Sulfur-containing six-membered heterocycles

Space restrictions mean that the reactivity of multiheteroatom systems or fused systems where both rings are heterocyclic cannot be covered in this section - the reader is referred to the relevant CHEC volumes. Space again dictates that the chemistries of oxygen- and sulfur-containing six-membered heterocycles, and the chemistry of monocyclic six-membered heterocycles with more than one heteroatom, are only briefly indicated alongside the description of pyridine/quinoline/isoquinoline chemistry, but especially where these are not shown by the pyridine prototypes, but again the reader should study the CHEC volumes for a full discussion. The inclusion of an extra heteroatom in a six-membered system exaggerates the effect of the first and so often it is possible to predict properties by extrapolation however, the same is not true for the five-membered systems, so these heterocycles with more than one heteroatom are considered in detail and separately. 

Ring contraction of other sulfur-containing six-membered heterocyclic compounds... 

Thermochemical data, and in particular the enthalpies of formation of oxygen-and sulfur-containing six-membered heterocycles, provide essential information on the factors responsible for the contrasting behaviour (structural, conformational and reactivity) between these types of compounds. Understanding of the experimental thermochemical observations has required theoretical modeling to confirm the relative importance of the steric, electronic, electrostatic and stereo-electronic interactions that are responsible for the enthalpies of formation for 1,3- and 1,4- dioxanes, 1,3,5-trioxane and their... 

Calorimetric and computational study of sulfur-containing six-membered rings including saturated S- and O-heterocycles 05CSR347. 

Part 2 (Volumes 2 and 3) deals with mono- and poly-cyclic compounds with one or more six-membered heterocyclic ring, with nitrogen, oxygen and sulfur as the heteroatoms. Volume 2 contains the six-membered rings with one nitrogen atom (Part 2A) and Volume... 

Ethylene disulfonyl-1,3-butadiene (43) is an example of an outer-ring diene with a non-aromatic six-membered heterocyclic ring containing sulfur. It is prepared by thermolysis of sulfolenes in the presence of a basic catalyst. It is very reactive [43] and even though it is electron-deficient, it readily reacted with both electron-rich and electron-poor dienophiles (Equation 2.15). 

Romers, C., Geometry and Conformational Properties of Some Five- and Six-Membered Heterocyclic Compounds Containing Oxygen and Sulfur, 4, 39. 

The sole example of a six-membered heterocyclic ylide containing one sulfur is the tetrahydrothiapyran compound 152 which is prepared from BrCH2CH2SCH2CH2Br exactly as for the oxygen analogue 139 <67AG(E)81, 69CB1802>. 

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). 

Pyridine and Piperidine. These are six-membered heterocyclic rings containing one nitrogen. Piperidine (azine) is aromatic and undergoes aromatic hydroxylation. Piperidine functions as a secondary amine and undergoes hydroxylation with either glucuronic acid or sulfuric acid. 

Volume 63 of Advances in Heterocyclic Chemistry comprises five chapters, from five different countries. In Chapter 1,1. D. Sadekov and V. I. Minkin (Rostov, Russia) have provided the first comprehensive survey since 1984 of six-membered heterocycles containing a tellurium atom. These compounds, which show many differences to their sulfur and selenium analogs, are of potential importance for materials applications. 

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