Synthesis of lamellarin alkaloid analogues from enaminone precursors
Date
2014-02-07
Authors
Scalzullo, Stefania Margherita
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Abstract
The synthesis of alkaloids from enaminones has been used extensively in the University of the Witwatersrand’s organic chemistry laboratories. In this thesis enaminone precursors are one of the main ways of accessing lamellarin analogues. The lamellarin alkaloids are an important family of marine alkaloids, owing to their vast biological properties. A brief background to marine alkaloids and their general potential is given, followed by a review of lamellarin alkaloids, their structural and biological properties and some of the major syntheses carried out over the past few years.
Two novel features form the basis of the synthetic methods described in the thesis. The first is an approach to forming the lamellarin alkaloids from enaminone precursors, which are prepared through the Eschenmoser sulphide contraction. The second method uses a novel pyrrole formation, which was initially conceptualized by Garreth L. Morgans in his PhD thesis (2008). The main target of the investigation was lamellarin G trimethyl ether.
In Chapter 3, the syntheses of a range of mono-, di- and tetra-substituted phenacyl halides are discussed. The phenacyl halides were used in the preparation of various enaminone precursors. The tetrasubstituted phenacyl halide 2-bromo-1-(2-hydroxy-4,5-dimethoxyphenyl)ethanone 3.17 is required for the synthesis of our target lamellarin G trimethyl ether. The phenacyl halides are important in both the model synthesis described in Chapter 4 and the synthesis toward lamellarins in Chapter 5.
Chapter 4 deals mainly with the synthesis of pyrrolizine systems. Methodology is described for the preparation of a variety of enaminones, pyrroles and tetracyclic lamellarin analogues. The closest pyrrolizine system to lamellarin G trimethyl ether, 11-(3,4-dimethoxyphenyl)-2,3-dimethoxy-9,10-dihydrochromeno[4,3-b]pyrrolizin-6(8H)-one 4.52, was the final and most complex tetracyclic model structure analogous to lamellarin G trimethyl ether. Indolizine and pyrroloazepine adaptations were also demonstrated and tetracyclic systems 10,11-dihydro-8H-chromeno[3,2-a]indolizin-12(9H)-one 4.39 and 9,10,11,12-tetrahydrochromeno[3',2':3,4]pyrrolo[1,2-a]azepin-6(8H)-one 4.40 were successfully prepared, even though the pyrrole formed in an unexpected way.
Finally in Chapter 5, the methodology established in the model study was used in the attempted synthesis of lamellarin G trimethyl ether. A second method was also investigated. Thus, various N-alkylated and N-H enaminones were successfully synthesized, from which novel and unexpected pyrrole-containing products 8-(3,4-dimethoxyphenyl)-2,3-dimethoxy-5H-chromeno[3',2':3,4]pyrrolo[2,1]isoquinolin-14(6H)-one 5.28 and (3-ethoxy-8,9-dimethoxy-2-phenyl-5,6-dihydropyrrolo[2,1-a]isoquinolin-1-yl)(phenyl)methanone 5.37 were formed, even though our desired product lamellarin G trimethyl ether could not be attained from either method.