Imide coupling

An alternative to using van der Waals forces to organise molecules at surfaces is to covalently bond monomers. Haq and Richardson,34 for example, have attempted to develop PMDA-ODA oligomers using controlled imide coupling... 

Bitzer, T. and Richardson, N. V. Demonstration of an imide coupling reaction on a Si( 100)— 2x1 surface by molecular layer deposition. Applied Physics Letters 71, 1890-1892 (1997). 

PP-PA6 Blends Containing Dispersed Core-Shell Microparticles. In the third type of PP-PA6 blend system, the PP-g-MA blend com-patibilizer and rubber was completely substituted by maleic anhydride-grafted rubbers such as EPR-g-MA and SEBS-g-MA. As reported previously (22, 23) and schematically represented in Figure 4, imide-coupling at the PP-PA6 interface, and surface-tension gradient and immiscibility between PP, PA6, and rubber are responsible for the accumulation of the rubber at the PA6 microparticle surface, which results in microparticles with a PA6 core and a rubber shell. Like PP-g-MA blend compatibilizers, maleic anhydride-grafted rub-... 

N-oxide salts (HBTU and TBTU, respectively) [39], or from l-hydroxy-7-azabenzotriazole (HOAt) such as N-[(dimethylamino)-lH-l,2,3-triazolo[4,5-fe] pyridino-l-y]methylene]-N-methy]methanaminium tetrafluoroborate N-oxide (HATU) [40], are well established reagents. They are especially devoted to peptide coupling reactions due to their efficiency and the low degree of undesirable race-mization produced in the final peptide compared to the use of classical carbodi-imide-coupling methods. Therefore, as the polystyrene-supported HOBt is an often used polymeric reagent (Section 7.6.3) [41], its transformation in a supported HOBt and tetramethylurea-derived aminium salt analog to HBTU and TBTU resulted directly. Thus, the reaction of polystyrene-2% divinylbenzene copolymer resin P-HOBt (20) with tetramethylchloroformamidinium tetrafluoroborate (21) (4 equivalents) in the presence of triethylamine gave polymeric N-[(lH-benzotriazol-l-yl)(dimethylamino)methylene]-N-methylmethanaminium tetrafluoroborate N-oxide (P-TBTU, 22) (Scheme 7.6) [42],... 

Other methods of esterification that do not utilize a carbodi-imide coupling method have also been explored. For example, the stereoselective esterification of 20-(S)-camptothecin, a hindered 3 ° alcohol, with amino acid derivatives was accomplished with the use of scandium triflate (Sc(OTf)3) in high yields while retaining the integrity of the amino acid stereocenter (eq 9). ... 

Carbodi-imide-coupling to an aminoalkyl derivative of polyacrylamide... 

Polymerization by G—G Goupling. An aromatic carbon—carbon coupling reaction has been employed for the synthesis of rigid rod-like polyimides from imide-containing dibromo compounds and aromatic diboronic acids ia the presence of palladium catalyst, Pd[P(CgH )2]4 (79,80). 

Peptide synthesis requires the use of selective protecting groups. An N-protected amino acid with a free carboxyl group is coupled to an O-protected amino acid with a free amino group in the presence of dicydohexvlcarbodi-imide (DCC). Amide formation occurs, the protecting groups are removed, and the sequence is repeated. Amines are usually protected as their teit-butoxy-carbonyl (Boc) derivatives, and acids are protected as esters. This synthetic sequence is often carried out by the Merrifield solid-phase method, in which the peptide is esterified to an insoluble polymeric support. 

A key step in the synthesis of the spiroketal subunit is the convergent union of intermediates 8 and 9 through an Evans asymmetric aldol reaction (see Scheme 2). Coupling of aldehyde 9 with the boron enolate derived from imide 8 through an asymmetric aldol condensation is followed by transamination with an excess of aluminum amide reagent to afford intermediate 38 in an overall yield of 85 % (see Scheme 7). During the course of the asymmetric aldol condensation... 

Samarium(II) iodide also allows the reductive coupling of sulfur-substituted aromatic lactams such as 7-166 with carbonyl compounds to afford a-hydroxyalkylated lactams 7-167 with a high anti-selectivity [74]. The substituted lactams can easily be prepared from imides 7-165. The reaction is initiated by a reductive desulfuration with samarium(ll) iodide to give a radical, which can be intercepted by the added aldehyde to give the desired products 7-167. Ketones can be used as the carbonyl moiety instead of aldehydes, with good - albeit slightly lower - yields. 

The low conductance of the Ji-conjugated system is due to the nodes at the imide nitrogen positions. Several additional trends have been observed. The conductance of pyridine-terminated PBI derivatives (P-PBI) is typically a factor of 2 larger than the thiol-terminated ones (T-PBI). Although the Au-S bond provides a stronger electronic coupling than the Au-N bond, the T-PBI has two extra methylene... 

Based on the established mechanism for titanium-catalyzed hydroamination, the authors propose a reversible reaction between a titanium imide complex and the alkyne to form metalloazacyclobutene 86, which in turn undergoes 1,1-insertion of the isonitrile into the Ti-C bond. The generated five-membered ring iminoacyl-amido complex 87 with the new C-C bond is protonated by the primary amine to afford the desired three-component coupling product, with regeneration of the catalytic imidotitanium species. Very recently, titanium-catalyzed carbon-carbon bond-forming reactions have been reviewed.122... 

The problem of the nucleophilicity of amides in glycosylation reactions is not limited to the sulfoxide method and has been shown to result in the formation of glycosyl imidates from intermolecular reaction with activated donors. It appears that this problem may be suppressed by the prior silylation of the amide [348,349]. Accordingly, it may be sufficient to operate the sulfoxide method with an excess of triflic anhydride when amides are present so as to convert all amides into O-triflyl imidates, which are then hydrolyzed on work-up. Despite these problems, several examples have been published of successful sulfoxide glycosylation reactions with acceptors carrying remote peptide bonds [344,345] and with donors coupled to resins via amide-based linkages [346,347], with no apparent problems reported. Sulfonamides and tertiary amides appear to be well tolerated by the sulfoxide method [340,350],... 

The use of N-glycosyl amides as glycosyl donors was reported by Pleuss and Kunz [240]. These amides were activated by Ph3P and CBr4 to produce bromo-N-imidates, which were spontaneously converted into the corresponding bromide concomitant with releasing nitrile, and then coupled with alcohols by activation with AgOTf (Scheme 5.86). 

FIGURE 2.11 Structures and nomenclature of compounds that serve as auxiliary nucleophiles. Generation of activated esters. Substituted hydroxamic acids are sometimes added to carbodi-imides or other reactions to improve the efficiency of couplings. The additive suppresses side reactions by converting activated species into activated esters (see Section 2.10) before they have time to undergo secondary reactions, p(Me2SO) HOBt 9.30, HOAt 8.70. 

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