom adjacent to phosphorus have only been known only since 1986 <86CB4io> and therefore no representative is contained in Comprehensive Heterocyclic Chemistry, First Edition (CHEC-I) <84CHEC-i(i)523>. They include now... 

V. Reactions with Five-Membered Ring Systems Containing One Heteroatom. . 194... 

VI. Five-Membered Ring Systems with Two or More Heteroatoms.206... 

The chemistry of 7r-hypervalent heterocyclic systems is one of the important fields in that of hypervalent compounds. A number of 7r-electron systems containing a 10—S—3 framework have been prepared, and their structures and reactivities have been investigated [79]. In general, all atoms bonded to central atom in 7r-sulfuranes such as trithiapentalene are in the same plane. The equatorial bond has the characterization of a double bonding system and the two apical bonds are a 3-center 4-electron bond. As the sp2 type carbon atom or heteroatom having a lone electron pair is located in the two five-membered ring systems, a 1271 conjugated system is constructed and plays an important part in the stability of 7r-sulfurane. 

A rare example of a vapor-phase carbonyl condensation reaction leading to a double heteroatom five-membered ring system involves the production of 4-methylthiazole (28). This compound is a precursor to thiabendazole, a fungicide used to protect citrus fruit (ca 300 tons is consumed annually). The 4-methylthia-zole process is reported to use CS2SO4-doped zeolite MFI [53]. 

Solid-acid catalysts can also be used to prepare a three-heteroatom five-membered ring system, but relatively mild reaction conditions are required. A recent example is the liquid-phase formation of 4-aminotriazole (29) by reaction of hydrazine and formic acid in the presence of a strong-acid resin [54]. 

This review covers 71-excessive five-membered ring systems containing one arsenic, antimony, or bismuth atom within the ring. Their benzo-, dibenzo-, unsaturated, partially unsaturated, and saturated analogues are also covered. These heteroatoms can have expanded valence shells and thus different coordination numbers (CN) are included as well. Although not strictly covered in this Chapter, mention is made of systems containing one of the above heteroatoms with another atom within the ring, but usually to show an important property of the system. 

Targets with one Heteroatom Five-membered heterocycles with one heteroatom, such as pyrrolidine and tetrahydrofiiran skeletons, exist widely in numerous natural products and bioactive compounds. Therefore, intense efforts have been devoted to the synthesis of these five-membered ring systems. Among the various methods existing for the synthesis of chiral pyrrolidine and proline derivatives, few can match the synthetic potential of 1,3-DC reactions of azomethine ylides with alkenes [3]. Generally, azomethine yhdes are unstable species, so they are normally generated in situ and trapped by unsaturated bonds (Scheme 2.1) [2b]. 

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