Ester precursor

The scope of Barton s thiohydroxamate ester chemistry has been significantly expanded by the finding that the intermediate alkyl radicals (R ) can be intercepted by a host of neutral molecules (see Scheme 25).42b 49c,52>53 Several different classes of compounds can thus be prepared from a common thiohydroxamate ester precursor. 

Fig. 31.11 I intermediates in the enantioselective hydrogenation of /7-amino ester precursors.

The ester precursor, CF2=CF—O—CF2CF(CF3)0—CF2CF2—COOMe, is a product of the DuPont Company. 

The desired long spacer necessary to allow multiple branch construction, was obtained with commercial tetra(ethylene glycol) which was readily modified into aminoacetal 21 in five steps (Scheme 8). Using 21 as an amine source, poly(amidoamine) (PAMAM) dendrons were prepared according to published protocol using reiterative cycles of methyl acrylate and ethylenediamine reactions. The terminal amines of each generation (G = 1, G = 2 and G = 3, 22a-c) were obtained quantitatively from each of the half generation methyl ester precursors. 

The imide-aryl ether copolymers were prepared via the poly(amic alkyl ester) route [53-56]. This intermediate route to polyimide is befleved to be more versatile than the poly(amic acid) analogue due to its enhanced solubility. Volksen et al. [53,54] reported that up to 75 % of a cosolvent could be used along with NMP, which is an important consideration in copolymerization of chemically dissimilar block copolymers. Furthermore, the poly(amic alkyl ester) precursor to the... 

The characteristics of selected copolymers, prepared in the fully imidized form and in the poly(amic alkyl ester) precursor to the polyimide, are shown in Table 10. The weight percentage, or loading, of the labile blocks in the copoly -mers was intentionally maintained low ( 20 wt%) in order to produce discrete... 

Experimentally a cyanate ester precursor mixture consisting of BPEC, 1 wt % BPE, and 100 ppm cobaltacetylacetonate was prepared and subsequently mixed with the cyclohexane phase separating solvent [86]. Essentially the same procedure as for the epoxy is used for sample preparation with the difference that the curing was done at 80 °C and post drying at 240 °C. 

Limited carbonyl NMR data are available for these anomeric amides. However, carbonyl shifts for hydrazines 217 and 218 were on average 3 ppm higher than their hydroxamic ester precursors. This reflects a higher degree of residual amide resonance in the hydrazines relative to A-acyloxy-iV-alkoxyamides where the difference was closer to 8.0 ppm. As reported for A-acyloxy-A-alkoxyamides (see Section IV.B.2), analysis of variance in the hydrazine and hydroxamic ester shifts indicates that substituents affect the hydroxamic ester carbonyl shifts ( 2.6) more than those of the hydrazines ( 1.3 ppm). 

The hydrolysis of other esters that show similar low selectivity for trapping by N3, CP or Br most likely proceeds through preassociation. This includes 46e, 46f, 71b, and 71c, (Schemes 28 and 29) and ester precursors of 75e and 75f (Scheme 32).5 -6 2,89 gjjj McClelland showed that a large... 

Increases in substrate levels that were accidentally produced by metabolic engineering also resulted in an olfaction-detectable increase in the methyl benzoate emission in transgenic carnation [30]. The metabolic flux from the anthocy-anin pathway was redirected towards benzoic acid, the methyl ester precursor, by antisense suppression of the flavanone 3-hydroxylase. 

Methylene enol ethers are usually lower boiling than their ester precursors. Low molecular weight products can be easily lost in evaporation therefore the toluene must be removed with care. 

The intramolecular Heck reaction (type d cyclization) has been used to access 3-benzazepine derivatives from o-bromobenzyl substituted /3-amino ester precursors in excellent yields. A key feature though was the use of microwave irradiation in the poly(ethylene glycol), PEG3400, as the solvent <2007EJ0201>. 

The company is interested in producing the polymer in bulk for sale to several customers that have expressed an interest in the product. The polymer will be produced by heating the amino acid methyl ester precursor according to specially staged process conditions followed by base hydrolysis of the C-terminal ester and stoichiometric protonation with dilute acid as shown in Figure 1.2. 

Morse D. and Meighen E. A. (1984) Aldehyde pheromones in Lepidoptera. Evidence for an acetate ester precursor in Choristoneura fumiferana. Science 226, 1434-1436. 

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