Enantioselective total synthesis of the potent antitumor agent dibromophakellstatin en route to dibromophakellin and palau'amine
Marine natural products biogenetically derived from the pyrrole-imidazole oroidin (i) display fascinating structural diversity and exhibit a wide range of significant biological activities. Hence, they have attracted great interest from the synthetic community leading to the development of new synthetic methodologies and providing an admirable set of strategies and tactics to assemble the most complex representatives of this class of alkaloids. In this work, a synthesis of the potent antitumor dibromophakellstatin (ii) was devised featuring a highly diastereoselective acylation of the C2-symmetric prolyl-prolyl anhydride (iii), an intramolecular Mitsunobu reaction installing the C6 aminal and a tandem Hofmann rearrangement/cyclization that delivered the targeted imidazolidinone of the phakellstatins. Both enantiomers of phakellstatin were prepared starting from either D- or L- proline.
Building on several findings made in the course of the synthesis of phakellstatin, namely the differential reactivity of the aminal centers (C6 and C9) and the formation of an oxidized phakellstatin, a second generation approach towards these deceitfully simple alkaloids was initiated. The second generation synthetic strategy towards phakellstatin (iv) and phakellin (v) utilizes a novel variation of Du Bois' C-H insertion involving a urea or a guanidine. This approach appears more amenable to the annulation of the phakellin substructure for the final stages of the synthesis of the potent immunosuppressant palau'amine (vi).
Advisor:Romo, Daniel; Bergbreiter, David; Sulikowski, Gary; Hu, James
School:Texas A&M University
School Location:USA - Texas
Source Type:Master's Thesis
Keywords:dibromophakellstatin total synthesis oroidin alkaloid hofmann rearrangement
Date of Publication:08/01/2003