Stepwise synthesis

A further link is then made with another sugar after removal of the protective group B, and so on. In these cases, X can be Br, B can be CO—C6H4NO2, and R can be CH2C6H5. 

The solvolysis of sugar derivatives with C-2 groups participating in 

In general, therefore, polycondensation does not lead to products of complete steric purity. The degrees of polymerization are also usually low, since the conversions and yields are small. 

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A further stepwise synthesis of this type, in which 2-bromomethyl-3-nitropyridine (105) was the starting material, is the only method so far reported which directly produces pyrido[3,2-d]pyrimidines without nuclear oxygen substituents (106). ... 

The conversion of the cyclic hexamer of fiiran and acetone into naphthafiirophane was reported <96TL6201>. Stepwise synthesis of core-modified, zncso-substituted porphyrins (e.g., 19) was achieved by acid catalyzed condensations <96TL197>. 

FIGURE 5 Stepwise synthesis of a triblock copolymer (PCL-PLA-PCL) of PCL and polylactic acid using aluminum coordination catalysts to minimize randomization of the block structure by transesterification. (From Ref. 43.)... 

Due to an interest in studying their unusual reactivity (vide infra), several attempts were made to maximize yields of the strained dimers 74. Lengthening reaction times and decreasing substrate concentrations in the cyclooligomerization experiments proved fruitless. In response to this situation, a stepwise synthesis of the tetrahexyl-substituted dimer was developed as shown in Scheme 18. Surprisingly, Hay coupling of 77 resulted in an improved yield of the tetramer (45 vs 8 %) and a substantial decrease in the yield of dimer (13 vs 30%). This product distribution was unexpected, since intramolecular reactions are typically much faster than intermolecular reactions. 

As mentioned in last year s Report, aromatic phosphoramidates have been used to protect 5 -phosphoryl groups in the stepwise synthesis of oligodeoxyribonucleotides. The appropriate monomer units are coupled with DCC and the phosphoramidate protecting group is removed when required with isoamyl nitrite. A rapid and general preparative method for oligonucleotides has been developed based on phosphoramidates of the highly lipophilic 4-aminophenyltriphenylmethane (25). Purification of... 

The repeated succession of similar reaction sequences for the stepwise synthesis of molecules which consist of repeated building blocks has been termed in literature as repetitive or iterative synthesis. To a molecule with a defined framework having one or more functional groups A (Fig. 1) an identical molecule unit is attached (Iter. 1). Subsequently the functional groups are then reobtained (Iter. 2) so that the reaction sequences can be repeated over and over with the molecule growing by one unit in each reaction cycle (Iter. 1 + 2). This growth... 

Fig. 16. Stepwise synthesis of terminal substituted dispiranes as precursors for new liquid crystals... 

Fig. 3. Stepwise synthesis of rifaximin (from De Angelis [39]). The reaction of rifamycin S (I) with pyridine perbro-mide (II) in 2-propanol/chloroform (70/30) mixture at 0°C gives 3-bromorifamycin S (III), which is then condensed with 2-amino-4-methyl-pyridine (IV) at 10°C. The o-quinonimic compound (V) is then obtained. This compound is finally reduced with ascorbic acid to rifaximin. 

Ashwell GJ, Urasinska-Wojcik B, Phillips J (2010) In situ stepwise synthesis of functional multijunction molecular wires on gold electrodes and gold nanoparticles. Angew Chem Int Ed 49 3508-3512... 

In addition, the fulvestrant could be glycosylated effectively at its 17-OH position with pivaloylated glycosyl trichloroacetimidates, which suppressed the competing transacylation side reaction and led to improved yields of the desired glycosides (Scheme 3.48d) [503]. In this synthesis, the inverse procedure (i.e. addition of a trichloroacetimidate donor to a mixture of an acceptor and a promoter) was found to be superior for glycosylations. Very recently, a stepwise synthesis of branched... 

Scheme 6 outlines the stepwise synthesis of poly(naphthalene) and poly-(tetrafluoronaphthalene). 

Following a similar approach, the first bis(triorganosilyl) diazene 402 that is stable at room temperature could be obtained.410 Thus, the reaction of (/-Bu2MeSi)HNNH(SiMe/-Bu2) 403, an analog of 399, with 2 equiv. of BuLi has provided (/-Bu2MeSi)LiNNLi(SiMe/-Bu2) 404, whose oxidation with bromine has led to the formation of the diazene /-Bu2MeSiNNSiMe/-Bu2 402 (Scheme 58). Recently, the stepwise synthesis of cyclic and open-chain silyl hydro-xylamines has been reported by Klingebiel < / /.411 14 For hypervalent silyl hydroxylamines, see Section 6.8. 

The self-organization of polysilanols in the presence of other hydrogen bond acceptors has been studied by several groups.512-516 Several other publications have dealt with the stepwise synthesis of siloxane and siloxanol chains.450,517-522 Recent work on fully condensed siloxane rings and silsesquioxane cages involves the non-aqueous hydrolysis of chlorosilanes - as well as mechanistic and structural studies. 

As this brief overview demonstrates, novel copolymers obtained by hybridization of the linear and globular architectural states are readily prepared through a variety of synthetic approaches. In general the dendritic components of the hybrid copolymers are well defined, with unique molecular and structural characteristics. In contrast, all the linear components prepared polymerization are less precisely defined and are polydisperse. Only the very short linear components, themselves prepared by stepwise synthesis just like the dendrons, are monodisperse and can be used to prepare well-defined, monodisperse hybrids. While architectural and structural precision may be of great importance for the determination of ultimate properties, some degree of structural variation is quite acceptable for practical applications in many areas including, for example, surface modification, sensing, or encapsulated delivery. 

Z. Chang, M. C. Kung, and H. H. Kung, Stepwise synthesis of siloxane chains, CherrL Comrrmn. 206 (2004). 

Figure 1.12 Stepwise synthesis scheme for chemical deposition of silver into an alumina membrane. (With kind permission from Springer Science + Business Media [38]. A. Huczko, Applied Physic A, 70, (2000), 365-376 Figure 1.7 Reprinted with permission from The Materials Research Society.)...

Figure 1.13 Stepwise synthesis of direct gold sputtering onto alumina membrane to produce nanostructures. (Reprinted with permission from Ref [41]. 2005 Elsevier.)...

The reasoning which led the author to make this first shot in the dark regarding the usefulness of combinations of solid compounds as ammonia catalysts was as follows If we assume that a labile iron nitride is an interminate in the catalytic ammonia synthesis, every addition to the iron which favors the formation of the iron nitride ought to be of advantage. In other words, the hypothesis was used that surface catalysis acts via the formation of intermediate compounds between the catalyst and one or more of the reactants. An experimental support for this theory was the fact that a stepwise synthesis via the formation and successive hydrogen reduction of nitrides had been carried out with calcium nitrides (Haber), and cerium nitrides (Lipski). Later, the author found molybdenum nitride as being the best intermediate for such a stepwise synthesis. 

Figure 4.5 Scheme for stepwise synthesis of oligonucleotides via coupling of phosphorami-dite-functionalized nucleosides. Reprinted from Noro et al. (2005). Copyright 2005 American Chemical Society. 

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