Cyclic heterocycles from

Electrosyntheses of heterocycles from nitroso derivatives prepared in a batch cell according to Scheme 34 need two conditions. The first one is a good stability of the hydroxylamine intermediate and the second one is a very fast cyclization of the nitroso compound to avoid the formation of an azoxy compound by condensation of the generated nitroso and the hydroxylamine. Electroanalytical studies using cyclic voltammetry can give information on the rate of cyclization. 

Thianthrenes have not been dealt with previously in Advances in Heterocyclic Chemistry. This chapter covers the chemistry of thianthrenes from 1960 to the end of Chemical Abstracts 1988 coverage. Earlier references are included only where they are of particular significance or serve to place later work in context. The family of linear tri-6-membered-cyclic heterocycles with group VIb elements located 1,4- in the central ring, of which thianthrene is a member, have been referred to as dibenzodi-chalcogenins, chalcogenanthrenes, and chalcanthrenes. This review will not attempt to draw comparisons between the chemistry of thianthrene and that of the other chalcanthrenes. 

A large number of a, 3-didehydro-a-amino acids have been identified as constituents of relatively low molecular weight cyclic compounds from microbial sources. However, the presence of a,p-didehydroalanine in bacterial as well as in mammalian histidine ammonia lyase and in phenylalanine ammonia lyase shows that the occurrence of a,p-didehydro-a-amino acids is not limited to small molecules alone 8 These residues are incorporated in natural sequences by posttranslation modification. a,p-Didehydro-a-amino acids have also been postulated to be precursors in the biosynthesis of several heterocyclic metabolites including penicillin and cephalosporin 9 Other well-known compounds containing ,( -di-dehydro-a-amino acids are nisin 10,11 (a food preservative112 ), subtilin (a broad spectrum antibiotic) 13 and some of the metabolites isolated from Streptomyces strains such as gri-seoviridin 14 ... 

Ghosh AC, Ramgopal M (1980) Cyclic Peptides from Penicillium islandicum. A Review and a Reevaluation of the Structure of Islanditoxin. J Heterocycl Chem 17 1809... 

Homolytic substitution of heteroaromatic compounds, 16, 123 Hydantoins, chemistry of, 38, 177 Hydrogen cyanide derivatives, synthesis of heterocycles from, 41, 1 Hydrogen exchange base-catalyzed, 16, 1 one-step (labeling) methods, 15, 137 Hydrogenated porphyrin derivatives hydroporphyrins, 43, 73 Hydroxamic acids, cyclic, 10, 199 1 -Hydroxyindoles, 1 -hydroxytryptophans, and 1-hydroxytryptamines,... 

The Pfitzinger reaction provides a simple and convenient method for the synthesis of condensed polycyclic compounds containing a heterocycle from readily obtainable starting materials. Cyclic ketones and, more rarely, diketones are often used as starting compounds. 

Such syntheses may be classified into four main categories from nonhetero-cyclic precursors from single pyridine substrates, from pyridine substrates with ancillary synthons, and from other heterocyclic substrates. 

Alkynes and ynamines have also been developed as useful participants in RCM approaches to pyrrolidine derivatives (Scheme 13). Mori and co-workers reported the formation of a cyclic dienamide via ene-ynamide RCM <020L803>. They also studied substituent effects on the course of RCM of enynes and reported the formation of both pyrrolidines and piperidines <02MI678>. The syntheses of 5- and 6-ring carbocycles and O-containing heterocycles from appropriate precursors were also reported in that work. Synthetically-useful dienylboronic acids have been prepared from acetylenic boronates <00AG(E)3101>. ... 

The most recent contribution in this area is by Shea who prepared tricyclic oxygen containing heterocycles from acyclic enynes 82 using a combination of intramolecular Nicholas and PKRs. They constructed [5.7.5] and [5.8.5] (83-84) systems involving the formation of a complexed cyclic alkyne. They used several PKR conditions that give different yields and diastereo-selectivities. Due to the strain of the intermediate, [5.6.5] systems (85) were obtained in poor yields (Scheme 24) [121]. 

Macrocycles are sometimes only formed in the presence of monomers. When all monomer has been consumed by polymerization, depolymerization also stops [333]. The tendency of common heterocycles to the production of giant molecules, and the possibility of generating cyclic fractions from their chains is illustrated in Table 6. 

In addition to several general reviews on enamine chemistry, all of which include heterocyclic syntheses, there is an extensive survey by Hickmott which is entirely concerned with the formation of heterocycles. More specialized reviews deal with heterocyclic enamines, enaminones, the photochemistry of enamides , heterocyclic jS-enamino esters , enamino thiones , the synthesis of indole alkaloids via enamines , formation of pyrimidines, pyridopyrimidines, pyridines and pyrrolizines from enamines , synthesis of lactams , formation of heterocycles from cyclic enamino ketones and 2-acetylcyclohexen-l-ones, the synthesis of 3-cyano-2(l -pyrimidine-thiones and -selenones from jS-enamino ketones and the chemistry of cyclic en-aminonitriles. ... 

The cyclamides are small cyclic peptides that characteristically contain multiple thiazole, thiazoline, oxazole, and oxazoline rings, which are derived from cysteine, serine, and threonine residues. Some of the first examples of this class to be described were the patellamides (53-55) from the tunicate Lissoclinum patella, although it was later determined they were produced by the symbiotic cyanobacterium Prochloron sp. The structures were solved by a combination of acid hydrolysis and GC analysis, coupled with 2D NMR. Smaller cyclic peptides from this class include the hexapeptides westiellamide (56) and microcyclamide (57) from M. aeruginosa Many members of the class possess cytotoxic properties, although their biological function or mechanism of action is not fully understood. In contrast to many cyanobacterial peptides, aside from the unusual heterocyclic residues, these peptides generally contain only ribosomal amino acids. 

Syntheses of cyclic iodonium salts and other heterocycles from alkenylio-donium salts 00UK118. 

The ease of ring formation helped us to make five-membered cyclic acetals from ketones and a seven-membered heterocyclic compound (see Chapter 6). In this chapter we have seen how three-membered epoxides are formed. Ring formation is generally preferred to bimolecular reactions forming open chain compounds providing that the ring is three-, five-, six-, or seven-membered. Four-membered ring are a special case discussed in Chapter 29 where all these points are developed more fully. 

To determine the stability and reactivity of heterocyclic compounds, it is useful to compare them with their carbocyclic analogues. In principle, it is possible to derive every heterocycle from a carbo-cyclic compound by replacing appropriate CH2 or CH groups by heteroatoms. If one limits oneself to monocyclic systems, one can distinguish four types of heterocycles as follows ... 

Reactions which yield cyclic products from open-chain starting materials by the elimination of water, hydrogen halides or other low molecular mass compounds are called cyclocondensations. They are important methods for synthesizing five-, six- und larger-membered heterocycles. 

S.2.2.7. Fused Heterocycles from Cyclic Dipolarophiles. Furan provides a useful example of a diene acting instead as a dipolarophile. As will be discussed in Chapter 6, this molecule has the lowest aromaticity of the 5-membered heterocycles. It is also useful as a diene in Diels-Alder cycloadditions, but here we will observe that one double bond can react with a 1,3-dipole (Scheme 5.34). 

An intermolecular version of the Sakurai reaction has been developed. It proceeds at temperatures as low as —78°C, using substoichiometric amounts of the Lewis acid, to form five-or six-membered oxygenated heterocycles from cyclic allylsilox-anes and aldehydes through a chairlike transition state (eq 65). Acetals may also act as electrophiles in this kind of reaction. /3-Borylallylsilanes also undergo nucleophilic allylation to lead to the preparation of functionalized alkenylboranes, which may participate in further transformations. ... 

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