Complex molecules, synthesis properties

The fact that the Diels-Alder reaction is accelerated by catalysts proved a boon to the use of such cycloaddition reactions in the service of complex molecule synthesis. A myriad of creative applications and general methods to effect stereoselective Diels-Alder reactions have been documented. An illustrative example is found in Grieco s reported total synthesis of ( )-quassin, one of the bitter principles of quassin wood with potent cytotoxic properties... 

A more detailed understanding of the biochemical pathways and enzymes involved in saponin biosynthesis will facilitate the development of plants with altered saponin content. In some cases, enhanced levels of saponins or the synthesis of novel saponins may be desirable (for example, for drug production 4 or improved disease resistance3,5,6), while for other plants, reduction in the content of undesirable saponins would be beneficial (for example, for legume saponins that are associated with antifeedant properties in animal feed7). This chapter is concerned with recent progress that has been made in the characterization of the enzymes and genes involved in the synthesis of these complex molecules and focuses on triterpenoid saponins. 

Heterocycles are of great interest in organic chemistry due to their specific properties. Many of these cycles are widely present in natural and pharmaceutical compounds. Electrochemistry appears as a powerful tool for the preparation and the functionalization of various heterocycles because anodic oxidations and cathodic reductions allow the selective preparation of highly reactive intermediates (radicals, radical ions, cations, anions, and electrophilic and nucleophilic groups). In this way, the electrochemical technique can be used as a key step for the synthesis of complex molecules containing heterocycles. A review of the electrolysis of heterocyclic compounds is summarized in Ref. [1]. 

These examples clearly show that the combination of modem methods and concepts of natural product biosynthesis and enzymology with effective techniques of chemical synthesis can help to solve challenging problems in the preparation of complex molecules. Rapidly evolving methods for the optimization of certain desirable characteristics of a particular enzyme (e.g., directed alteration of enzyme properties like substrate specificity, selectivity, turn-over rates, and stability) will continue to facilitate the design of desired enzyme reactivity [241]. Increasing interdisciplinary cooperation between chemists, biochemists, and biologists will thus be vital for the successful continuation and development of modem natural product research. 

Trifluoromethyl hypofluorite gained its initial popularity, however, from its reactions with enol derivatives. Such reactions continued to be important during the last decade. The synthesis of complex molecules such as fluoroerythromycin (equation 137)244 and 2-fluoro (25)245 and 16-fluoroestradiols (26) (equation 138)246 are good examples of using CF3OF to produce compounds with useful therapeutic properties. 

The synthesis of enantiomerically pure compounds is the challenging problem for organic chemists. The synthesis becomes obsolete if the intermediates produce racemic mixtures. The problem is particularly acute when the asymmetric centers do not reside in a rigid cyclic or polycyclic framework. To be able to carry out efficient syntheses of complex molecules, chemists have to control the sense of chirality at each chiral center as it is introduced in the course of synthesis. Monoalkyl- or dialkyl-boranes exhibit a remarkable chemo-, stereo-, and regioselectivity for the hydroboration of unsaturated compounds. This property, coupled with the capability for asymmetric creation of chiral centers with chiral hydroboration agents, makes the reaction most valuable for asymmetric organic synthesis. In some of the cases, however, this has been achieved by diborane itself as shown in the synthesis of monensin by Kishi et al. A stereospecific synthesis of its seven carbon. component has been accomplished by two hydroboration reactions (Eq. 129) 209. ... 

The properties of hypothetical closed carbon cages have been the snbject of discussion for many years, and the highly symmetric truncated icosahedral (soccer ball) geometry of the archetypal species buckminsterfullerene (Ceo) had particular appeal. It was suggested that Ceo would be stable and possibly superaromaticbut the deliberate synthesis of such a complex molecule remained too daunting a challenge. ... 

The medicinal properties of the sarpagine-related alkaloids remain of great interest, as well as the nature of their stmcture and stereochemistry. The construction of these structurally complex molecules remains of paramount importance to synthetic chemists. Several studies have been recently published delineating approaches for the synthesis of sarpagine-related alkaloids. In this chapter we focus on key syntheses of the sarpagine-related alkaloids published since 2000. 

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