Other Six-Membered Heterocycles

11 Synthesis of a Particular Class of Compounds and a Critical Comparison of the Various Routes Available Pyrazoles, Indazoles, and Their Derivatives as Starting Materials for the Syntheses of Fused Ring Systems 

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The three possibilities of synthesizing a C5 chain, namely, C2 +Ci + C2. C2 -t C2 -I- Cl, or Cl + C3 + Cl, lead to pyrylium salts having identical substituents in positions 2 and 4 in the second case, or 2 and 6 in the first and third cases. Despite this limitation, such syntheses are very convenient because they make the pyrylium salts easily accessible (more so than other six-membered heterocyclic aromatics) from aliphatic starting materials. 

The Al—Bi heterocycle [Me2AlBi(Tms)2]3 43 [78] exhibits almost the same structural parameters as were found in other six-membered heterocycles of the type [R2AlE(Tms)2]3 (E = P, As, Sb). 

Waggoner, K. M. Hope, H. Power, P. P. Angew. Chem. Int. Ed. Engl. 1988, 27, 1699. Six-membered heterocycles [RMER ]3 have been subject to detailed preparative and theoretical studies since they are isoelectronic to borazine B3N3Hs. However, it was demonstrated that only B3P3-heterocycles exhibit a considerable delocalization of the 6 k-electrons, whereas the delocalization in other six-membered heterocycles of the desired type contributes very little to their stabilization. Power P. P. J. Organomet. Chem. 1990, 400, 49. 

There are two important general types of reaction by which six-membered heterocycles containing two or more heteroatoms can be transformed into other six-membered heterocyclic systems, namely reactions which involve an ANRORC mechanism, and reactions which proceed by a Diels-Alder/retro-Diels-Alder type of mechanism. Transformation of 1,3-oxazinones and -thiazinones into pyrimidones (equations 193 and 194) has been extensively used, especially in the conversion of isatoic anhydride into quinazolinones (e.g. equation 195). 2,4-Diaryl-l,2,3,5-oxathiadiazines, which are readily accessible by reaction of sulfur trioxide with aryl isocyanates, are useful precursors to pyrimidines and 1,3,5-triazines (equation 196). 

Other Six-Membered Heterocycles Including Acyclic Compounds 31... 

Meldrum s acid, 172 Tin(IV) chloride, 300 Containing two oxygens N-Bromosuccinimide, 49 Other six-membered heterocycles Diethoxytriphenylphosphorane, 109 Miscellaneous heterocycles of any ring size... 

Monocyclic 1,2,3-triazines 1 behave like other six-membered heterocycles as dienes in Diels-Alder reactions with inverse-electron demand. Substrates are electron-rich dienes and acetylenes. 11 103, 284-290 jn mosl caseSi the formation of the isolated products can be explained by an attack of the dienophile at N1 and C4 or N3 and C6 of 1. In a few cases, products were obtained, the formation of which had to be explained by an intermediate formation of an azacyclobutadiene (azete) or by an N2 and C5 attack.284,285... 

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