Five-membered heterocycles compounds

Saturated five-membered heterocyclic compounds are non-planar, existing in half-chair or envelope conformations. The far-IR spectra of THE and 1,3-dioxolane (127) show both to have barriers of ca. 0.42 kJ moP ... 

All the available evidence suggests that five-membered heterocyclic compounds containing a potential hydroxyl group between the two hetero atoms in the 1- and 3-positions exist predominantly in the oxo form (cf. 89 90). 

Azides add to double bonds to give triazolines. This is one example of a large group of reactions ([3-l-2]-cycloadditions) in which five-membered heterocyclic compounds are prepared by addition of 1,3-dipolar compounds to double bonds (see Table 15.3). These are compounds that have a sequence of three atoms A—B—C,... 

Since Huisgen s definition of the general concepts of 1,3-dipolar cycloaddition, this class of reaction has been used extensively in organic synthesis. Nitro compounds can participate in 1,3-dipolar cycloaddition as sources of 1,3-dipoles such as nitronates or nitroxides. Because the reaction of nitrones can be compared with that of nitronates, recent development of nitrones in organic synthesis is briefly summarized. 1,3-Dipolar cycloadditions to a double bond or a triple bond lead to five-membered heterocyclic compounds (Scheme 8.12). There are many excellent reviews on 1,3-dipolar cycloaddition, in particular, the monograph by Torssell covers this topic comprehensively. This chapter describes only recent progress in this field. Many papers have appeared after the comprehensive monograph by Torssell. Here, the natural product synthesis and asymmetric 1,3-dipolar cycloaddition are emphasized.630 Synthesis of pyrrolidine and -izidine alkaloids based on cycloaddition reactions are also discussed in this chapter. 

Heteroatom transfer in metallacyclopentadienes was first developed in the context of cobalt chemistry in the mid-1970s [27]. Cobaltacyclopentadienes were converted into various five-membered heterocyclic compounds such as pyrrole and thiophene, and into six-mem-bered heterocyclic compounds such as pyridine and pyridone derivatives. In the case of zirconacydopentadienes, the heteroatom compound must bear at least two halide substituents, since the Cp2Zr moiety is re-converted to the stable Cp2ZrX2. Indeed, this is the driving force behind the heteroatom transfer of zirconacydopentadienes. 

Due to the migration of C=C bond, the conjugated esters formed can accept the attack of a second nucleophile. Thus, by applying different difunctionalized benzenes, benzo-fused five-membered heterocyclic compounds 526 can be prepared... 

Oxadiazoles are five-membered heterocyclic compounds containing one oxygen and two nitrogen atoms. 

Paal-Knorr synthesis It is a useful and straightforward method for the synthesis of five-membered heterocyclic compounds, e.g. pyrrole, furan and thiophene. However, necessary precursors, e.g. dicarbonyl compounds, are not readily available. Ammonia, primary amines, hydroxylamines or hydrazines are used as the nitrogen component for the synthesis of pyrrole. 

L. L. Bambas, Five-Membered Heterocyclic Compounds, p. 35 et aeq. Interscience, New York, 1952. 

A theoretical understanding of furan and its congeners followed in the wake of the progress with the benzene problem, but for the five-membered heterocyclic compounds Bamberger s centric formula was always the most reasonable one, as Kekule structures could not be written at all. It is not now clear to what extent the heterocycles were really regarded as benzene analogues, but a remark made by Hantzsch, that coumarone is the furfurane of the naphthalene series was certainly percipient. Furan is variously said to be aromatic, superaromatic or not aromatic at all, for the debate continues. 

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... 

L. E. Bambas Five Membered Heterocyclic Compounds with Nitrogen and Sulfur, p. 219, Interscience Publishers, Inc., New York, 1952. 

Lablache-Combier, A. (1976) Photoisomerization of five membered heterocyclic compounds, in Photochemistry of Heterocyclic Compounds (ed. O. Buchardt), John Wiley, New York, pp. 123-206. 

Treatment of 139 with selenium and tellurium did not lead to any reaction. However, upon addition of a small amount of a triethylphosphane, the corresponding five-membered heterocyclic compounds 155 and 156 were isolated in high yield (equation 40)142. 

C. Methods for the Synthesis of Five-Membered Heterocyclic Compounds. .. 160... 

Dipolar cycloaddition reactions are most commonly applied for the synthesis of five-membered heterocyclic compounds.86 87 [3+2] cycloaddition reactions of transition-metal propargyl complexes have been reviewed.88 Addition of diazomethane to carbene complexes (CO)5Cr= C(OEt)R results in cleavage of the M = C bond with formation of enol ethers H2C = C(OEt)R,3 89 but (l-alkynyl)carbene complexes undergo 1,3-dipolar cycloaddition reactions at the M = C as well as at the C=C bond. Compound lb (M = W, R = Ph) affords a mixture of pyrazole derivatives 61 and 62 with 1 eq diazomethane,90 but compound 62 is obtained as sole... 

Selenophene (C4H4Se) is a five-membered heterocyclic compound containing one heteroatom. The first compound of this series to be prepared was 2,5-dimethylselenophene, obtained in 1885 by Paal1 from acetonylacetone by heating it with phosphorus pentaselenide. Unsubstituted selenophene was obtained much later it was mentioned... 

The aromaticity of a heterocycle depends on how effectively the lone-pair of the heteroatom contributes to the aromatic sextet. The aromaticity of five-membered heterocyclic compounds may be estimated from their reactivity in the Diels-Alder reaction.94 Spectrophotometry shows that furan, thiophene, and selenophene resemble benzene in that with maleic anhydride 1 1 complexes are formed which are stable up to 150°C in the case of thiophene, decompose at 150°C with selenophene (whereby selenium is formed together with a diene which gives a further adduct with another molecule of maleic anhydride), and produce the usual adduct at 20°C with furan. Thus, only furan is a normal diene as regards the Diels-Alder reaction. 

From sulfur- and nitrogen-containing five-membered heterocyclic compounds... 

From sulfur- and nitrogen-containing five-membered heterocyclic compounds Photocyclization of l,3-dithiole-2-ones 723 affords thieno[3,2-r ]dithiins 725. The reaction probably involves cycliza-tion of the intermediate dithiones 724 (Scheme 110) <20010L3573, 2003JOC7115>. 

By heating 30 (dry or in less polar solvents), high yields of oxygen and starting pyrrole were obtained, along with a small amount of lactam (28). Deoxygenation is not normal behavior of the hydroperoxide function, but it had analogy in other five-membered heterocyclic compounds.35... 

The (3 -I- 2) photocycloaddition occurs between three-membered heterocyclic compounds and electron deficient alkenes in the presence of 1,4-dicyanonaphthalene to afford five-membered heterocyclic compounds... 

In the thiophene series, phosphorus sulfide converts 1,4-diketones and sodium salts of succinic and alkylsuccinic acids to the five-membered heterocyclic compounds. The yields are low, usually 20-30% from the succinates and 60% from the diketones. 

Five-membered heterocyclic compounds of the furan, thiophene, and pyrrole series are interconvertible by one-step catalytic processes. ... 

Similar elimination reactions are employed in the synthesis of the benzologs of five-membered heterocyclic compounds. For example, /3-phenylethyl alcohols having hydroxyl, sulfhydryl, and amino groups in the ortbo position ate cyclized to the dihydro derivatives of benzofuran, benzothiophene, and benzopyrrole, respectively. Likewise, dehydro-halogenation of /3-(o-hydroxyphenyl)-ethyl bromide gives dihydtobenzo-furan in 72% yield. ... 

From Five-Membered Heterocyclic Compounds From /V- (Methanesulfonyl)oxy plithalimide... 

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