The past decade has seen intense interest in the characterization, synthesis and medicinal development of polyketide/macrolide natural products such as the bryostatins, discodermolides, spongistatins, epothilones, etc. as extraordinarily potent anti-cancer agents. In many cases, only exceedingly limited quantities of these (often marine) natural products are available from natural sources/fermentation, and this has greatly hampered efforts to more fully evaluate their biological/medicinal profile. The recent synthesis of ~60 g of discodermolide by a large group of Novartis chemists with enormous effort, time, and expense both underscores the need for large amounts of these compounds and provides a useful benchmark as to the state of the art in terms of synthetic accessibility. It is imperative, therefore, that synthetic organic chemists continue to provide significantly simpler methods for the synthesis of such compounds. Success toward this ambitious goal would impact not just the supply of these compounds, but efforts to develop analogs with improved pharmacological profiles as well. The present proposal details the development of reactions that will establish high levels of structural and stereochemical complexity in operationally trivial, environmentally sound, and scalable processes. This will be achieved through the orchestration of tandem reactions that assemble simple, principally hydrocarbon fragments into large segments of the target natural products that contain as many as three stereocenters. Importantly, the conceptual foundation for these methods involves the use of Lewis acidic silanes, compounds that are extraordinarily straightforward and inexpensive to synthesize on large scales. Following the development of these methodologies, we will demonstrate their effectiveness by accomplishing brief and efficient syntheses of important natural products such as zincophorin, dictyostatin, and the C(1)-C(28) ABCD fragment and the C(29)-C(51) EF fragment of the spongistatins. The targets have been selected for their biological importance and because it is our goal to improve the step economy with which such structures may be prepared.
Polyketide natural products are a rich source of biologically active compounds with extraordinary medicinal potential as antibiotics and as anti-cancer agents. However, very often they are available from natural sources in only minute quantities, insufficient for their full biological profile to be investigated. This proposal seeks to develop efficient methods for the synthesis of gram-scale quantities of such compounds to aid in their biological evaluation and development.
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