Coupling-addition

First of all, there are the two products of O-coupling addition of methoxide ion to the diazonium ion, the (Z)- and (jE)-diazo methyl ethers. As discussed in Section 6.2, they are formed in reversible reactions with half-lives of the order of a fraction of a second (Z) to a minute (E). The two diazo ethers are, however, decomposed rapidly to the final dediazoniation products. We show in Scheme 8-47 the products obtained by Broxton and McLeish (1983 b) in the dediazoniation of 4-chloro-3-nitrobenzenedi-azonium ion (8.64) with methoxide ion in CH3OH. The products are 4-chloro-3-nitro-anisole (8.65, 49 9o), 2-chloro-nitrobenzene (8.66, 449o), and 2-nitroanisole (8.67). 

An example of the use of 1,4-butanediol diglycidyl ether for the activation of soluble dex-tran polymers is given in Chapter 25, Section 2.3. One end of the fezs-epoxide reacts with the hydroxylic sugar residues of dextran to form ether linkages, which terminate in epoxy functionalities. The epoxides of the activated derivative then can be used to couple additional mol-ecules-containing nucleophilic groups to the dextran backbone. 

Three-component coupling. Addition of AIBN to a benzene solution of an allylic stannane, an alkyl iodide, and an electron-deficient alkene initiates a radical process that results in 1,2-addition of the alkyl and allyl groups to the alkene.4 Example ... 

LA Carpino. l-Hydroxy-7-azabenzotriazole. An efficient peptide coupling additive. J Am Chem Soc 115, 4397, 1993. 

This topic has been reviewed, for the period up to 1967, in some detail by Berlin and Hellwege.<1969,144) Various interesting examples will be considered later but many aspects have been discussed in Section III together with stereochemical aspects of spin-spin couplings. Additional instances will now be considered. 

Unfortunately, A-(9-fluorenylmethoxycarbonyl)aziridine-2-carboxylic acid cannot be used in peptide synthesis, since N-deprotection of the respective peptides with secondary amines leads to oxazoline or dehydroamino acid side products. Similarly, N-(tert-butoxy-carbonyl)aziridine-2-carboxylic acid is inappropriate due to the instability of the aziridine moiety to TFA treatment. Attempts to convert A-tritylaziridine-2-carboxylic acid into homogenous and stable active esters as useful intermediates in peptide synthesis leads to positive results only in the case of the pentafluorophenyl ester. 47 Consequently, this active ester seems to be the method of choice for acylating peptides. The related Abhydroxysuc-cinimide and A-3-hydroxy-4-oxo-3,4-dihydro-l,2,3-benzotriazine ester could not be isolated in pure form and have therefore been used as crude products. 47 Access to 2-carbonylazir-idine peptides is also possible by carbodiimide-mediated coupling. Additionally, alkylamides of A-tritylaziridine-2-carboxylic acid are prepared by the azide method,1 5 yet this method fails in peptide coupling steps. 85 ... 

Unlike LS coupling, additional classification of states of the subshell of equivalent electrons in jj coupling in the cases of practical interest causes no problems. In fact, it is well known [18] that for the degenerate states of the jN configuration, at j < 7/2, to be classified it is sufficient only to use additionally the seniority quantum number v. 

The intramolecular coupling of an aldehyde and alkene at —78 °C, promoted by Sml2, in the presence of a ketone, has led to a tandem coupling-addition process. This reaction has proved to be synthetically very useful for the preparation of poly-oxygenated compounds (equation 139)523. This reductive coupling process may also be promoted in dry methanol,... 

The coupling reaction is perhaps the most useful among the different electrochemical reaction types, since it normally has few or no counterparts in conventional laboratory practice. It provides a simple and direct route to bifunctional dimeric compounds from monofunctional monomeric compounds. Anodic coupling processes are formally of two kinds, either a coupling-elimination or a coupling-addition reaction, as shown in Eq. (10) and (11). 

In many respects, electrochemical coupling is the most useful of the reaction types described here, since many of its varieties have few or no counterparts in conventional laboratory practice. Bifunctionalization of dimeric carbon skeletons from monomers is perhaps the most attractive practical synthetic procedure among the myriads of possibilities. Two simple versions of anodic coupling exist formally, namely coupling-elimination (29) and coupling-addition (30). 

D. Coupling-addition reactions [eqn (30) Coupling with addition CH30 0 6 See reaction... 

The coupling addition of crotyl bromide to a triad of conformationally unrestricted a-oxy aldehydes in water, aqueous THF, and anhydrous THF has been examined. The proportion of. sy/z-isomers reaches a maximum (syn anti = 5.6 1) when the neighboring hydroxyl group is unprotected and water is the reaction medium (Scheme 33). Crotyl bromide adds to the hydroxy aldehyde with a preference for the adoption of the cyclic chelated transition states 31 and 32 (Scheme 34). 

As in previous years, the applications of Grignard reagents in organic synthesis have been a major area of research in 2008. Recent reports can be divided broadly into fundamental mechanistic studies,26,27 metal-catalysed coupling, addition and substitution reactions,28 33 the exploration of novel types of reactions involving Grignards34 40 and the development of new... 

Scheme 60 3-Iodo furans by sequential Sonogashira coupling-addition-cycloconden-sation [114]...

Scheme 61 Mechanistic rationale of sequential Sonogashira coupling-addition-cyclocondensations...

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