Heterocyclization and Aromatization

An additional paradigm of enzymatic heterocyclization can be found in cyclic polyethers such as momensin. In these antibiotics, characteristic tetrahydrofuran 

Stimulated by the biosynthetic pathways, biomimetic heterocyclization methods have also been developed with high efficiency. For example, using an oxo-bridged diphosphonium salt prepared in situ from PhsPO and Tf20, a variety of hetero- 

Although genetic studies have identified a number of putative aromatic PK aromatases and/or cyclases, in vitro characterization of these enzymes is usually difficult since the postulated Hnear or monocyclic poly 3-ketone templates are often unstable to be isolated or synthesized. For these reasons, the generality and synthetic applications of these enzymes have not been studied in most cases. However, there is plenty of evidence to suggest that enzymatic aromatization and cyclization take place for a variety of linear precursors with various chain lengths [32]. 

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CaveU and Chapman made the interesting observation that a difference exists between the orbital involved in the quatemization of aromatic nitrogen heterocycles and aromatic amines, which appears not to have been considered by later workers. The lone pair which exists in an sp orbital of the aniline nitrogen must conjugate, as shown by so many properties, with the aromatic ring and on protonation or quatemization sp hybridization occurs with a presumed loss of mesomerism, whereas in pyridine the nitrogen atom remains sp hybridized in the base whether it is protonated or quaternized. Similarly, in a saturated compound, the nitrogen atom is sp hybridized in the base and salt forms. 

The amido-, thioamido-, sulfonamido-, and semicarbazido-benzofuroxans possessing different lateral chains (het-eroaliphatic, heterocyclic, and aromatic) were prepared by traditional methods by transformation of functional group of benzene fragment. The typical chemistry of benzofuroxanes did not take place in these cases <2002AP15>. 

Diazoacetic esters, reactions with alkenes, alkynes, heterocyclic and aromatic compounds, 18, 3 26, 2 a-Diazocarbonyl compounds, insertion and addition reactions, 26, 2 Diazomethane ... 

D. Model Studies on Enzymatic Oxidation of Heterocycles and Aromatic... 

The Reactions of Diazoacetic Esters with Alkenes, Alkynes, Heterocyclic and Aromatic Compounds V. Dave and E. W. Warnhoff, Org. React., 1970,18, 217-402. Reactions of Singlet Oxygen with Heterocyclic Systems H. H. Wasserman and B. H. Lipshutz, Org. Chem., 1970, 40, 429-509. 

The electronic similarity suggests that unsaturated boron heterocycles and aromatic systems have 7r-electron transitions in common. In fact, such similarities are abundant, and are one main reason for the statement that certain boron heterocycles possess aromatic properties. The UV spectrum of 5,6-dihydrodibenz[c,e][l,2]azaborine (15) is reminiscent... 

H20, alcohols, amines, nitriles, simple heterocyclics and aromatics... 

Reviews (a) V. Dave and E. W. Wamhoff, The Reactions of Diazoacetic Esters with Alkenes, Alkynes, Heterocyclic and Aromatic Compounds, in W. G. Dauben, ed., Organic Reactions, Vol. 18, Chap. 3, John Wiley Sons, New York, 1970. (b) G. Maas, Top. Curr. Chem., 137, 75 (1987). (c) J. Salaun, Chem. Rev., 89, 1247 (1989). (d) A. Demonceau, A. J. Hubert, and A. F. Noels, Basic Principles in Carbene Chemistry and Applications to Organic Synthesis, in A. F. Noels, M. Graziani, and A. J. Hubert, eds., Metal Promoted Selectivity in Organic Synthesis, p. 237, Kluwer Academic, Dordrecht, 1991. 

Many types of aliphatic, heterocyclic, and aromatic derived quaternary ammonium compounds are produced both in plants and invertebrates. Examples include thiamine (vitamin Bj) (4). See also Vitamin choline... 

The mass spectra of benzo[6]thiophene and several of its derivatives have been recorded,120-124 but only two systematic analyses of such spectra have been made.121,128 The relatively high stability of benzo[6]thiophene to electron impact120,125 has been compared with that of other heterocyclic and aromatic systems.125... 

Intramolecular electron transfer in a stepwise manner from the amine substrate to die silver(III) center in a 1 2 complex, [Ag(OH)4] -iV,/V-dimcthylanilinc, has been observed.44 The kinetics of oxidation of some aliphatic, heterocyclic, and aromatic aldehydes towards bis(dihydrogentellurato)cuprate(III) and argentate(III) in alkaline medium have been studied.45 A negative salt effect was observed in the oxidation of aminoacetic acid by diperiodatocuprate(III) complex in alkaline medium.46 The oxidation of glutamic acid by thallium(III) perchlorate is catalysed by Ru(M), Os(III), and Nd(III) in a free radical mechanism and the rate is inversely dependent on [H+] concentration.47... 

Leonid Belen kii was born in Moscow, and he graduated from M. V. Lomonosov Moscow State University in 1953 with Professor A. P. Terentiev as supervisor in organic chemistry. Since 1955, he has worked as junior, senior (since 1966), and leading scientist (since 1988) at N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, where he obtained his Ph.D. degree (1963) under the direction of Professor Ya. L. Gol dfarb and his Degree of Dr. Chem. Sci. (1974) and rank of Professor in Chemistry (1991). His scientific interests include all aspects of chemistry of heterocyclic and aromatic compounds, particularly electrophilic substitution in benzene, thiophene, furan, and azole series as well as organosulfur chemistry. 

Heterocyclic and aromatic aldehydes (e.g., furan-2-carboxaldehyde, thiophen-2-carboxaldehyde, the pyridine carboxaldehydes, benzaldehyde, and diverse substituted benzaldehydes). 

Organic acids, phosphates and phosphonates amino acids, aminocarboxylates, (aza)crown ethers, phenols, heterocyclic and aromatic compounds... 

Additional carbamate-modified polysaccharides, (I), were prepared by Ohnishi [1] and used in separating enantiomeric racemates of both non-aromatic heterocyclics and aromatic derivatives. 

Furfural-salicylic acid and furfural-P-naphthalenesulfonic acid resins possess thermal stabilityexceeding that of KU-1 (sulfonated phenol-formaldehyde resin) and approaching that of KU-2 (sulfonated SDVB copolymer). The heat resistance of toluenesulfonic acid-furfural resin is quite similar to that of KU-2 resin. The improved thermal stability of furfural-based cation-exchange resins is attributed to the presence of heterocyclic and aromatic rings in these resins. 

Table 10 Heterocyclic and aromatic sulfones used in the Julia-Kocienski olefination reaction ...

Several factors affect the volatility and stability of a peptide derivative, not least of these being the number and nature of the constituent amino acids. Heterocyclic and aromatic amino acids reduce volatility while those containing sulphur tend to decrease the thermal stability. Small naturally occurring peptides which are not derived from proteins often contain only aliphatic amino acids which lack functional groups in the side chains. Peptides of this type of up to about ten amino acids, after conversion to suitable derivatives, are amenable to analysis by mass spectrometry, e.g. [164]. A variety of derivatives has been reported and include N-trifluoroacetyl peptide esters [136,165], N-acetyl peptide esters [166-168], aromatic N-acyl peptide esters [169-172], and per-methylated N-acyl peptides [173]. The principal modes of the electron impact induced fragmentation of these peptide derivatives are well established and have been summarised in recent reviews [174,175]. Although the spectra of the permethylated derivatives [176] are perhaps the simplest and easiest to interpret and are now frequently used, the N-acyl peptide esters have been widely and successfully employed. 

A partial list of xenoblotlca that form bound residues In plant tissues Is presented on Table Vlll. Many heterocyclic and aromatic... 

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