Heterocycles with One Heteroatom

A large acceleration of some difficult Diels-Alder reactions was promoted under MWI on a graphite support (91TL1723, 96LA743). Thus, hetero Diels-Alder reactions of glyoxal monoacetal and methyl glyoxylate with 2-methyl-1,3-pentadiene 1 (R = H R = Me) have been carried out under pressure in an MW oven to give a 

MW has also accelerated the tetrahydropyranylation of alcohols, phenols 13, and thiols 14 in the presence of hydrated zirconia to give 15 in 80-98% yields within 3-15 min. This method has distinct advantages over the other existing methods, characterized by mild reaction conditions, absence of solvent, shorter reaction time, and easy as well as quick isolation of the products in excellent yields. The hydrated 

Oxidation of the cyclic sulfide 4-oxothiophene to the respective sulfoxide 4-ox-othiophenesulfoxide was achieved by heating with iron(III)nitrate impregnated on montmorillonite Kio clay in methylene chloride for 12h to give a 79% yield. Much faster reaction took place using clayfen under MWI to give 83% yield within 

Subjecting 4-hydroxybenzaldehyde 29 (R = H) to 3-methyl-2-butenal (30) in the presence of pyridine in a sealed glass tube to MWI for 25 min yielded chromene 31 in 68% yield. Under similar conditions, the condensation of 2,4-dihydroxybenzalde-hyde 29 (R = OH) with 30 gave chromenes 32 and 33 in 49% and 12% yields, respectively (99BCJ259). 

Intramolecular Diels-Alder cycloaddition in triene 53 was achieved by heating with toluene in a sealed glass tube in an MW oven for 2.5 h to afford (1S)(2R)(5R)(6R) (8R)(95)(12R)-l,7,7-trimethyl-6-methoxy-4-oxatetracyclo[7.3.1.0]-lO-tridecen-3-one (54) in 97% yield (93JOC2186). Compound 54 is a precursor in the synthesis of the sesquiterpene (-I- )-longifolene. 

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Chemistry of furazanes fused with six- and seven-membered heterocycles with one heteroatom 99UK154. 

Table 4 Angularly fused (5 5 5) heterocycles with one heteroatom...

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

In the following sections, heterocycles are discussed in the same order as in an earlier monograph,55 namely in the order of increasing number of ring atoms. Within a certain ring, heterocycles with one heteroatom are discussed first, in the increasing order according to the atomic number on the periodic... 

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

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

The TT-electron excess of the five-membered rings is accompanied by a high rr-donor character. The best measure of rr-donation is the value of first ionization potential, IP, which for all aromatic heterocycles with one heteroatom of pyrrole type reflects the energy of highest occupied rr-orbital. IP, values decrease in the sequence pyrrole > indole > carbazole furan > benzo[/ ]furan > dibenzofuran thiophene > benzo[/ ]thiophene (Section 2.3.3.9, Tables 21 and 23). Thus, the more extensive the rr-system, the stronger is its electron donor ability. Furan and thiophene possess almost equal rr-donation, which is considerably lower than that of pyrrole. 

The heterocycles (la)-(ld) are heteropentalenes they have a central C—C bond, and are isoelectronic with the 107r pentalene dianion (2). They are formally derived by the fusion of two of the well-known simple heterocycles including furan, pyrrole, thiophene and selenophene. Considering the two rings as rings A and B, this entire group of five-five fused heterocycles with one heteroatom in each can be conveniently referred to as A,B-diheteropentalenes. 

Pyrrole, furan, and thiophene are five-membered aromatic heterocycles with one heteroatom. In pyrrole, the nitrogen is sp2-hybridized and contributes two electrons to the 6n aromatic ring. Furan and thiophene are isoelectronic with pyrrole, the [—(N )=] unit being replaced by —( 0 )— and -( S )- units, respectively. Pyrrole, furan, and thiophene are electron-rich (there are six n electrons distributed over five atoms) and undergo electrophilic... 

Systems considered are the parent heterocycles with one heteroatom (furan, thiophene, selenophene, and pyrrole), their monoaza and polyaza derivatives, and the monobenzo derivatives of the above systems. Dibenzo derivatives such as dibenzofuran and carbazole are not taken into account, since they have no position in the five-mem bered ring susceptible to electrophilic attack. Likewise, no consideration is given to more complicated systems, formed by the fusion of two heterocyclic rings such as thienothiophenes or pyrrolopyridines, for which, in any case, no quantitative work is available. 

From five-membered ring heterocycles with one heteroatom 938... 

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. 

FIVE-MEMBERED AROMATIC HETEROCYCLES WITH ONE HETEROATOM... 

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) ...

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