Heterocycles, reaction with literature reviews

The present review is intended to highlight the use of acetylenic esters in the synthesis of different heterocycles, primarily involving nucleophilic additions. The reactions of heterocyclic bases with acetylenic esters, however, will not be covered here, except in a few cases where it is more appropriate for inclusion here, depending on the type of reactions. The reactions of heterocyclic bases form the subject matter of a separate article. The survey covers the literature up to December 1973. 

Thermal (2 + 2)-cycloaddition reactions have never been reviewed so far, although occasionally a few reactions have been discussed in other review articles.20,21 The literature on this subject is summarized here in four subsections. First the mechanistic aspects of thermal (2 + 2)-cyclo-addition reactions are dealt with and subsequently a review is given of (2 + 2)-cycloadditions of heterocycles with olefins and compounds having other double bonds, with acetylenes, and with heterocumulenes. The reactions with acetylenes are discussed under two separate headings, covering (1) reactions with nonaromatic heterocycles and (2) reactions with heteroaromatics. The reactions included are exclusively inter-molecular (2 + 2)-cycloadditions. No examples are known of intramolecular thermal (2 + 2)-cycloadditions of two isolated -electron systems or of thermal electrocyclizations of conjugated 4/r-electron systems of heterocyclic compounds (Appendix). 

This review is divided into sections depending on the quantity of sulfur atoms in the formed heterocycle. We have examined all the literature data for the last 150 years up to March 2007, including the latest achievements concerning 1,2,3-dithiazoles fused with other heterocycles. We do not discuss the Herz reaction since it is well known and has been reviewed (1957CRV1011). 

It is well known that sugars react with aqueous ammonia to produce heterocyclic compounds in low yield. The products of the reaction of the mono- and di-saccharides with concentrated, aqueous solutions of ammonia are dependent on three factors (I) the length of time during which the reaction proceeds, (2) the temperature of the reaction, and (3) the catalyst used. This matter has been briefly but comprehensively reviewed.1 The present article is more detailed it covers the literature to the end of July 1970, and has been prepared for the use of chemists who may not be specialists in the carbohydrate field, as well as of those who are. 

Heterocycle-forming polymerization reactions have been the most extensively reviewed of the three methods for obtaining heterocyclic polymers. A series of monographs are available that cover the literature through 1971 quite well (B-72MI11109). These monographs have served to provide an excellent foundation for our efforts. Whenever possible, we have cited more recent references, especially any reviews. It is the purpose of this section to depict succinctly the rich variety of chemistry that has been employed to form heterocyclic polymers of this type. This section, then, is a capsule summary of most of the heterocycles that have been generated concomitant with formation of a polymer. 

The preparation of pyrimidines and hydropyrimidines from thioureas is well established.139,217,218 Since the latest review (1962) covering this reaction,218 several reports of the preparation of heterocyclic compounds by previously reported procedures have appeared in the literature. These involve reactions of thioureas with a,/3-unsaturated ketones,219-224 jS-ketoesters,225-228 aliphatic ketones,229-231 j8-dicar-bonyl compounds,232 and ethyl ethoxymethylenecyanoacetate.233 Selenoureas have also been reported to react with /3-ketoesters to give the analogous 2-selenopyrimidines.234,235 Two reports have appeared of the cyclization of l-/3-carboxyethyl-2-thioureas to hexahydro-pyrimidines in low yields in the presence of acetic anhydride 238,237 however, tetrahydrothiazines are the predominant products in these reactions. 

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