PART 2 FIVE-MEMBERED HETEROCYCLES

PART 2 FIVE-MEMBERED HETEROCYCLES Chapter 2 Pyrroles and Pyrrolidines... 

Histidine is one of the 20 naturally occurring amino acids commonly found in proteins (see Chapter 4). It possesses as part of its structure an imidazole group, a five-membered heterocyclic ring possessing two nitrogen atoms. The pAl for dissociation of the imidazole hydrogen of histidine is 6.04. 

A five-membered heterocyclic ring packs a relatively large number of polarized bonds into a relatively small molecular space. This provides a convenient framework to which to attach necessary side chains. In some cases, the framework itself is believed to be part of the pharmacophore. 

Hence, microwave irradiation in DMA for 12 min at 120 °C or 30 min at 140 °C, depending on the substrate, resulted in 20 or 21 in 90-96% yields, compared to yields around 50% and a 24 h reaction time for conventional heating. As can be seen in Eq. 2 in Fig. 5, two possibilities exist to connect the heteroaryl bromide part with the five-membered heterocycle (indicated by a and fi. Table 2) that results in a six-membered 2-pyridone ring. Still, a very high selectivity was obtained yielding the 2-pyridones 21, in which the new bond had been introduced at Ca. It should be noted that the obtained structures 20 and 21 represent heterocyclic derivatives of 2-quinolones rather than true 2-quinolones. 

Replacement of a carbon in the thiazolium or isothiazolium cation by nitrogen to yield the thiadiazolium salts (36) and (37) increases the deprotonation rate by between 10 and 10. These compounds as well as 1,3,4-thiadiazole (38) and 1,2,3-thiadiazole (39) have been studied by Olofson et as part of a series of five-membered heterocyclic... 

As it has been already mentioned in the Introduction, aminoazoles besides the role of 1,3-binucleophiles can take part in the MCRs as 1,1-binucleophiles with participation of exclusively exocyclic NH2-group. Usual products of such multi-component interaction are five-membered heterocycles having azole ring as a substituent. 

Finally, M. R. Grimmett from the University of Otago in New Zealand deals comprehensively with the halogenation of five-membered heterocycles in the first part of a survey that will ultimately cover the halogenation of all heterocycles. 

A significant part of the examples of transition metal catalyzed formation of five membered heterocycles utilizes a carbon-heteroatom bond forming reaction as the concluding step. The palladium or copper promoted addition of amines or alcohols onto unsaturated bonds (acetylene, olefin, allene or allyl moieties) is a prime example. This chapter summarises all those catalytic transformations, where the five membered ring is formed in the intramolecular connection of a carbon atom and a heteroatom, except for annulation reactions, involving the formation of a carbon-heteroatom bond, which are discussed in Chapter 3.4. 

Besides ring closure reactions proceeding through C-H activation, a major part of this chapter is devoted to the formation of five membered heterocycles in annulation reactions. 

The most general approach to synthesis of five-membered heterocyclic compounds involves cycloaddition of a 1,3-dipole to an appropriate unsaturated substrate, the dipolarophile. Intermolecular cycloadditions result in the formation of one new ring only. When the 1,3-dipole and the substrate are part of the same molecule, cycloaddition is intramolecular and leads to a new bicyclic system. Thus, intramolecular... 

Parts II (Five-Membered Heterocyclic Rings) and 111 (Six-Membered Heterocyclic Rings) are organized along classical lines Monocyclic five-membered rings with one heteroatom (Chapter 6), monocyclic five-mem-bered rings with two or more heteroatoms (Chapter 7), polycyclic com-... 

To limit the size of this report, a restriction was placed on the compounds that were eligible for inclusion. Firstly, to be included, the compound had to contain at least one five-membered heterocycle in its structure. Secondly, the number of atoms in the internal ring pathway of the macrocycle was required to contain at least 17 atoms (one more than the number of atoms present in the inner core of porphine Fig. 1). Thus this definition, which is in two parts, eliminates conjugated macrocycles containing other small heterocycles such as pyridines. It also rules... 

The best known mesoionic compounds have five-membered rings, and initially it was advocated by Baker, Ollis, Ramsden and other authors that only five-membered heterocycles which cannot be satisfactorily represented by any one covalent or ionic structure possessing a sextet of TT-electrons in association with the five atoms comprising the ring may be called "mesoionic". Here, following Katritzky, mesoionic means a mesomeric betaine. The first such compounds to be discovered were sydnones, followed by miinchnones and then by diazolones. In all these compounds the Z-type atom is part of a carbonyl group, and two Y-type atom chains separate two odd-numbered chains of X- and Z-atom chains. Only the main resonance structures are displayed in formulas (Figure 11). 

A 2-phosphaallyl anion as part of a five membered heterocycle [27, 35] When in the already discussed reaction between ethyl benzoate and lithium bis(trimethylsilyl)phosphanide the molar ratio of the educts is changed finally from 1 3 to 1 1, dark green, metallically lustrous, air sensitive crystals of... 

Thus, the photoisomerization of five-membered heterocycles seems not to be limited to one intermediate only but several intermediates may take part in this isomerization, depending on the nature of the ring system and substituents. 

Five-membered heterocycles. Part IV. Impact of heteroatom on benzazole aromaticity. 

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