Four-membered heterocycles with two

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). 

The most common structural motif of aluminum and gallium hydrazides is the centrosymmetric four-membered heterocycle with two exocyclic N—N bonds. The N-N distances of all these compounds fall in a very narrow range of about 145.5 pm on average with the most significant deviation (143.7 pm) detected for the diphenylhydrazido gallium derivative 14. The E2N2 heterocycles are planar with almost ideally equidistant E-N separations. As expected, the Al—N bonds of these compounds (196 to 199 pm) are shorter than the Ga—bonds (199 to 207 pm). 

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. 

The only four-membered heterocycles with two endocyclic double bonds are the highly reactive azetes approaches to their synthesis are discussed in CHEC 5.09.5.3.1 and CHEC-II 1.18.5.6. 

Several X-ray diffraction studies on four-membered heterocycles with two nitrogen atoms have been reported. In CHEC-II(1996) an account of various X-ray studies has been described <1996CHEC-II(1B)911>. 

Synthesis of four-membered heterocycles with two nitrogen atoms by C-C bond formation was reported in the early 1990s via a cesium fluoride-mediated cyclization of azine 236 (Scheme 36) <1991IC535>. 

This is the most successful method, which is widely used in the synthesis of the four-membered heterocycles with two nitrogen atoms. 

Four-membered heterocycles with two nitrogen atoms are widely used in research and industry. Numerous examples of 1,3-diazetidines are extensively used in the polymer industries. 

---