Heterocycles with four heteroatoms

Hetero Diels-Alder reactions of chiral 1-aryl-1,2-diaza-1,3-butadienes 809, derived from acyclic carbohydrates, with diethyl azodicarboxylate 810 in dry benzene 

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Table 3 Linearly fused (5 5 5) heterocycles with four heteroatoms...

Five-Membered Heterocycles with Four Heteroatoms... 

D. Heterocycles with Four Heteroatoms in the Ring 1. Tetrazoles... 

Five-membered heterocycles with four heteroatoms are also known. Apart from tetrazole, there are four structurally isomeric oxatriazoles and thiatriazoles ... 

Synthesis of Five-Membered Heterocycles with Four Heteroatoms 4.1.4.1 Tetrazoles... 

The fusion of four membered-heterocycles with two heteroatoms onto face a of the quinoline offers two isomeric combinations of the tricyclic ring system l,2-heterocyclo[2,3-a]quinoline and l,3-heterocyclo[3,2-a]quinoline. The later one of these two ring systems is the only one that examples of it namely, l,3-thiazeto[3,2-a]quinoline have been reported. Moreover, examples of those fused on faces ij or j are not known (Fig. 3). 

STRUCTURE OF FOUR-MEMBERED HETEROCYCLES WITH TWO HETEROATOMS... 

This method is by far the most successful and most widely used in the synthesis of four-membered heterocycles with two heteroatoms. In principle these formal additions can be envisioned to occur by (a) two displacements, (b) thermal concerted [2 + 2] cycloadditions, (c) photoinduced [2 + 2] additions, (d) stepwise zwitterionic intermediates or dipolar reactants, and (e) radical intermediates. In some cases the mechanisms have been elucidated but in many others the actual mode of addition is not known. For this reason, no attempt has been made to formally divide the latter four (b-e) into categories and only the first [(a), two displacement reactions] is covered separately. 

All four-membered heterocycles with three heteroatoms can be classified into four types compounds with three similar heteroatoms 1 symmetrical 2, and unsymmetrical 3 heterocycles with two different heteroatoms and derivatives with three different heteroatoms 4 <1996CHEC-II(1B)1189>. 

All four-membered heterocycles with three heteroatoms described in this work belong to the ring systems shown in... 

No single chapter in the first edition of Comprehensive Heterocyclic Chemistry (CHEC-I) was devoted to 6,6-fused heterocycles containing four heteroatoms arranged 1 3 with no heteroatoms occupying the bridgehead position. Indeed, the ring index of that edition only contained one example and subsequent reviews have referred to just one other <93PHC256>. This chapter describes such 6,6-... 

Section 3.1 is a brief overview Section 3.2 deals with six-membered heterocycles, including those with more than one heteroatom in the ring Section 3.3 deals with five-membered heterocycles with one heteroatom Section 3.4 deals with five-membered heterocycles with more than one heteroatom in the ring Section 3.5 covers small (three- and four-membered) and large (>six) ring heterocycles. 

Semi-empirical and molecular mechanics calculations have been widely used in the study of eight-membered heterocycles with three heteroatoms. Theoretical studies have centered on conformations of this class of heterocines, which usually belong to four low-energy conformational families classified as boat-chair, crown, boat, and chair forms (see Section 14.08.4.2). 

The importance of five-membered heterocycles with one heteroatom, of the benzo and dibenzo condensed systems and of the partially or completely reduced compounds as natural products, pharmaceuticals, and starting materials or auxiliaries for syntheses is much greater than for three- or four-membered heterocycles, apart from oxirane. 

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