Nomenclature ring strain

Structures and nomenclature for the most important five-membered monocycles with one or more heteroatoms are depicted in Scheme 1. The aromaticity scale in five-membered heterocycles has been long debated.97-101 The decreasing order of aromaticity based on various criteria is (DRE values in kcal/ mol) benzene (22.6) > thiophene (6.5) > selenophene > pyrrole (5.3) > tellurophene > fur an (4.3). Pyrrole and furan have comparable ring strains (Scheme 38). The aromaticity of furan is still controversial 100 the NMR shielding by ring current estimated it at about 60% of the aromaticity of benzene, and other methods reviewed earlier102 estimated it at less than 20%. 

Such compounds are of use in the study of ring strain and also in synthetic approaches to starting materials for more complex systems. Several review articles have highlighted this77. The ring systems formed by these reactions are generally quite complex. In order to classify the reactions, a simple approach has been adopted. Not all the complexity is described in this nomenclature and only the atoms involved in the ring system formed are included. 

We observed that cyclic esters (lactones) may be formed when the carboxyl electrophile and hydroxyl nucleophile are in the same molecule (see Section 7.9.1). Similarly, cyclic amides are produced when carboxyl and amine groups are in the same molecule, and are again most favoured when this results in the generation of strain-free five- or six-membered rings. Cyclic esters are termed lactones, whereas cyclic amides are in turn called lactams. The nomenclature of lactams is similar... 

So what can we conclude about the stability of polyprismanes from strain calculations We carried out the above illustrative calculations on [3,4]prismane. Here are resrrlts of the homodesmotic strain calculations of Minyaev et al. [1] at the B3LYP/6-311G(2df,p) level [stack height, ring size], nomenclature of Fig. 12.1, SE and SE/C-C, in kJ moU ... 

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