Biological functions of natural and synthetic

Biological functions of natural and synthetic bioactive heterocycles... 

Biological Functions of Natural and Synthetic Bioactive Fleterocycles... 

Synthetic porphyrins have been used extensively as model systems for investigating the complex biological functions of natural porphyrin-containing systems. Given the capabilities of porphyrins to bind and release gases and to act as the active center in catalytic reactions in biological systems, porphyrin-... 

III. OCCURRENCE OF N-OXYGENATED C=N FUNCTIONALITIES IN NATURAL AND SYNTHETIC COMPOUNDS AND BIOLOGICAL ACTIVITY... 

Synthetic polysaccharides are prepared mainly to obtain models which may be helpful in establishing the biological functions of natural polysaccharides. It is desirable to find systems in which the polymerization of anhydro sugars proceeds stereospecifi-cally, i.e. to prepare uniform and well defined structures. 

Several methods have been developed to generate the transient highly reactive o-QMs. In addition to thermal and base initiation, some Lewis acids and transition-metal salts/complexes of Os, Rh, Ir, Mn, and Pd have been reported to mediate the generation of o-QMs. The resulting 2-aryl-chromans can be functionalized to provide the core structures of natural and synthetic compounds exhibiting a wide array of biological properties (Scheme 7.56). 

Most reports concerning 2//-pyran-2-ones (a-pyrones) involve non-fluorinated derivatives, which perform important biological functions in nature and have unlimited synthetic potential for the construction of a variety of arenes and heteroarenes [28], However, very few deal with 2-pyrones containing fluoroalkyl groups. It is evident that the C-2, C-4 and C-6 positions of the 2-pyranone ring are electrophilic in nature and prone to nucleophilic attack. The presence of polyfluoroalkyl substituents on the pyrone ring favours these reactions. At the same time, R -containing 2-pyrones behave as cyclic dienes in cycloadditions. 

A vast array of piperidine containing cores, both natural and synthetic, are of biological and medicinal interest. These heterocyclic scaffolds have been the subjects of considerable synthetic efforts, especially for the construction of optically active compounds. In this context, Khan et al. reported a catalytic bromodi-methylsulfonium bromide (BDMS) three-component reaction of 1,3-dicarbonyls with aromatic aldehydes and aromatic amines for a facile access to highly functionalized piperidines (Scheme 24) [104]. This strategy is an interesting illustration of... 

Since the start of the twentieth century, steroids have continued to be the focus of the research activities of natural product chemists, synthetic chemists, biochemists and clinicians. The reasons are several-fold and related to the fascination of the chemical complexity of sterols and their biochemical functions in living organisms. Sterols and steroids are excellent compounds for the organic chemists to practise their skills upon in the development of new reactions and synthetic procedures. The biological functions of sterols, for example as an essential constituent of membranes, have proved thought-provoking to lipid biochemists. 

A large number of macromolecules possess a pronounced amphiphilicity in every repeat unit. Typical examples are synthetic polymers like poly(l-vinylimidazole), poly(JV-isopropylacrylamide), poly(2-ethyl acrylic acid), poly(styrene sulfonate), poly(4-vinylpyridine), methylcellulose, etc. Some of them are shown in Fig. 23. In each repeat unit of such polymers there are hydrophilic (polar) and hydrophobic (nonpolar) atomic groups, which have different affinity to water or other polar solvents. Also, many of the important biopolymers (proteins, polysaccharides, phospholipids) are typical amphiphiles. Moreover, among the synthetic polymers, polyamphiphiles are very close to biological macromolecules in nature and behavior. In principle, they may provide useful analogs of proteins and are important for modeling some fundamental properties and sophisticated functions of biopolymers such as protein folding and enzymatic activity. 

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