Reactions of heterocycles

Takamizawa et al. developed a general ring-expansion reaction of heterocycles that, applied to thiazolium salts, yields 1,4-thiazines (496, 497) thiamine (220) reacts with dialkyl acylphosphonates (221) to give the tricyclic 1,4-thiazine (222) (498), which is easily hydrolyzed to dihydro-1,4-thiazinone (223) (499) (Scheme 106). In the case of thiazolium slats containing no functional groups (224), 1,4-thiazine derivatives (226) were directly obtained in fairly good yields (Scheme 107). 

In this section formation of pyrazoles from the reaction of heterocycles with compounds other than hydrazines will be discussed. 

The structures of chalcogen-nitrogen compounds are frequently unpredictable. For example, the reactions of heterocyclic systems often result in substantial reorganization of their structural frameworks, e.g. ring expansion or contraction. The formation of acyclic products from ring systems (or vice versa) is also observed. 

V. Aldol Reactions of Heterocyclic Enamines and Their Importance for the... 

The double bonds of either enamines or their salts readily undergo many reactions. We shall divide the reactions of heterocyclic enamines on the basis of the mechanism involved. 

Reactions of Heterocyclic Enamines with Other Electrophilic Reagents... 

From the preparative point of view, reactions of heterocyclic aromatic compounds with nucleophilic reagents are very important, especially the reactions of their quaternary salts containing a formal enamine grouping in the molecule. 

These compounds show the typical reactions of heterocyclic N-oxides and their structure was proved by methylation which takes place on N-1. Quinazoline 3-oxide is soluble in water and melts at 155°C. It has basic properties and its pKa value in water is 1.47. ... 

Modern concepts have been extended to the chemistry of heterocyclic compounds more slowly than to the chemistry of aromatic and aliphatic systems, but efforts are now being made to classify and explain the properties and reactions of heterocyclic compounds in terms of these newer ideas (cf. reference 11). However, many of the most important heterocyclic compounds are potentially tautomeric, and elucidation of their tautomeric composition must precede a logical treatment of their properties. Further, many natural products such as the nucleic acids and alkaloids contain potentially tautomeric groups and information of this type is needed for a detailed explanation of th reactions which they undergo,... 

Identification of the product(s) resulting from the reaction of heterocyclic compounds with diazomethane has been used in attempts to elucidate their tautomeric composition (for summaries, see references 7 and 41). This work was based on the assumption that if a compound which is capable of existing in both an —NH and an —OH form produced only the =NMe derivative when it w as treated with diazomethane, it existed entirely in the =NH form. On the other hand, formation of the —OMe derivative was interpreted to mean that a finite amount of the compound existed in the —OH form. In some cases the tautomer present in the solid state w as concluded to be different from that present in solution for example, 41 42 gave a higher proportion of the 3,4-dimethoxy derivative when ethereal diaz-... 

The reactions of carbenes, which are apparently unique in displaying electrophilic character in strongly basic solutions, include substitution, addition to multiple bonds, and co-ordination with lone pairs of electrons to form unstable ylides. This last reaction is of obvious relevance to a consideration of the reactions of heterocyclic compounds with carbenes and will be summarized. 

Prior to 1960 little work had been done on reactions of heterocyclic compounds involving hetarynes, i.e. intermediates with a triple bond in the nucleus containing the hetero atom. Since then interest in hetarynes has grown and investigations in this area are developing rapidly using information available from carbocyclic aryne chemistry. Therefore, a short survey of the chemistry of arynes is presented before summarizing typical problems encountered in hetaryne chemistry. 

The Reactions of Heterocyclic Compounds with Carbenes C. W. Rees and C. E. Smithen... 

Use of Lewis acids in free-radical reactions of heterocycles and heterocycliza-tions 98AG(E)2562. 

Control of chemical activity and selectivity by catalytic antibodies in reactions of heterocycles 95MI11. 

Microwave activation of reactions of heterocycles 98S1213, 99T10851. 

Reactions of heterocycles under high pressure 97LA623, 97T2669, 98H(47)1135. 

Recyclization reactions of heterocycles with participation of malononitrile and its derivatives 99UK45. 

Regioselectivity in reactions of heterocyclic anions of allyl-type with electrophiles 99CRV665. 

Stereoselectivity of chiral homoenolate equivalents in reactions of heterocycles 99S365. 

Reactions of heterocycles in aqueous media with participation of indium compounds as reagents or catalysts 99T11149. 

Recent advances in Cr(II)- and Cr(irt)-mediated reactions of heterocycles 99S1. 

Reactions of heterocycles with fluorinated organometallic compounds 97MI31. 

Tandem reactions of heterocycles combining Diels-Alder reactions with sig-matropic rearrangements 98S227. 

The reactions of heterocyclic 1,2-dicarbonyl compounds with diazomethane, which were studied by Eistert and his co-workers in... 

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