Synthetic operations/procedures

The increasing current interest in processes that are able to generate complex molecular architectures in one pot has stimulated the development of domino-like procedures that create three bonds in one synthetic operation, with considerable advantages in terms of synthetic efficiency and environmental friendliness. This section describes some examples of the application of these concepts to the preparation of quinolizidine compounds. 

The aminoxyl radicals lend themselves to synthetically interesting procedures of oxidation, both in the radical form itself and in the oxoammonium form (from TEMPO). Major advantages appear to be the mild operating conditions, the range of substrates susceptible to transformation and the selectivity in the oxidation of specific structural motifs. 

As catalytic semihydrogenation of alkynes to Cis-alkenes is not only a very important synthetic operation (ref. 1) but also of industrial interest, it is a challenging task for both synthetic and catalytic chemists. For instance, the importance of the problem is illustrated by numerous recent publications on different aspects of the selective hydrogenation of many compounds related to the propargyl alcohol structure (refs. 2-7). In this respect, 1,4-butenediol, obtained by the liquid-phase semihydrogenation of 1,4-butynediol, is a raw material for insecticides and Vitamin Bg (refs. 2,8,9). Furthermore, the total and selective liquid-phase hydrogenation of this compound is one of the procedure for making butanediol, the top 95 chemical produced in the United States (refs. 10,11), whose major use is in the manufacture of polyesters. 

There is still much debate as to whether synthetic operations should be divided into Stage I and Stage II operations. This is because synthetic operations at whatever scale they are performed should be scaleable and translate into the later technical application. It becomes clear from the above that the guideline for Stage II synthetic operations will in general be such that the unit operations employed for the material s synthesis will directly lead to scaleable synthetic operations that closely link to laboratory procedures and later the technical production. In extreme cases one may even employ catalyst libraries that stem completely from commercially produced materials to ensure straightforward production of a material in case it proves to be a hit. 

One-pot procedure integrates several glycosylation steps into one synthetic operation to furnish the target oligosaccharide in a short period of time without... 

Methods available before 1971 for the preparation of thiol esters are briefly summarized in a review article.4 Since then, several newer techniques have been developed, to meet a certain set of criteria required for recent synthetic operations. This development may be summarized as follows. Whenever an acid chloride is available, the reaction of the T1(I) salt of a thiolate of virtually any kind, including alkane-, benzene-, 2-benzothiazoline-, and 2-pyridinethiol, proceeds efficiently and near-quan-titatively. However, if selective thiol ester formation in the presence of hydroxy or other functional groups in the same molecule is required, three main procedures are available. First, reaction of an acid (1), with... 

Like their ubiquitous 0(O-acetal relatives, N, 0-acetals can exceed the narrow bounds of passive protection and participate in synthetic operations of far greater significance. Wc will illustrate the point by the enantioselective a-alkylation of proline [Scheme 8.163]357 360 without the use of a chiral auxiliary. The procedure is another example of the self-regeneration of stereocentres and begins... 

The term synthon is more familiar to organic chemists and describes fragments from which varions molecnles can be bnilt by synthetic procedures. Sometimes the term was also used to describe structural units within supermolecules which can be formed and/or assembled by known or conceivable synthetic operations involving intermolecular interactions To avoid confusion, the term tecton was introduced to describe the molecular units of supramolecular structures assembled through noncovalent forces. Thus, the tecton has been defined as any molecule whose interactions are dominated by particular associative forces that induce self-assembly of an organized network Here, we will use the term synthon for... 

Emulsion polymerization first gained industrial importance during World War II when a crash research program in the United States resulted in the production of styrene-co-butadiene [SBR] synthetic rubber. The Harkins-Smith-Ewart model [5-6] summarized the results of early research, which focussed on this and similar systems. Current thinking is not entirely in accord with this mechanism. It is still worthwhile to review it very brielly here, however, because it is still widely referenced in the technical literature and because some aspects of the model provide valuable insights into operating procedures. 

In general, the reduction of tosyl hydrazones with boron hydrides is a convenient alternative to the classical procedures of Wolff-Kishner, Clemmen-sen, and Raney nickel reductions when one wants to delete the carbonyl of ketones and aldehydes after using it for the very many synthetic operations it allows. 

As in the previous volume, only a limited number of procedures for metallations and subsequent functionalizations are given. Although we have tried to choose the procedures as representative as possible, the user of this book may need additional information for designing reaction conditions for a particular synthetic operation. In this respect the present chapter on reactivity of polar organometallic intermediates may be helpful. It is based upon the results of several experiments carried out during the preparation of this volume and other books in this series, and on selected literature data. 

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