Medium ring-sized heterocycles

Medium Ring Size Heterocycles Fused with Other Heterocyclic Systems 294... 

The intramolecular cyclization of hydroxy ketones and thioketals, performed under mild conditions, is often used to produce oxygen-containing medium-ring size heterocycles. Upon treatment with silver perchlorate... 

Green Synthetic Approaches for Medium Ring-Sized Heterocycles of Biological Interest... 

Synthesis of Medium Ring-Sized Heterocycles 294 6. Eluorous Synthesis 309... 

It is not surprising, therefore, that medium ring-sized heterocyclic compounds have attracted the focused attention of synthetic and medicinal chemists and the like [19]. However, because of enthalpic and entro-pic factors, medium ring-sized compoxmds are generally difficult to access. Most of the classical cyclization... 

GREEN SYNTHETIC APPROACHES EOR MEDIUM RING-SIZED HETEROCYCLES... 

TABLE 1 Representative Examples of Naturally Occurring and Pharmaceutically Important Benz-Annulated Medium Ring-Sized Heterocyclic Compounds... 

RCM reaction had major impact in the field of medium ring-sized heterocycle synthesis as demonstrated by numerous publications in this area [60,61]. However, the reaction also suffers from many of the drawbacks of common organic transformations, and thus has drawn attention for greener applications. Grubbs and coworkers developed a highly active water-soluble olefin metathesis catalyst of the type 99 (Figure 5) for this purpose [62]. 

In recent years, the use of heterogeneous catalysis has received useful applications in various organic transformations due to several advantages over conventional homogeneous catalysis. Several solid acid catalysts [108] such as Amberlyst [109], solid-supported fluoroboric acid [110], polyaniline sulfate [111], polystyrene-supported sulfonic acids [112], sulfated zirconia [113], and silica [114] in conjunction with other greener techniques such as MW, US, environmentally benign solvents, solvent-free conditions, and so on have been screened, evaluated, and compared with respect to yields, reaction time, reaction temperature, ease of purification, reusability, toxicity, and other hazards for sustained applications. Many of these efforts have found applications in the medium ring-sized heterocycles such as oxepines, azepines, diazepines, oxazepines, thiazepines, and so on. 

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