Diseño y síntesis de productos naturales y dispositivos moleculares

Resumo

ABSTRACT Section A Titanocene(III) complexes, essentially titanocene(III) chloride (Cp2TiCl), have recently emerged as a powerful tool in organic synthesis. They are soft single-electron-transfer (SET) reagents capable of promoting different kinds of reactions, such as homolytic epoxide and oxetane openings, Barbier-type reactions, Wurtz-type reactions, Reformatsky-type reactions, reduction reactions, and pinacol coupling reactions. All these reactions have been applied in functional group interconversions in some natural product synthesis, showing the potential of this reagent. Chapter I A drawback of these procedures is the lack of chemoselectivity in the electron transfer step. For this reason, aromatic and ¿,ß -unsaturated carbonyl compounds are usually unsuitable substrates in Barbier-type reactions. In this chapter, we demonstrate that this shortcoming can be resolved with a complex, described by Prof. Andreas Gansaüer, titanocene carboxylate(III), able to improve not only the regioselectivity of Barbier type procedures but also tolerate epoxides moieties in the starting materials. Chapter II In recent years, bioinspired radical cyclizations have emerged as a powerful tool for the efficient synthesis of different terpenic structures. It is also worth noting that, in many cases, these cyclizations represent a complementary approach to more developed biomimetic cationic cyclizations. Moreover, it is expected that new chemo- and stereoselectivities could be obtained by taking into account the significant differences between their corresponding reactive intermediates and the catalysts used. One of the main differences is the nature of the alkenes present in the polyprenic starting material. Electron deficient alkenes are not usually suitable functionalities for cationic cyclizations of simple polyprenic precursors. On the other hand, these electron deficient alkenes can efficiently react with carbon centered radicals. Thus, the expected final products using conventional cationic cyclizations and radical ones might not be the same. In this Chapter, we focused our attention on Ti(III)-catalyzed radical cyclizations of epoxypolyprenes presenting an internal keto functionality in their structures. This approach provides an unprecedentedly straightforward access to natural terpenoides with pendant unsaturated side chains. Additionally, in the case of bi- and tricyclizations, decalins with cis stereochemistry have been obtained as a consequence of the presence of the ketone Section B Tubular organic nanostructures offering precise control over inner and outer surface functionality represent attractive building blocks for the bottom-up approach in nanotechnology, as well as for the development of future materials and biological applications. In this section, the current state of the art in creating organic nanotubes is summarized, focusing on the chemistry behind tubular structure formation. The discussion of general design principles, which are altogether inspired by Nature and combine covalent and noncovalent synthesis, is followed by detailed treatments of the individual approaches especially including helices and their assemblies. Chapter III Helical structures are widespread is nature and have fascinated to chemists for years. Such helical structures have been extensively studied and have been used in many interesting applications. Therefore, the development of new helical structures is desirable. Within this context, we recently reported that ortho-OPEs can be stapled into helical conformation, even in a chiral way. Nevertheless, this approach is limited by the solubility of starting materials. In this Chapter, we have developed a new methodology where allyl protected starting materials can be easily handled and can be stapled using metathesis reaction. Dihydroxylation of such stapled derivatives and subsequent functionalization of the corresponding diol can affect the equilibrium between the two P and M conformers. An increase in the steric hinder of the staple will result in a perfect conformational preference paving the way for the synthesis of helical nanoporos.