Synthetic Problems to be Solved

In what follows, we try to shed the light on some mysterious features of Pechini-type methods, to overview major drawbacks in synthetic routines described in the past decade and put forward some problems to be solved in order to design advanced oxide materials in an optimal way. 

The main problems related to the use of vinyl or dienyl sulfoxides in cycloadditions can be grouped into two different categories, synthetic and mechanistic aspects respectively. Availability of the starting materials, stability of the obtained cycloadducts, and final elimination of the sulfinyl group, are the three main problems to be solved from a synthetic perspective. Reactivity of the substrates and the endo/exo selectivity of the cycloadditions will remain as important questions to be answered from a mechanistic viewpoint. 

The next problem to be solved is that of developing a method by which the computer can use the structural information perceived in the previous step to generate a synthetic tree by the appUcation of suitable transforms. Since the identification of applicable transforms is made on the basis of target structure and is independent of offspring structure, Corey et al. have used a method 8) that classifies transforms according to the nature of the critical structural features of a target molecule to which the transforms may be keyed. By way of illustration a few of the more important classes of transforms are shown below ... 

Based on the retro-synthetic analysis, the problems to be solved in total synthesis were proposed as following ... 

In this book, an effort has been made to provide comprehensive yet concise commentaries on the metdianisms of each reaction, as well as on their synthetic applications. These provide an accurate prescription for their use and should be useful for the development of a broader perspective on carbonyl olefination. The final chapter is concerned with asymmetric carbonyl olefination, which is one of the frontiers of organic synthesis. As this subject exemplifies, the established methodologies are not necessarily perfect and there still remain many problems to be solved in the field of carbonyl olefination. It is hoped that this book will be of wide use to all chemists engaged in organic synthesis, both in industrial laboratories and in academic institutions. 

Some of the problems can be solved with specially selected oil grades. Another solution is synthetic oils, but cost is a problem particularly with silicone oils. Alternatives must be reviewed to match service life of the lubricant with lubrication requirements in the compressor. 

The difficulty becomes accentuated by several other requirements that cannot be achieved through the selection of a single filter medium. Therefore, selection is often reduced to determining the most reasonable compromise between different, mutually contradictory requirements as applied to the filter medium at a specified set of filtration conditions. Because of this, some problems should be solved before final medium selection. For example, should attempts be made to increase filtration rate or filtrate purity Is cost or medium life more important In some cases a relatively more expensive filter medium, such as a synthetic cloth, is only suitable... 

The above example demonstrates the potentialities of mass spectrometry as a method for elucidating the position of isopropylidene groupings. This problem has often to be solved in synthetic monosaccharide chemistry, especially when the compound may form several isomeric isopropylidene derivatives and the chemical methods are tedious and, sometimes, unreliable. 

However, there are still a number of problems that will have to be solved Synthetic equivalents will have to be found for those cosubstrates that do not undergo this cycloaddition. The possibilities of inversion the polarity (e.g. by the introduction of a nitro-group at the unsaturated cosubstrate) remain to be examined as does the extension of these cycloaddition reactions to an intramolecular version leading to cyclopentane annulation. Although the regioselectivity may be manipulated by changing the catalyst (e.g. Pd(0) versus Ni(0)), there is still room for improvement and a deeper understanding of the mechanistic details of these reactions is needed. But the activity and the interest in this field assure that most of these problems will be solved in the near future. 

This chapter presents an overview of existing strategies for asymmetric catalytic epoxidation of unfunctionalized olefins with synthetic catalysts. The significant progress in metal-catalyzed oxo transfer and dioxirane epoxidation has increased the accessibility of several classes of optically active epoxides and concurrently highlighted important problems yet to be solved. 

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