Selectivity in synthesis

There are numerous other examples of radical clock reactions in the literature used both for simple rate determinations to facilitate the quest for selectivity in synthesis and for more detailed probing of mechanistic pathways. 

In the phosphorus(V) acids area, there has been continued interest in the preparation of phosphates from elemental phosphorus, and also of compounds derived from the calixarenes. Apart from these, there has been little of real significance in the phosphate field. A much greater interest has been shown in enantio- or diastereo-selectivity in synthesis and reactivity of phosphonates and... 

Another important skill is understanding the factors that influence selectivity. In synthesis, selectivity is everything, because the synthetic chemist seeks a single product over all others. In order to maximize the formation of that single product, we need to understand the factors that control selectivity. In this chapter, selectivity is discussed... 

Current research in LHASA is focused on developing new methods assisting users in choosing the most appropriate strategy for their target structures. Furthermore, the development of criteria for the evaluation of synthesis routes and/or algorithms for selecting optimal synthesis routes within the tree are discussed [34]. 

The benzylidene derivative above is used, if both hydroxyl groups on C-2 and C-3 are needed in synthesis. This r/vzns-2,3-diol can be converted to the sterically more hindered a-cpoxide by tosylation of both hydroxy groups and subsequent treatment with base (N.R. Williams, 1970 J.G. Buchanan, 1976). An oxide anion is formed and displaces the sulfonyloxy group by a rearside attack. The oxirane may then be re-opened with nucleophiles, e.g. methyl lithium, and the less hindered carbon atom will react selectively. In the following sequence starting with an a-glucoside only the 2-methyl-2-deoxyaltrose is obtained (S. Hanessian, 1977). 

Most of the resin systems used in commodity composites are slight modifications of the standard commercial mol ding grade material. Usually certain selected properties, such as purity or molecular weight range or distribution, are enhanced or carehiUy selected. In addition, special additives, such as flow controllers, thermal stabilizers, or antioxidants, are often added by the resin manufacturer prior to shipment. Many of the conventional or commodity-type resins used in thermoplastic composites are Hsted in Table 1 and the preparation of each of these is described. AH resins and blends described in the hterature are not Hsted, and the synthesis described is not the only procedure available, but is usually the most common commercial process. 

In an early synthesis a methyl carbonate, prepared by reaction of a phenol with methyl chloroformate, was cleaved selectively in the presence of a phenyl ester. ... 

The low position selectivity in the epoxide opening step of this early synthesis was improved in a later study (Ref. 2). 

The success of the halo ketone route depends on the stereo- and regio-selectivity in the halo ketone synthesis, as well as on the stereochemistry of reduction of the bromo ketone. Lithium aluminum hydride or sodium borohydride are commonly used to reduce halo ketones to the /mm-halohydrins. However, carefully controlled reaction conditions or alternate reducing reagents, e.g., lithium borohydride, are often required to avoid reductive elimination of the halogen. 

The vapor-phase catalytic replacement of chlorine by fluorine with hydrogen fluoride as the fluorine source has been the subject of a number of patents for the synthesis of Freons or Genetrons This topic has been carefully reviewed in the literature [2, p 97ff] One advantage of using a catalyst with hydrogen fluoride is to allow some degree of selectivity in the displacement of a specific chlorine from... 

Bis(phe nylthiomethyl)dihydropyran, CSA, CHCI3, 54-93% yield. This dihydropyran can be used for the resolution of racemic diols or for regio-selective protection, which is directed by the chirality of the dihyropyran. Other 2,2 -substituted bisdihydropyrans that can be cleaved by a variety of methods are available, and their use in synthesis has been reviewed. ... 

Although the Paterno-Buchi reaction is of high synthetic potential, its use in organic synthesis is still not far developed. In recent years some promising applications in the synthesis of natural products have been reported. The scarce application in synthesis may be due to the non-selective formation of isomeric products that can be difficult to separate—e.g. 6 and 7—as well as to the formation of products by competitive side-reactions such as Norrish type-I- and type-II fragmentations. 

A syn-selective asymmetiic nih o-aldol reaction has been reported for structurally simple aldehydes using a new catalyst generated from 6,6-bis[(tiiethylsilyl)ethynyl]BINOL (g in Scheme 3.18). The syn selectivity in the nitro-aldol reaction can be explained by steric hindrance in the bicyclic transition state as can be seen in Newman projection. In the favored h ansition state, the catalyst acts as a Lewis acid and as a Lewis base at different sites. In conbast, the nonchelation-controlled transition state affords anti product with lower ee. This stereoselective nitro-aldol reaction has been applied to simple synthesis of t/ireo-dihydrosphingosine by the reduction of the nitro-aldol product with H2 and Pd-C (Eq. 3.79). 

Aziridination remains less well developed than epoxidation. Nevertheless, high selectivity in inline aziridination has been achieved through the use of chiral sulfi-nimines as auxiliaries. Highly successful catalytic asymmetric aziridination reactions employing either sulfur ylides or diazo esters and chiral Lewis acids have been developed, although their scope and potential applications in synthesis have yet to be established. 

For a successful application in synthesis, several problems have to be solved regioselectivity, whether the C-C bond is formed with the 1- or 3-position in an unsymmetrical ambident anion, EjZ selectivity in the formation of the double bond, and simple diastereoselectivity, since two new stereogenic centers are created from prostereogenic compounds. Further, different types of induced stereoselectivity or enantioselectivity may be required. Allylmetals with a wide choice of substituents are accessible by various methods (Sections D. 1.3.3.3.1.-10.). 

The iridium(III) complexes are broadly similar to the rhodium(III) ammines a selection of synthesis is shown in Figure 2.84. 

With nonracemic chiral diazoacetates the insertion process occurs with evident match/mismatch characteristics. This has been demonstrated in reactions of optically pure 2-methylcyclohexyl diazoacetates (Eq. 9) [85] and in carbon-hydrogen insertion reactions of steroidal diazoacetates (Eq. 10) [86], as well as with the synthesis of pyrrolizidines 36 and 37 [84]. The mechanistic preference for formation of a /J-lactone in Eq. 10 over insertion into the 4-position is not clear,but there are other examples of /J-lactone formation [87]. In these and related examples, selectivities in match/mismatch examples are high, and future investigations are anticipated to show even greater applicability. 

Abstract For many years after its discovery, olefin metathesis was hardly used as a synthetic tool. This situation changed when well-defined and stable carbene complexes of molybdenum and ruthenium were discovered as efficient precatalysts in the early 1990s. In particular, the high activity and selectivity in ring-closure reactions stimulated further research in this area and led to numerous applications in organic synthesis. Today, olefin metathesis is one of the... 

The aim of this volume is to convince the reader that metal carbene complexes have made their way from organometallic curiosities to valuable - and in part unique - reagents for application in synthesis and catalysis. But it is for sure that this development over 4 decades is not the end of the story there is both a need and considerable potential for functional organometallics such as metal carbon multiple bond species which further offer exciting perspectives in selective synthesis and catalysis as well as in reactions applied to natural products and complex molecules required for chemical architectures and material science. 

Alkali promoters are often used for altering the catalytic activity and selectivity in Fischer-Tropsch synthesis and the water-gas shift reaction, where C02 adsorption plays a significant role. Numerous studies have investigated the effect of alkalis on C02 adsorption and dissociation on Cu, Fe, Rh, Pd, A1 and Ag6,52 As expected, C02 always behaves as an electron acceptor. 

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