STRUCTURE OF SMALL HETEROCYCLIC RINGS

STRUCTURE OF SMALL HETEROCYCLIC RINGS 5.01.2.1 Known Small Heterocycles... 

Figure 3 Structures and names of small heterocyclic rings with exocyclic double bonds.

Cyclic metalloimines, although they are outside the limits of the topic we are considering, are readily polymerized with opening of the ring [38]. The rings of ethylimine and its N-substituted derivatives are often used for this purpose. Complexation significantly alters the electronic structure of small heterocycles. Their structure can be represented as shown in 56. 

Structure of Small and Large Rings Table 2 Structures of Known Four-membered Heterocyclic Compounds... 

Limited knowledge exists with respect to detailed molecular structures of simple pyrans and heteropyrans. The semiempirical CNDO/2 calculations, using a full geometry optimization procedure, predict 4 and 5 to exhibit small deviations of the heterocyclic rings from the planar conformations.34 Unfortunately, similar calculated data for thiopyrans 6, 7, and 9 were not presented.45 Accounts of several X-ray diffraction investigations on compounds possessing pyran or thiopyran fragments are available.46 51... 

Trimethylene oxide, the simplest oxetane, was first prepared by Reboul in 1878. ° Subsequent analysis of the structure of this heterocycle revealed some interesting features. Thus, in contrast to what is observed with cyclobutanes, the oxetane ring is less puckered (Figure 13.1). The replacement of a methylene unit by an oxygen reduces the otherwise unfavorable eclipsing interactions that are minimized by out-of-plane distortion in the cyclobutanes. - As small saturated heterocycles, oxetanes display chemical as well as physical characteristics whose origins can be traced to then-... 

Lewars EG (1984) Structure of small and large rings. In Katritzky AR, Rees CW, Lwowski W (eds) Comprehensive heterocyclic chemistry, vol 7. Pergamon, New York, pp 95-130... 

Azetidine ring is an important structure because it is present in many compounds of pharmaceutical interest however, its manipulation must be done very carefully owing to the reactivity of these heterocycles of small size. An interesting application of the use ofbiocatalytic processes is the resolution of azetidine esters (Scheme 7.11). The procedure to choose for the resolution of these compounds is the enzymatic ammonolysis of the corresponding N-substituted azetidines [26]. 

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