Heteroatoms in Each Ring

Compounds described in this section include compounds of general types 8.84-8.88, though studies have not yet been carried out on all of them. In addition, there are compounds with nitrogen at bridgeheads, with structures A-D being typical examples. 

Thienopyrazine, -pyridazine, -pyrimidine NH, Pyrrolopyrazine, -pyridazine, -pyrimidine 

For imidazoll,2-a]pyridine, two sets of data are shown (from different authors) and the values disagree both in magnitude and positional order (the same is true of the it densities) this reflects the lack of reliability of current MO methods. For pyrrolopyridines (azaindoles) (8.89), the 3-position is predicted to be the most reactive site in each case (83T2851). 

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Reports of bicyclic systems with one nitrogen atom containing two additional heteroatoms in each ring are scant in the literature. Fourteen such ring systems have been reported previously 1-14 <1996CHEC-II(8)733>. The literature examples available during the last decade include 1,2,4-triazino[3,4-Z ][ l,3,4]thiadiazine-4,7-dione 15, [l,2,4]tri-azino[3,4-/d [ 1,3,4]thiadiazinc 16, l,9a-dihydro-8//-l, 2,4-triazino-[3,4-Z ] [1,3,4] thiadiazine-4-oncs 17, [l,3,5]tria-... 

D. Systems with Two Heteroatoms 1. One Heteroatom in Each Ring... 

The structural diversity associated with bicyclic 5-5 fused heterocyclic systems containing two heteroatoms in each ring has been noted previously in both CHEC(1984) <1984CHEC(6)1027> and CHEC-II(1996) <1996CHEC-11(7)115>. In recent years this diversity has also been reflected in the number of applications that have been found for these compounds which include organic metals, molecular clips and receptors and molecular magnets. These fields will be examined in due course through this chapter. 

By far, the greatest interest in 5-5 heterocyclic systems containing two heteroatoms in each ring has been in the field of supramolecular chemistry and in the preparation of molecular receptors of various types. One of the most widely used heterocyclic backbones for these receptors has been glycoluril 5 and its derivatives. 

The aim of this chapter is to present the chemistry of five-five fused aromatic heterocyclic systems containing one heteroatom in each ring, represented by (la)-(ld), wherein X and Y may be the same or different heteroatoms and represent O, N, S and Se. 

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. 

Another chapter of CHEC-I includes 5 5 fused-ring systems with two or more heteroatoms <84CHEC-I(5)111>, although heterocycles with two nonbridgehead heteroatoms in each ring, which are the subject of this chapter, are only briefly considered there. 

This chapter covers bicyclic ring systems with one bridgehead N atom and with one extra heteroatom in each ring, and their benzo-fused derivatives. Of the 78 theoretically possible bicyclic systems, 33 are known, as are 53 benzo-fused derivatives. 

In the next two sections a-linked ring systems are designated by the heteroatom in each ring whenever appropriate. For example, the system derived from thiophene-pyrrole-thiophene is designated as SNS. Substitution on the nitrogen atoms is indicated appropriately [e.g., SN(Me)S]. 

In azole chemistry the total effect of the several heteroatoms in one ring approximates the superposition of their separate effects. It is found that pyrazole, imidazole and isoxazole undergo nitration and sulfonation about as readily as nitrobenzene thiazole and isothiazole react less readily ica. equal to m-dinitrobenzene), and oxadiazoles, thiadiazoles, triazoles, etc. with great difficulty. In each case, halogenation is easier than the corresponding nitration or sulfonation. Strong electron-donor substituents help the reaction. 

Compounds with one heteroatom in each heterocyclic ring... 

Compounds with two heteroatoms in each heterocyclic ring 1246... 

Compounds with one heteroatom in each heterocyclic ring 10.23.9.1.1(1) Synthesis of the heterocyclic ring... 

The heterocycles (la)-(ld) are heteropentalenes they have a central C—C bond, and are isoelectronic with the 107r pentalene dianion (2). They are formally derived by the fusion of two of the well-known simple heterocycles including furan, pyrrole, thiophene and selenophene. Considering the two rings as rings A and B, this entire group of five-five fused heterocycles with one heteroatom in each can be conveniently referred to as A,B-diheteropentalenes. 

The azoles (oxazole, imidazole, and thiazole) are five-membered aromatic heterocycles that have two heteroatoms in the ring. One of the heteroatoms in each of these heterocycles is an sp2-hybridized nitrogen that contributes one electron to the 6n aromatic system and has a basic nonbonded lone pair. The other heteroatom (oxygen, nitrogen, or sulfur) contributes two electrons to the 6n system. The imidazole skeleton is present in the amino acid histidine. The thiazole ring occurs in thiamin (vitamin B. ... 

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