Five-membered heterocycles defined

The known types of five-membered heterocycles are presented at the beginning of each main section. Possible structural types with all heteroatomic combinations conceivable are almost unbounded, their presentation here would be worthless and is therefore avoided. The nomenclature of these heterocycles is defined by lUPAC rules (both Hantzsch-Widmann and replacement nomenclature are used) which are adhered to throughout the chapter. 

Figures 15.45 and 15.46 illustrate impressively that the significance of 1,3-dipolar cycloadditions extends beyond the synthesis of five-membered heterocycles. In fact, these reactions can provide a valuable tool in the approach to entirely different synthetic targets. In the cases at hand, one can view the 1,3-dipolar cycloaddition of nitrile oxides to alkenes as a ring-closure reaction and more specifically, as a means of generating interestingly functionalized five- and six-membered rings in a stereochemically defined fashion.

Mesoionic compounds - These are defined as five-membered heterocycles that cannot be represented satisfactorily by any one covalent or polar structure and possess a sextet of electrons in association with the five atoms comprising the ring <1976AHC(19)1>. Two types of mesoionic compounds have been recognized and these are described as Type A and Type B. Mesoionic heterocycles are a subclass of heterocyclic mesomeric betaines <1985T2239>. For examples see Sections 2.41.2, 2.4.14, and 2.4.54. 

Well-defined substitution patterns in the target molecules can be constructed by a combination of fluorine-free or fluoro-substituted (1,3 ee) components with fluorine-free and fluoro-substituted (1,2 nn) compounds. A representative example for the introduction of fluorine and fluoro-substituted groups into five-membered heterocycles via both educts is the reaction of fluoro-substituted chalcones and pentafluorophenyl hydrazine (88JIC773). 

A large class of organic reactions that lead to the formation of five-membered heterocyclic molecules, have been designated as 1,3 dipolar cycloaddition reactions °. Huisgen has defined the 1,3 dipole to be a species which is represented by zwitterionic resonance structures (i.e. the standmd Lewis octet structures) and which undergoes 1,3 cycloadditions to a multiple bond system, the dipolarophile , as in structures (18) and (19). 

A wealth of x-ray diffraction data is available on this group of heterocycles and selected structural parameters are summarized in Table 2. In all the systems having generalized structures (25) or (26), the six-membered ring has an envelope conformation with the sulfur atom out of the plane (ca 0.2-0.5 A) defined by the other five atoms. The five-membered rings are planar and quasiplanar with the nonsulfur atoms of the six-membered ring. The study of compound (25 R1 = NH2, R2 = H,... 

Five-membered rings, too, are relatively easy to form depending on conditions, glucose derivatives can easily be formed in the furanose form, i.e. based on tetrahydrofuran. The polyfunctional and stereo-defined sugars can be used to prepare sterochemically related nitrogen heterocycles. ... 

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