Abstract
Alkaloids are a very important class of basic natural products containing at least one nitrogen atom, which are produced by a large variety of organisms including bacteria, fungi, plants, and animals. Most of the alkaloids have special biological activities and are also the main active ingredients of most herbal medicines, so they are important sources for drug discovery. However, the natural sources of these alkaloids are usually scarce and the isolation in large quantities is difficult, which limits the research of their bioactivities further. Thus, it is very important to develop efficient and economical synthetic routes for their synthesis and derivatization. In this thesis, we describe the efforts toward the synthesis of quinazolinone alkaloids and their analogues, and substituted imidazo[1,2-a]pyridines en route to imidazolealkaloids. Chapter 1 describes the studies toward pyrrolo-/ pyrido-quinazolinone alkaloids and their analogues. A brief introduction to the history and biological importance of vasicinone, luotoninA, and rutaecarpine has been provided in this chapter. The key tricyclic intermediates accessed from simple substrates and Dieckmann condensation form the lynchpin to assemble quite a few natural pyrrolo and pyrido quinazolinone alkaloids and their analogues. In Chapter 2, the synthetic studies toward the substituted imidazo[1,2-a]pyridine en routet o imidazole alkaloids have been described. We developed a facile entry to 3-aryl/ alkenyl/alkynyl substituted imidazo[1,2-a] pyridines from the readily available benzyl/ allyl/ propargylhalides and 2-aminopyridines as substrates via formimidamide chemistry. This new methodology is devoid of caustic or expensive reagents, such as transition metal complexes. Also, an extension of this chemistry to synthesize the 2,3-disubstituted imidazo[1,2-a]pyridines has been presented.