In heterocyclic synthesis

Table 1 lists some of the common binucleophiles utilized in heterocyclic synthesis, the numerical prefixes referring to the relative positions of the nucleophilic centers to each other. Higher order binucleophiles, e.g. 1,5-systems, come readily to mind and the above illustrative examples rapidly increase in scope when the incorporation of these structural elements into heterocyclic systems is considered. This last group offers many opportunities for ring annulations. 

Small unsaturated rings are usually very reactive undergoing ring opening in a number of ways, and this characteristic has been utilized in heterocyclic synthesis. In their role as dienophiles or dipolarophiles, the initial cycloaddition is usually followed by a valence tautomerism resulting in a six-membered or larger ring system. Several examples exist, however, where this does not occur, and these are described below. 

An interesting application of a phosphorus ylide in heterocyclic synthesis is in a ring annulation. The diazopyrazole (592) when treated with various phosphorus ylides gave the 3//-pyrazolo[5,l-c][l,2,4]triazole derivatives (593) with elimination of triphenylphosphine (79TL1567). 

Phosphonium hexafluorophosphate, benzotriazolyl-N-hydroxytris(dimethylamino)-in peptide synthesis, 5, 728 Phosphonium salts chromene synthesis from, 3, 753 reactions, 1, 531 Phosphonium salts, vinyl-in pyrrole synthesis, 4, 343 Phosphonium ylides in heterocyclic synthesis, 5, 165 Phosphoramide, triethylene-as pharmaceutical, 1, 157 Phosphoramide, triethylenethio-as pharmaceutical, 1, 157 Phosphorane, pentaphenyl-synthesis, 1, 532 Phosphoranes, 1, 527-537 Berry pseudorotation, 1, 529 bonding, 1, 528... 

Iminophosphoranes versatile tools in heterocyclic synthesis 95AHC(64)159. 

In the first chapter, N. M. Ahmad and J. J. Li (Pfizer, Ann Arbor, USA) discuss the use of palladium in quinoline synthesis, thus filling an important gap in a recent monograph on the uses of palladium catalysis in heterocyclic synthesis authored by the same group. This is followed by an account of pyrimidine-pyridine interconversions by H. C. van der Plas (Wageningen University, The Netherlands) the immense variety of heterocyclic chemistry is illustrated by the large number of diverse strategies for such transformations. 

Dipolar cycloaddition of azides with olefins provides a convenient access to triazolines, cyclic imines, and aziridines and hence is a valuable technique in heterocyclic synthesis. For instance, tricyclic -lactams 273 - 276 have been synthesized using the intramolecular azide-olefin cycloaddition (lAOC) methodology (Scheme 30) [71]. 

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