Five-membered heterocycles with one heteroatom

Five-Membered Heterocycles with One Heteroatom 6.24.3.1 Pyrroles... 

Fig. 3.6 Five-membered heterocycles with one heteroatom obtained by solid-phase chemistry.

Pyrazoles Fused to Five-Membered Heterocycles WITH One Heteroatom... 

Fig. 6.1. Plots of reactivity versus electron demand for a series of five-membered heterocycles with one heteroatom (indicated on the plots).

Section 3.1 is a brief overview Section 3.2 deals with six-membered heterocycles, including those with more than one heteroatom in the ring Section 3.3 deals with five-membered heterocycles with one heteroatom Section 3.4 deals with five-membered heterocycles with more than one heteroatom in the ring Section 3.5 covers small (three- and four-membered) and large (>six) ring heterocycles. 

Electrophilic substitution is an important type of reactions for five-membered heterocycles with one heteroatom and enables compounds with various substituents to be obtained. The present work is devoted to certain features of substrate and positional selectivities in electrophilic substitution reactions of derivatives of pyrrole, furan, thiophene and selenophene, and also the corresponding benzannulated systems, which had not been explained until recently. In a recent review (05RKZ(6)59), these problems were mainly discussed for thiophenes, while in a previous review (94H(37)2029) only monocyclic pyrrole, furan and thiophene derivatives were considered. 

To explain the noncorrespondence in the series of substrate and positional selectivities in electrophilic substitution reactions of five-membered heterocycles with one heteroatom, a hypothesis was put forward (79MI2, 80KGS1587). Namely, the formation of jS-substituted... 

Table 1 Differences in total energies ( tot) of five-membered heterocycles with one heteroatom and cationic ir-complexes formed on their protonation for ot- (d ) and /i-positions AEp), as well as values of AE p = AE —AEp (all in kcal/mol) ...

The photoreactions of five-membered heterocycles with one heteroatom give structural isomers. In some cases Dewar isomers were proposed as intermediates, but only a small number were isolated. 

DIMEDONE-ANNULATED FIVE-MEMBERED HETEROCYCLES WITH ONE HETEROATOM... 

An example of cycloaddition reactions that involve transformation of five-membered heterocycles with one heteroatom into benzo[c]-fused heterocycles through a sequence of Diels-Alder reactions is presented in Scheme 1. 

By examining bond order uniformity, it is clear that benzyne is quite a reactive species if a triple bond is involved in the reaction, but not very reactive if a double bond is involved in the reaction. The furan ring had the highest bond order deviation and, as a result, it should be the most reactive five-membered heterocycle with one heteroatom studied here. 

The parent compounds of the monocyclic five-membered heterocycles with one heteroatom are aromatic. When considering the three most important systems only, it appears that the aromaticity increases as follows furan < pyrrole < thiophene (< benzene). This sequence also applies to the respective benzo[Z ] condensed systems. 

The importance of five-membered heterocycles with one heteroatom, of the benzo and dibenzo condensed systems and of the partially or completely reduced compounds as natural products, pharmaceuticals, and starting materials or auxiliaries for syntheses is much greater than for three- or four-membered heterocycles, apart from oxirane. 

Five- and six-membered heterocycles are of utmost importance to both life and drug discovery. The most common five-membered heterocycles with one heteroatom are pyrrole, fiiran, and thiophene. 

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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