| This thesis describes the synthesis of the dimeric pyrrole-imidazole alkaloids sceptrin, ageliferin, nagelamide E, oxysceptrin, nakamuric acid, nakamuric acid methyl ester, axineallamine A, and axinellamine B. The total synthesis of racemic sceptrin was accomplished by the fragmentation of an oxaquadricyclane to establish the cyclobutane core, followed by attachment of the pyrrole sidechains and elaboration of the 2-aminoimidazole units. Microwave irradiation of sceptrin yielded the natural products ageliferin and nagelamide E. An enantioselective synthesis of sceptrin was effected by means of an enzymatic desymmetrization and careful modification of the oxaquadricyclane fragmentation conditions. The absolute configuration of ageliferin was determined by synthesis from enantiopure sceptrin, highlighting the possibility that sceptrin may be the biosynthetic precursor of ageliferin. A computational study of the formation of ageliferin indicates that this reaction likely proceeds via a radical fragmentation mechanism. Biosynthetic considerations led to the synthesis of oxysceptrin, nakamuric acid, and nakamuric acid methyl ester from sceptrin. Attempts to obtain the cyclopentyl core of the axinellamines, massadines, and palau'amines culminated in the development of a ring contraction of ageliferin. However, the stereochemistry of the spiro ring junction formed in this reaction was opposite that found in the natural products. Finally, the total synthesis of axinellamines A and B by closure of a linear intermediate and studies on oxidation of intermediates formed in this route are discussed. |
