Tandem amination catalysis in the synthesis of diazaphenoxazine compounds of pharmaceutical interest

Thesis Overview

<p> </p><p><strong>1.0 &nbsp; INTRODUCTION</strong></p><p>Phenoxazine<strong>(1)</strong>&nbsp;is a compound analogous in structure to phenothiazine<strong>(2)</strong>&nbsp;with</p><p>oxygen in place of sulphur.Its other systematic names are 10H-phenoxazine and 2,2,5,6-dibenzo-1,4-oxazine1.They are tricyclic nitrogen-oxygen heterocycles2. Owing to the wide range of application of phenoxazine compounds, the synthesis of their derivatives and isolation of the natural phenoxazines have been a subject of great interest over the years3. Phenoxazine compounds have a wide range of applications, particularly as drugs and dyes.</p><p>&nbsp;</p><p>The naturally occurring phenoxazine derivatives have been classified as <em>Ommochromes</em>, fungal metabolites, <em>Questiomycins</em>&nbsp;and <em>Actinomycins4.</em></p><p>Phenoxazines are generally grouped into linear phenoxazines and angular phenoxazines. The linear phenoxazine, as the name implies, has a linear arrangement of rings like compounds <strong>3</strong>&nbsp;and <strong>4</strong>&nbsp;below.</p><p>&nbsp;</p><p>&nbsp;</p><p>&nbsp;</p><p>Angular phenoxazines have their skeleton extended by adding fused benzene rings to a, c, h or j faces such as compounds <strong>5</strong>, <strong>6</strong>, <strong>7</strong>, and <strong>8</strong>&nbsp;below.</p><p>&nbsp;</p><p>&nbsp;</p><p>&nbsp;</p><p>&nbsp;</p><p>There are still other structural arrangements with additional annular nitrogen atom(s). Where additional one, two or three nitrogen atom(s) are added, they are known as monoaza, diaza or triaza analogues respectively. Examples are below;</p><p>&nbsp;</p><p><strong>TANDEM CATALYSIS</strong></p><p>Tandem catalysis is the application of transition metal complexes as catalyst. Tandem catalysis has led to the development of simple and efficient methods of carbon-carbon and carbon-heteroatom bond formation5. Transition metal catalysed reactions have been used extensively in both ring synthesis andfunctionalisation of heterocycles6. As well as completely new modes of reactivity, variants of older synthetic methods have been developed using the milder and more selective processes that involve the use of transition metal catalysts7.</p><p>&nbsp;</p><p>Although there are many processes catalyzed by a range of transition metals, palladium-catalyzed processes vastly outnumber the others such as Ni, Rh, Cu, Fe.8</p><p>The rate, yield and scope of palladium-catalyzed cross-coupling reactions are influenced by both the reaction parameters and the choice of ligands, allowing for a wide substrate scope through judicious choice of these parameters8.</p><p>&nbsp;</p><p>The naturally occurring phenoxazine derivatives are numerous9. The<em>Ommochromes</em>&nbsp;such as <em>xanthommatin</em><strong>(9)</strong>&nbsp;are acidic pigments found in different arthropods and are responsible for the colouration in the wings, cuticle and eyes of insects10.</p><p>&nbsp;</p><p>&nbsp;</p><p>&nbsp;</p><p>Some fungal metabolites derived from phenoxazine ring have been isolated from various wood-rotting fungi and from moulds. The colouration in these organisms has been attributed to these phenoxazine derivatives of type <strong>10</strong>11-13.</p><p>&nbsp;</p><p>With the exception of the <em>actinomycin</em>&nbsp;antibiotics which have antibacterial and anti-tumor activities, these naturally occurring phenoxazines are notparticularly useful compounds.</p><p>Interest in the naturally occurring phenoxazines &nbsp; has declined considerably giving way to a systematic synthesis of phenoxazineanalogs modeled after the biologically active azaphenothiazines14.</p><p>Following reportson the pharmacological activities of phenoxazine, attention was diverted form their dyeing properties to a study of their biological activities.15-17 From tests carried out in laboratory animals and in man, it was found that many phenoxazine derivatives showed pronounced pharmacological activities as CNS depressants18, sedatives19, antiepileptics20, tranquilizers21, antituberculosis22, anti- tumour23, antibacterial24, spasmolytic25, anthelminthic26 and parasiticidal agents27.</p><p><strong>1.2 &nbsp; &nbsp; STATEMENT OF THE PROBLEM</strong></p><p>It was interest to ascertain if tandem catalysis procedures could be extended to C- substituted side chain derivatives of diazaphenoxazine. The extension would validate the generality of optimized tandem molecular amination protocols in terms of substrate scope.</p><p>To the best of our knowledge, there is practically no report of the synthesis of side chain amino derivatives of diazaphenoxazine through tandem catalysis.Thus, a concise application of tandem methodologies would allow access to five hitherto unknown heterocyclic scaffolds of the diazaphenoxazine type shown below.</p><p>&nbsp;</p><p>&nbsp;</p><p><strong>1.3 &nbsp; &nbsp; OBJECTIVES OF THE STUDY</strong></p><p>The specific objectives of this study were to:</p><p>(i) Synthesize the key intermediate, 3- chloro-1 9- diazaphenoxazine.</p><p>(ii) Prepare single crystals of this intermediate for X-ray analysis.</p><p>(iii) Transfrom 3-chloro-1,9- diazaphenoxazine to varions 3- substituted aminoheterocyclic derivatives via Buchwald – Hartwig tandem amination protocol.</p><p>(iv) Characterize the new derivatives using spectroscopic techniques.</p><p>&nbsp;</p><p><strong>1.4 &nbsp; JUSTIFICATION OF THE STUDY</strong></p><p>Among the motivations to carry out the research work is the wide range of application of phenoxazine derivatives18-27 and the need to synthesize new derivatives which may have better and desirable properties.</p><p>&nbsp;</p> <br><p></p>