Stepwise addition method

Another approach to the calculation of group electronegativities Xg is based on a stepwise addition method [Zhou, Nie et al, 2007] ... 

Stepwise addition method (Model polymerization), + MOVED... 

The trimer and the pentamer, obtained by the stepwise addition method using THE as solvent, and the monomer, obtained by the same method using MEK as solvent. 

Figure 5.52 (I) Schematic drawing of the rectangular shape origami with zoom-in view on the right and side views illustrating the zigzag 120° dihedral angle between double-stranded helices. (II) Schematic pathway to the formation of tubular DNA structures following method 1, in which both sets of linkers are added simultaneously, or following the stepwise addition method 2, creating 1-3 connections followed by 2-4 linkers (or inversely). Arrows show the way stair-like DNA ribbons self-assembled into tubes.

Polymerization methods [I], [II], and [III] (Fig. 1) indicate, respectively, the dropwise addition of VAc and initiator the dropwise addition of VAc and the stepwise addition of initiator the batch method, in which all ingredients of water, VAc, PVA, and initiator were put into the reaction vessel before starting polymerization. In method [I], when the temperature of the PVA solution in the flask attained 70°C, dropwise additions of 20 g of an aqueous solution containing initiator and 250 g of VAc were started. In method [II], the process was similar to method [I], except the initiator was added stepwise. When the temperature of the contents in the flask was raised to 70°C, 24 g of an aqueous solution containing half the prescribed amount of initiator was first added. 

The synthesis of aziridines through reactions between nitrenes or nitrenoids and alkenes involves the simultaneous (though often asynchronous vide supra) formation of two new C-N bonds. The most obvious other alternative synthetic analysis would be simultaneous formation of one C-N bond and one C-C bond (Scheme 4.26). Thus, reactions between carbenes or carbene equivalents and imines comprise an increasingly useful method for aziridination. In addition to carbenes and carbenoids, ylides have also been used to effect aziridinations of imines in all classes of this reaction type the mechanism frequently involves a stepwise, addition-elimination process, rather than a synchronous bond-forming event. 

Difficulties due to side reactions (cyclization) and a broad molecular weight distribution accompanying the polycondensation of active esters led to the application of methods wherein the polymers are built up stepwise. In 1968, Sakakibara et al.31) introduced the solid-phase technique using Merrifield s resin. By stepwise addition of tert-pentyloxycar-bonyl tripeptides, they have synthesized (Pro-Pro-Gly)n with n = 5, 10, 15 and 20. 

There are two gereral routes to mikto-arm star polymers. The first method makes use of the stepwise addition of living polymers to multifunctional chloro-silane compounds [59-62], The Athens group uses the sequential addition of living polymers to multifunctional chlorosilane compounds under tight stoichiometric control [63, 64],... 

The method is normally limited to the production of s unmetrically substituted biguanides by the stepwise addition of unlike amines, biguanides bearing different 1- and 5-substituents (e.g. isoCsHisNH ... 

Another method for preparation of higher oligomers takes advantage of site isolation on a solid phase.6 It proceeds via alternate addition of a pair of symmetrical difunctional monomers in a stepwise addition obviating the need for deprotection steps (Scheme 6). The sequence is extended one unit at a time, unlike solution approaches that tend to involve growth from both ends. This method has not yet been fully investigated, so it is difficult to speculate about its effectiveness or to foresee potential problems. 

Synthesis of the peptide Ac-A-Bn-C-OH was performed by a stepwise solid-phase fragment condensation approach, as shown in Scheme 3, using the general procedures of the automated solid-phase method. The synthesis began with Boc-C-resin support and continued with the stepwise addition of protected peptide fragments until the completed peptide was assembled on the resin. The supported peptide Ac-A-Bn-C-resin was obtained after deprotection of Boc-A-Bn-C-resin and acetylation of the free amino groups. 

The search for alternatives to the stepwise addition of new addends to a mono-, bis-, or tris-adduct of Cr,o for the preparation of highly functionalized fullerenes has been a challenge since the beginning of fullerene chemistry. Here we present the principal approaches introduced to control the regiochemistry of multiple additions to fullerenes by the use of tethered addend systems. The tethered macro-cyclization method proved to be a major breakthrough and has provided relatively facile access to otherwise strongly disfavored addition patterns. 

The stepwise addition of N-carboxy a-amino acid anhydrides to amino acids or peptides in organic solvents is well documented (for review see Refs. 69 and 70). More recently the method has been applied to peptide synthesis in aqueous solutions (71,72,73). Novel syntheses of N-substituted carboxyanhydrides of a-amino acids have been carried out (74,75) and the crystal molecular structures of several of them have been determined (76,77,78). 

The fruitfulness of the idea of a stepwise addition with an independent variation of the addends was brilliantly illustrated by Normant s studies, which resulted in the elaboration of a general method of alkene synthesis based on the reaction of alkyne carbometallation. Basically this reaction represents a case of the well-known nucleophilic addition to a carbon-carbon triple bond. In the Normant reaction, however, the initial addition of a nucleophile (an organome-tallic reagent) across the triple bond results in the formation of a stabilized carbanion-like intermediate equivalent to a vinyl carbanion. This intermediate can similarly be further reacted with an external electrophile. Most typically, copper-modified Mg or Li reagents, which are unable to react with acidic acetylenic hydrogens, are used in this sequence. 

Older methods of fractionating protein admixtures in aqueous solution usually involved the stepwise addition of ammonium sulfate, or an organic solvent, such as acetone or ethanol. These methods have been used in the crystallization of several starch-metabolizing en-... 

The structure proposed for secretin was synthesized by Bodanszky and coworkers3° by two different methods. In one, the heptacosapeptide chain was formed by stepwise addition of individual amino acid units and in the other, four peptide fragments were combined. However, none of the synthetic preparations have been as potent as the purified secretin from natural sources. 

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