INTRODUCTION: Any of a major class of organic chemical compounds characterized by the fact that some or all of the atoms in their molecules are joined in rings containing at least one atom of an element other than carbon. The cyclic part of heterocyclic indicates that at least one ring structure is present in such a compound, while the prefix hetero refers to the noncarbon atoms, or heteroatoms, in the ring. In their general structure, heterocyclic compounds resemble cyclic organic compounds that incorporate only carbon atoms in the rings but the presence of the heteroatoms gives heterocyclic compounds physical and chemical properties that are often quite distinct from those of their all-carbon-ring analogs.
General features of Heterocyclic Compounds: The most common heterocycles are those having five- or six-membered rings and containing heteroatoms of nitrogen (N), oxygen (O), or sulfur (S). The best known of the simple heterocyclic compounds are pyridine, pyrrole, furan, and thiophene. A molecule of pyridine contains a ring of six atoms-five carbon atoms and one nitrogen atom. Pyrrole, furan, and thiophene molecules each contain five-membered rings, composed of four atoms of carbon and one atom of nitrogen, oxygen, or sulfur, respectively 1.
Another example , ocatanol 9 can be used to mimic the amphiphilic nature of lipid, because it has a polar head group ( primary alcohal) and a long hydrocarbon chain as tail, such as that oh fatty acids which make up part of a lipid membrane. Armed with this understanding, the organic chemist can ‘tailor’ a structure to meet a particular need by modifying the heterocyclic component.
Heterocyclic compunds are also finding an increasing use as intermediate in organic synthesis[10-12]. Very often this is because a relatively stable ring system can be carried through a number of synthetic steps and then cleaved at the required stage in a synthesis to reveal other functional groups. For e.g. 4-chloro-5(4H)-oxazolones are useful intermediates 13 in organic synthesis. In particular hydrolytic cleavage affords α-chloro-αacylaminoketones. More ever they are the logical intermediate to prepare 4-(phospho-ranylidene)-5(4H)-oxazolones, that are very important and useful ligands.
An important feature of the structure of many heterocyclic compunds is that it is possible to incorporate functional groups either as subsituents or as part of the ring itself. For example, basic nitrogen atoms can be incorporated both as amino substituents and as part of a ring. This means that the structures are particularly versatile as a means of providing, or of mimicking a functional group. For example the uses of tetrazole ring system as a mimic 8 of a carboxylic and functional group because of its similarity in acidity and in steric requirement. Tetrazole group is superior in terms of metabolic stability, bioavailibilty and four nitrogen atoms present in the tetrazole ring can create a greater charge distribution.
Synthesis of DOTA-Conjugated Multivalent Cyclic-RGD Peptide Dendrimers via 1,3-Dipolar Cycloaddition and their Biological Evaluation: Implications for Tumor Targeting and Tumor Imaging Purposes.
As described in the review, most hydroxamates have very lengthy synthetic route and therefore this review explored various synthetic route to hydroxamates and presents a wider literature for easy modifications of the existing hydroxamates so as to exploit their broad biological applications.
In the first instance, compound 190 was synthesized from D-leucine (189) following a modified literature  as presented in scheme 38.
Synthesis of Photocleavable Linear Macromonomers by ATRP and Star Macromonomers by a Tandem ATRP-Click Reaction: Precursors to Photodegradable Model Networks.
 In the synthesis of this class of compound, a building block 198 is first synthesized from chiral succinate (192) by condensation of the carboxylate with an appropriately protected hydroxylamine derivative followed by trans-esterification of the benzyl ester.
The bidentate Zn2+ chelating properties of the hydroxamic acid has the dual effect of a strong zinc coordination as well as expulsion of the nucleophilic water molecule from the active site, thereby preventing hydrolysis to occur.-terminal peptide hydroxamic acids are readily available through modified solid phase peptide synthesis (SPPS) protocols.
Removal of these protecting groups gave rise to the final hydroxamic acids (187 and 188) using 5% TFA/CH2Cl2.The synthesized bifunctional compounds showed an improved inhibitory activity against MMP-2 and MMP-14 as well as improved anti proliferative activity on A2780 ovarian cancer cell line.
The heterocyclic moiety was synthesized by the reaction of α-amino nitriles (163) with chlorosulphonyl isocyanate and hydrochloric acid to give hydantoin (164).
B., Synthesis of azide-alkyne fragments for click' chemical applications; formation of oligomers from orthogonally protected trialkylsilyl-propargyl azides and propargyl alcohols.
The new hydroxamic acid containing benzodiazepine (72) was synthesized and has demonstrated biological activity in MCF-7 and PC-3 tumor cell lines.