Urethane linker

The same urethane linker group that is a feature of conjugates 138 and 139 (Section 5.03.12) has also been used to provide SIN-1 13 conjugates of two vitamin E analogues, 6-tochopherol and Torolox , that undergo enzymatic bioactivation in the presence of porcine liver esterase to release nitric oxide <2006MI363>. 

Preformed polymers can also be employed to prepare imprinted core-shell particles [143]. The group of Chang recently prepared a poly(amic acid) bearing oestrone as a template molecule covalently bound to the polymer through a urethane linker (see Fig. 2). A layer of this polymer was subsequently deposited on silica particles (10 pm diameter) prefunctionalised with amino groups at their surface. Thermal imidisation of the polymer yielded finally a polyimide shell (thickness about 100 nm) on the silica particles. Subsequent template removal yielded the imprinted cavities, which exhibited selective rebinding of oestrone in HPLC experiments. 

A urethane linker, 40, was introduced by Sproat and Brown [109] as an alternative to the succinyl linkage and is prepared by reacting LCAA-CPG with tolylene-2,6-diisocyanate. A 19-mer oligonucleotide constructed on this urethane-linked CPG support was successfully deprotected by extended aqueous ammonia treatment at 56°C for 48 h. 

Fig. 4 Hydrogen-bonded thermoplastic elastomer based on self-complementary interactions of ureido-pyrimidinone (UPy). (a) Poly(ethylene-co-butylene) with OH end groups, (b) Poly(ethyl-ene-co-butylene) functionalized with UPy. (c, d) Lateral intraactions through tt—tt stacking of UPy and hydrogen bonding of urethane linkers, (e) AFM image of nanofibers formed through these lateral interactions. Reprinted with permission friun [90-92]. Copyright 2000, 2006, 2008 Wiley-VCH Verlag GmbH, Wiley Periodieals, Ine., and American Chemical Society...

The linker group is immobilized as a urethane to the amino-functionalized carrier (59). It facilitates the attachment of a variety of molecules such as alkyl halides, alcohols or amines bound as carboxylic acid esters and amides. 

In a parallel study, Wipf and Fritch11041 have shown that also urethane-protected (Boc), and even amino acid segments, are tolerated as acyl compounds on the aziridine nitrogen. The best results were obtained with alkylcopper reagents derived from CuCN and an alkyl-lithium in the presence of boron trifluoride-diethyl ether complex. Some 6-alkylated compounds (11-15%) were isolated as well. This work was extended to a solid-phase procedure that resulted in resin-bound alkene isosteres that could immediately be used in further peptide synthesis.11051 For this purpose, the 2-nitrophenylsulfonyl (oNbs) group was used for nitrogen protection and aziridine activation. It could be readily cleaved with benzenethio-late, which was compatible with the acid-sensitive Wang linker used. 

For the complete solid-phase synthesis of H- and A-Ras peptides, the hydrazide linker turned out to be the linker of choice (14). This linker is cleaved by oxidation to an acyldiazene that is then attacked by a suitable nucleophile. The linker is orthogonal to classic urethane protecting groups such as Boc, Fmoc, and... 

An example of a non-covalent MIP sensor array is shown in Fig. 21.14. Xylene imprinted poly(styrenes) (PSt) and poly(methacrylates) (PMA) with 70 and 85% cross-linker have been used for the detection of o- and p-xylene. The detection has been performed in the presence of 20-60% relative humidity to simulate environmental conditions. In contrast to the calixarene/urethane layers mentioned before, p-xylene imprinted PSts still show a better sensitivity to o-xylene. The inversion of the xylene sensitivities can be gathered with PMAs and higher cross-linker ratios. As a consequence of the humidity, multivariate calibration of the array with partial least squares (PLS) and artificial neural networks (ANN) is performed, The evaluated xylene detection limits are in the lower ppm range (Table 21.2), whereas neural networks with back-propagation training and sigmoid transfer functions provide the most accurate data for o- and p-xylene concentrations as compared to PLS analyses. 

A recent example of the parallel synthesis of different quinazoline-2,4-dione derivatives (46) demonstrates how to combine the advantages of fluorous synthesis with those of solid-phase chemistry without using expensive perfluorinated solvents [22] (Scheme 3.22). In the beginning, a fluorous benzyl alcohol (47) is adsorbed on fluorous reversed-phase silica gel (FRPSG). Then, in a sequence of splits and reactions, the linker group, which remains bound to the FRPSG by fluorophilic interactions, is converted into a library of differently substituted carboxamido-urethanes (48). These are cyclized and the liberated quinazoline-2,4-diones (46) are eluted from the support with the fluorophobic solvents water and CHjCN/... 

Comparison of Diol Cross-Linkers in Castable Urethane Elastomers... 

There are regulatory and handling problems in using methylene bis(2-chloroaniline) as a chain extender (curative or cross-linker) for toluene diisocyanate (TDI)-terminated prepolymers, to produce urethane elastomers (1,2). There is, therefore, a strong interest in achieving similar elastomer properties with other curatives and methylene diphenyl diisocyanate (MDI)-terminated prepolymers(3,4). 

Most of the linkers described for the synthesis of 3 -aminoalkyl ohgonucleotides employ the carbamate (urethane) linkage to the polymer. A 2,2 -dithiodiethanol-derivatised silica gel support (Section 19.3.2.3) has been transformed by Asseline and Thuong [287] into the N-ro-hydroxyhexylcarbamate (Figure 19.12) via the consecutive treatments with l,T-carbonyldiimidazole and 6-amino-l-hexanol followed by the phosphoramidite synthesis of a DNA undecamer. Deprotection by 0.1 M DTT in concentrated aqueous ammonia unmasked the [3-mercaptoethyl carbamate, which fragments under basic conditions with the elimination of thiirane to furnish 3 -aminohexyl oligodeoxyribonucleotide. The structure of the latter has been confirmed by the reaction with acridine-9-isothiocyanate. 

Raw Materials for Urethane Coatings Isocyanate Components Di- and Polyhydroxy Components Chain Extenders and Cross-linkers Catalysts... 

Chain Extenders and Cross-linkers. In addition to the two principal components of most urethane coatings, isocyanate and polyol components, a number of di- or polyfunctional, active hydrogen components may be used as chain extenders or cross-linkers. The most important classes of compounds for this use are diols or polyols (monomers or oligomers), diamines, and alkanolamines. Typical examples of diols are ethylene, dlethylene, dlpropylene glycol, 1,4-butanedio1, 1,5-hexanediol, neopentyl glycol,... 

In the U.S. converting industry, one- and two-component solvent-based adhesives are still the most widely used, accounting for nearly 80% of adhesive formulations. Many of these adhesives are typically polyester-or polyether-based urethanes (see Chapter 4) with isocyanate fimctionaHty that cure by reacting either with atmospheric moisture or a cross-linker. Polyether-based urethanes, while less expensive than polyester-based, do not provide the same strength and do not perform well in some processing environments or end uses. 

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