Heterocyclic and polycyclic

Table 5.5 Chemical shifts and couplings in some common heterocyclic and polycyclic aromatic systems.

To summarize the amides are most suitable for the formation, by ortholithiation, of condensed heterocycles and polycyclic aromatics (in which subsequent rings are formed by intramolecular attack on the amide group). In other cases the removal of the amide group may be problematic, though if carboxylic acids, aldehydes or hydroxymethyl-substituted compounds are required, alternative amide substituents may be used. 

It is now evident that it is desirable to divide five-membered meso-ionic heterocycles into two general classes type A and type B. This review is mainly concerned with the presently known 44 members of type A (Table I) and eight members of type B (Table II). Brief reference is made at the end of the review to various six-membered heterocycles and polycyclic systems that have been described as meso-ionic, but it is now firmly proposed that this practice should be discontinued. 

S. Hanessian and R. Ldger, Expedient assembly of carbocyclic, heterocyclic, and polycyclic compounds by Me3Sn radical mediated carbocyclizations of dienes and trienes A novel oxidative cleavage of the C-Sn bond, J. Am. Chem. Soc. 7)4 3115 (1992). 

Sharma, R. K., Chan, W. G., Seeman, J. I., and Hajaligol, M. R. (2003). Formation of low molecular weight heterocycles and polycyclic aromatic compounds (PACs) in the pyrolysis of a-amino acids. J. Anal. Appl. Pyrolysis 66, 97-121. 

Carbocycles, heterocycles and polycyclic arenes can serve as carbene ligands for the synthesis of complexes with benzannulated arenes (equation 133) . ... 

Solution The criterion for aromaticity in a compound is the satisfaction of Hiickel s rule, which states that "only monocyclic carbocycles containing 4n + 2 tt electrons (where n is an integer) in a closed, planar, conjugated circuit are capable of enhanced thermodynamic stability." Actually, this rule can also be applied to heterocycles and polycyclic carbocycles. For instance, naphthalene,... 

Chemical Shifts of Selected Heterocyclic and Polycyclic Aromatic Compounds... 

Synthesis of Heterocyclic and Polycyclic Compounds by Cycloaddition Reactions... 

---