Rings s. a. Heterocyclics

N-condensed, by double ring closure 17, 445 —, 2-imino- 16, 508 0-Heterocyclics (s. a. iert Amino-O-heterocyclics)... 

Pschorr ring closure 15, 561 Pseudoacids s. Hydroxylactones Pseudoaromatic rings s. Di-thiylium salts. Metal complex compounds, ar., Tropenium salts Pseudoazulenes 15, 468 N-Pseudoazulenes 14, 791 0-Pseudoazulenes 14, 338 Pseudobase adducts and their reactions 13, 584 —, dehydrogenation 14, 769 —, p-methylation of quinolines via — 14, 769 Pseudobases 11, 323 Pseudobases, N-heterocyclic (s. a. 0-Alkylpseudobases, N-heterocyclic) 15, 304... 

Tire second chapter deals with rings systems which comprise two or more fused five- or six-membered heterocyclic rings. It is authored by Dr. I. Shcherbakova of NFS Pharmaceuticals, Salt Lake City, U.S.A., together with Drs. Elguero and Katritzky. 

This bistability is favoured by structures, which will adopt n—stacked motifs, i.e. lamellar molecules. In addition the tendency to adopt the planar structure is enforced by the presence of electronegative heteroatoms (currently restricted to N), which lead to a propensity of in-plane electrostatic S- N contacts between rings. These may comprise heterocyclic S- N contacts or S- N contacts to other functional groups such as the pyrazine or cyano nitrogen atoms. 

Most of the (5,5) (2N2)-fused heterocyclic systems are fully substituted aromatic systems and therefore they do not have any hydrogens attached to the ring. Very little is reported on C-unsubstituted compounds. Methine groups are generally part of a ring azomethine moiety and are either linked to a fusion atom C or N or to a nonfusion atom N or S (Table 1). 

Fluorine and chlorine are the only important choices for the labile substituents on a heterocyclic reactive system but numerous other leaving groups have been patented, mostly as substituent on an s-triazine ring. These include sulpho, cyano, thiocyanato, azido and trichloromethyl [26], as well as more elaborate groupings (Figure 7.2). Such derivatives are... 

A wide variety of cyclic S-N compounds containing two or three coordinate sulfur is known (i ), The binary S-N rings vary in size from four (S2N2) to ten (SsNs" ") atoms and examples of anions (SsNs") and cations (S3N2 , S N3 , S N ) are known in addition to neutral molecules (2). The excess of electrons in these planar, electron-rich heterocycles is often accommodated in low lying tt orbitals but, in some cases (e.g. S N, S Ns"), the number of tt electrons is reduced by formation of transannular S-S bonds to give cages ( , ). 

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