Five-membered heterocycles with O and

Five-membered heterocycles with O and S (Se, Te) atoms 97PF1C192, 98PHC195, 99PHC200. 

Insertion of a C=0, C=S or S=0 group between an amino and a hydroxy function of a 1,2-aminoalcohol produces a five-membered heterocycle with O and N as ring heteroatoms linked by -CO-, -CS- and -SO-groups. Although the reaction proceeds in two steps, it can often be carried out as a one-pot process. 

Despite the fact that the history of hetarynes is older than that of the benzynes (cf. 2), physical data on these compounds are scarce. Numerous trapping experiments furnished evidence for the formation of brradicaloid intermediates in the field of five-membered heterocycles (didehydrofurans, -thiophenes, and -pyrroles)." Direct spectroscopic data on these species, however, do not exist, which may be attributable to the increased ring strain in the five-membered o-arynes, associated with a strong tendency to undergo ring-opening reactions. 

Five-Membered Heterocycles with Vicinal Te and O Heteroatoms... 

The second chapter by I. D. Sadekov, G. M. Abakarov, and V. I. Minkin of Rostov State University (Russia) is entitled Five-Membered Heterocycles with Vicinal Te and O Heteroatoms, a relatively new field of chemistry, which has undergone rapid expansion in the last two decades. 

The number of five-membered heterocycles with four hetero atoms is limited to those with nitrogen atoms. Thus, the tetrazole acyclonucleo-sides were formed from sugars N,N -diphenylformazanes by the action of AT-bromosuccinimide or lead tetraacetate [133,134]. The synthesis of the tetra-O-benzoyl-D-lyxononitrile and its conversion to (o-lyxo-tetritol-l-yl)-tetrazole derivatives upon reaction with sodium azide was reported [133, 134]. A pseudo C-nucleoside including a tetrazole ring was achieved by conversion of 3- 0-benzyl- 1,2-O-isopropylidene-D-ribo-pentodialdehydo-1,4-furanose to the respective nitrile and then reaction with sodiiun azide [86]. 

Five-membered heterocycles with vicinal Te and O heteroatoms 06AHC(92)55. 

A theoretical, comparative study of the tautomerism of 56 five-membered heterocyclic rings announced in (76AHC(Sl)l) has appeared (81MI40402). The stabilities of the three forms for 5-pyrazolones, 5-pyrazolethiones and 5-aminopyrazoles have been calculated by a simple Hiickel o) iterative method. The relative energies and the substituent and solvent effects are in agreement with the experimental results. 

With X = RN the C-1 electrophilic center is most active, and five-membered heterocycles 131 are generated (69ZOR1179). With X = O there is a competition between C-1 and C-3 electrophilic centers, the role of the latter increasing on going to sulfur analogs (X = S) in accord with a decrease in the - -M effect in this series (81UK1252). 

In the first chapter, D. S. Donald and O. W. Webster summarize much fundamental heterocyclic chemistry dealing with the preparation of heterocycles from hydrogen cyanide and its derivatives, mostly previously available only in the patent literature. In the second, the account of the ringopening of five-membered heteroaromatic anions by T. L. Gilchrist brings together the numerous transformations that can succeed the removal of a proton from a carbon atom in a five-membered heterocyclic ring. 

In Table 32 the v(C = O) and other characteristic bands are given for some saturated five-membered heterocycles, and compared with the corresponding absorption frequencies for cyclopentanone. Adjacent NH groups and sulfur atoms have the expected bathochromic effect on v(C = O), whereas an adjacent oxygen atom acts in the reverse direction. The CH2 vibrations of cyclopentanone are repeated to a considerable extent in the heterocyclic analogues. 

The reactivity of these five-membered systems with nucleophiles has received note. Zatsepina and Tupitsin and their co-workers recorded the use of deuterium exchange rates in methyl groups of five-membered heterocycles in ethanolic KOEt as a source of o values, with a p value of 4.7.271 Spinelli and his co-workers revealed that even for tri- and tetrasubstituted five-membered heterocycles the Hammett equation employing er values where appropriate gives good correlations. The systems studied are 32-37, in all cases the slow step... 

Abstract This chapter is devoted to recent progress in the chemistry of the 5 5 fused heterocyclic systems. There are four possible modes of 5 5 fusions of the simple five-membered heterocycles leading to four structures containing one heteroatom in each ring. The heteroatoms may be the same or different and may be O, NH, S, Se, Te, P, As, or Sb. The fully conjugated hetero analogs of pentalene dianion have a central C-C bond and are isoelectronic with the 10-7t-electron pentalene dianion. The scope of the chapter is outlined with a survey of various structural types and nomenclature of the parent compounds and their derivatives. New synthetic procedures and synthetic applications of title compounds are presented. This review has concentrated on the new developments achieved from 1997 to September 2007. 

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