Subject sulfur heterocycles

The additions of thiyl radical to alkenes and alkynes can also occur intra-molecularly, thus leading to sulfur heterocycles. This subject has been explored by Surzur s group [32-33]. Based on steric and stereoelectronic factors, the Baldwin-Beckwith rules [34] predict the 5-exo ring closure as the favored pathway. Scheme 6 shows that the pent-4-enylthiyl radical, generated from the allyl sulfide, yields both five- and six-membered rings, in the ratio of 1 19. The predominance of the larger cycle, in contrast with the above-mentioned rules, is due to the reversibility of the reaction. The product distribution reflects a thermodynamic control. 

The reaction has been the subject of reviews,316,45 from which it will be seen that the reaction can be applied to synthesis of five- and six-membered oxygen-heterocycles, sulfur-heterocycles, bicyclic compounds, and compounds containing an additional heteroatom in the ring. 

The nature of the sulfur-nitrogen bond, and in particular the sulfur-nitrogen double bond in inorganic heterocycles, has been discussed, and the reader is referred to the two recent reviews on the subject (79AG(E)91, 8lAG(E)444). 

Similar reactions are known of compounds in which the carbon-nitrogen bond is part of a heterocyclic nucleus.17 18 The oxygen atom of the reactive system may be replaced by a sulfur atom, with, however, some reduction in the tendency toward rearrangement. Allyl p-tolyl sulfide rearranges (XI —> XII) to the extent of 27% (50% based on sulfide not recovered) when subjected to refluxing at 228-264° for four hours.19... 

N, S and 0 heterocyclic compounds, along with noncyclic sulfur compounds and hydrocarbons, are predominant in "meaty" flavor volatiles. The mechanisms of heterocyclic formation by Maillard and pyrolysis reactions have been reviewed by Vemin and Parkanyi (57) and the Maillard reaction itself is a recurring subject of review (58). Since other speakers contributing to this volume will discuss these aspects of meat flavor, they will not be repeated in this presentation. 

This chapter describes heterocyclic systems which are isoconjugate with the pentalenyl dianion (13) and which cannot be represented by classical Kekule structures. Molecules of this type (e.g. 9-12) can only be represented by dipolar structures, or possibly by structures involving tetracovalent sulfur atoms, and are described as conjugated mesomeric betaines or non-classical heteropentalenes. The structure and chemistry of this type of heterocycle have been the subject of two earlier reviews (77T3203, 77HC(30)317) and their classification as conjugated mesomeric betaines and relationship to other dipolar heterocycles has also been discussed (83UP43700). 

Abstract Synthesis methods of various C- and /V-nitroderivativcs of five-membered azoles - pyrazoles, imidazoles, 1,2,3-triazoles, 1,2,4-triazoles, oxazoles, oxadiazoles, isoxazoles, thiazoles, thiadiazoles, isothiazoles, selenazoles and tetrazoles - are summarized and critically discussed. The special attention focuses on the nitration reaction of azoles with nitric acid or sulfuric-nitric acid mixture, one of the main synthetic routes to nitroazoles. The nitration reactions with such nitrating agents as acetylnitrate, nitric acid/trifluoroacetic anhydride, nitrogen dioxide, nitrogen tetrox-ide, nitronium tetrafluoroborate, V-nitropicolinium tetrafluoroborate are reported. General information on the theory of electrophilic nitration of aromatic compounds is included in the chapter covering synthetic methods. The kinetics and mechanisms of nitration of five-membered azoles are considered. The nitroazole preparation from different cyclic systems or from aminoazoles or based on heterocyclization is the subject of wide speculation. The particular section is devoted to the chemistry of extraordinary class of nitroazoles - polynitroazoles. Vicarious nucleophilic substitution (VNS) reaction in nitroazoles is reviewed in detail. 

In this section, the reactivity of carbon- and heteroatom-containing substituents attached directly or indirectly to the heterocyclic ring of 1,3-dithiole derivatives is discussed. Here, chemical manipulations on these systems exclude ring carbon and sulfur atoms, which are the subject of Section 4.12.6. Special 1,3-dithioles called Jt-extended TTFs, containing two or more 1,3-dithiole moieties linked together via =C-(C=C) -C= or similar conjugated spacers, are discussed with typical TTFs in Section 4.12.11, although formally they constitute an example of functionalization of the 1,3-dithiole by 2rt-extended substituents, terminated by another 1,3-dithiole moiety. 

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