Transformation reactions steps

For SFRP and RAFT polymerizations, directly using the polymers in the next cationic ring-opening polymerizations without any modification of the terminal group is impossible since the terminal groups are nitroxide and dithioester groups, respectively. Several transformation reaction steps are then required, and low transformation efficiency results. 

Absolute configurations of the amino acids are referenced to D- and L-glyceraldehyde on the basis of chemical transformations that can convert the molecule of interest to either of these reference isomeric structures. In such reactions, the stereochemical consequences for the asymmetric centers must be understood for each reaction step. Propose a sequence of reactions that would demonstrate that l( —)-serine is stereochemically related to l( —)-glyceraldehyde. 

A more recent application of the Knoevenagel reaction is its use in domino reactions. The term domino reaction is used for two or more subsequent transformations, where the next reaction step is based on the functionality generated in the preceding step. Such reactions are also called tandem reactions or cascade reactions. 

Many block and graft copolymer syntheses involving transformation reactions have been described. These involve preparation of polymeric species by a mechanism that leaves a terminal functionality that allows polymerization to be continued by another mechanism. Such processes are discussed in Section 7.6.2 for cases where one of the steps involves conventional radical polymerization. In this section, we consider cases where at least one of the steps involves living radical polymerization. Numerous examples of converting a preformed end-functional polymer to a macroinitiator for NMP or ATRP or a macro-RAFT agent have been reported.554 The overall process, when it involves RAFT polymerization, is shown in Scheme 9.60. 

Abstract Recent advances in the metal-catalyzed one-electron reduction reactions are described in this chapter. One-electron reduction induced by redox of early transition metals including titanium, vanadium, and lanthanide metals provides a variety of synthetic methods for carbon-carbon bond formation via radical species, as observed in the pinacol coupling, dehalogenation, and related radical-like reactions. The reversible catalytic cycle is achieved by a multi-component catalytic system in combination with a co-reductant and additives, which serve for the recycling, activation, and liberation of the real catalyst and the facilitation of the reaction steps. In the catalytic reductive transformations, the high stereoselectivity is attained by the design of the multi-component catalytic system. This article focuses mostly on the pinacol coupling reaction. 

The present section describes domino processes which combine two or three initiating anionic reaction steps with a following nonanionic transformation. 

In an anionic/radical domino process an interim single-electron transfer (SET) from the intermediate of the first anionic reaction must occur. Thus, a radical is generated which can enter into subsequent reactions. Although a SET corresponds to a formal change of the oxidation state, the transformations will be treated as typical radical reactions. To date, only a few true anionic/radical domino transformations have been reported in the literature. However, some interesting examples of related one-pot procedures have been established where formation of the radical occurs after the anionic step by addition of TEMPO or Bu3SnH. A reason for the latter approach are the problems associated with the switch between anionic and radical reaction patterns, which often do not permit the presence of a radical generator until the initial anionic reaction step is finished. 

Moreover, by using only TEMPO without addition of 2-714 the Michael adduct 2-713 is transformed into the isopropenylcyclopentane 2-715b as the major product. The process can also be extended by another radical reaction step [364]. 

In the final section of this chapter, we would like to present some domino processes with a particular high number of reaction steps, and partly unusual transformations. 

The transformation of the chain end active center from one type to another is usually achieved through the successful and efficient end-functionalization reaction of the polymer chain. This end-functionalized polymer can be considered as a macroinitiator capable of initiating the polymerization of another monomer by a different synthetic method. Using a semitelechelic macroinitiator an AB block copolymer is obtained, while with a telechelic macroinitiator an ABA triblock copolymer is provided. The key step of this methodology relies on the success of the transformation reaction. The functionalization process must be 100% efficient, since the presence of unfunctionalized chains leads to a mixture of the desired block copolymer and the unfunctionalized homopolymer. In such a case, control over the molecular characteristics cannot be obtained and an additional purification step is needed. 

Recently, an enantioselective total synthesis of ( )-18,19-dihydroantirhine has been reported by Kametani s group (140). They started from the chiral cyclopentanone derivative 211, obtained from the previously prepared (/ )-1,2-isopropylideneglyceraldehyde (141). Utilizing a number of reaction steps, 211 was transformed to 215 with the desired stereochemistry at the future C-15 and... 

An efficient synthesis of ( )-yohimbine has been published by Stork and Guthikonda (222). Reaction of the pyrrolidine enamine of A-methylpiperidone with methyl 3-oxo-4-pentenoate gave 411 in good yield. Reduction of 411 with lithium in liquid ammonia furnished trans-TV-methyldecahydroisoquinolone 412. This building block was transformed in simple reaction steps to secoyohimbane 413 from which ( )-yohimbine could be obtained by oxidative cyclization with... 

Photooxidation of imino ether 540 in the presence of potassium cyanide led to the a-amino carboxamide 542, which could be transformed by simple reaction steps to 5-carboxamidoyohimbine (545) (271). 

Thus, the synthesis of dendrimers consists of two constantly repeating reaction steps. The first step deals with the linkage of a branching unit to two other units - the construction step. In the second reaction, groups are transformed, so that they can react further - the activation step. This procedure is also referred to as an iterative (repetitive) strategy.131... 

Halogenated aliphatics can be partially or completely degraded under anaerobic conditions through a transformation reaction called reductive de-halogenation. Often a co-metabolic degradation step, reductive dehalogenation... 

More likely, there are ephemeral intermediate species with short residence times, and the reaction proceeds in several steps with several intermediates. In such a reaction pathway, changes in the relative rates of the reaction steps can result in changes in the fractionation. Furthermore, there may be multiple pathways by which a chemical transformation can occur. For example, transformation of Se(IV) to Se(0) could proceed via simple abiotic reaction, or via uptake of FlSeOj by a plant, reduction to Se(-ll) within the plant, incorporation into amino acids, death and decay of the plant, release of the Se(-II), and oxidation to Se". The overall transformation, from Se(lV) to Se(0), is the same, but because the two reaction pathways differ greatly, the overall isotopic fractionation may be greatly different. 

The needles and twigs of the European yew contain 10-deacetylbaccatin III, shown at the bottom of the page. A few reaction steps can transform 10-deacetylbaccatin III into TAXOL . Instead of destroying an entire Pacific yew tree for a single treatment of TAXOL , scientists can now use the needles from a European yew. The parent tree is not harmed. 

In the case of compound 2a, the rate-determining step of the overall transformation is the methoxycarbonylation reaction (step 2). The similarity of k-2 and kg... 

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