Bulk Butyl ester

This tertiary ester was developed to reduce aspartimide and piperidide formation during the Fmoc-based peptide synthesis by increasing the steric bulk around the carboxyl carbon. A twofold improvement was achieved over the the standard Fbutyl ester. The Mpe ester is prepared from the acid chloride and the alcohol and can be cleaved under conditions similar to those used for the r-butyl ester. ... 

The methyl, ethyl, and butyl esters of acrylic and methacrylic acids are polymerized under the influence of heat, light, and peroxides. The polymerization reaction is exothermic and may be carried out in bulk for castings, or by emulsion, or in solution. The molecular weight decreases as the temperature and catalyst concentration are increased. The polymers are noncrystalline and thus very clear. Such resins are widely used because of their clarity, brilliance, ease of forming, and light weight. They have excellent optical properties and are used for camera, instrument, and spectacle lenses. 

Palladium black gave the highest yield (81%) in the reduction of enamide 88 with complete stereoselectivity for C-4 epimer 102. Proof that the selectivity observed was a steric effect associated with the bulk of the C-2 tert-butyl ester was obtained by repeating the reduction with methyl ester 89. Using palladium black as a catalyst under identical conditions to those used in the reduction of 88, a 70% overall yield of products 106 and 107 in a ratio of 1 13 (isolated yield/ratio after chromatographic purification) was obtained (Scheme 45). 

Steric bulk means that f-butyl esters are resistant to nucleophilic attack at the carbonyl group, and that includes hydrolysis under basic conditions (nucleophilic attack by HO"). But they do hydrolyse relatively easily in acid, because the mechanism of hydrolysis of f-butyl esters in acid is quite different. It does not involve nucleophilic attack at the carbonyl group and is a favourable Sjsjl reaction at the f-butyl group (Chapter 17). 

Another type of initiator that has been evaluated for increasing polystyrene production rates are the multifunctional peroxides. Examples include 2,2-bis [4,4-bis(tert-butylperoxy)cyclohexyl]propane (I) [9], peroxyfumaric acid, 0,0-te/Y-butyl O-butyl ester (II) [10], ter t-butyl peritaconate (III) [11], and poly (monopercarbonates) (IV) (Figure 7.4) [12]. Although all of these initiators indeed show extremely fast production rates of high MW polystyrene, they all suffer from a flaw, i.e. the polystyrene produced is branched and special precautions must be taken to keep the continuous bulk polymerization reactors from fouling [13]. This is likely why none are currently used commercially for polystyrene manufacture. 

The synthesis of the chiral copper catalyst is very easy to reproduce. The complex catalyses the asymmetric alkylation of enolates of a range of amino acids, thus allowing the synthesis of enantiomeric ally enriched a,a disubstituted amino acids with up to 92% ee. The procedure combines the synthetic simplicity of the Phase Transfer Catalyst (PTC) approach, with the advantages of catalysis by metal complexes. The chemistry is compatible with the use of methyl ester substrates, thus avoiding the use of iso-propyl or ferf-butyl esters which are needed for cinchona-alkaloid catalyzed reactions[4], where the steric bulk of the ester is important for efficient asymmetric induction. Another advantage compared with cinchona-alkaloid systems is that copper(II)(chsalen) catalyses the alkylation of substrates derived from a range of amino acids, not just glycine and alanine (Table 2.4). 

Several dialkoxyaluminum complexes have been applied to the reaction of methyl acrylate (11a) and cyclopentadiene (2)7. These studies also demonstrate that the aging time influences the arrangement of the catalytic species in solution. The enantiomeric excess observed in these reactions does not exceed 70% for methyl acrylate (11a). When the steric bulk in the ester alkyl group is increased, the enantiomeric excess can be improved for example, using ligand 18 the ethyl lib and /-butyl esters 11c produce ee s of 73% and 81%, respectively7. 

Competition between the two carboxy residues in the cyclization of alkyl 1,3-dihydro-l-hydroxy-a,a-disubstituted-3-oxo-l-benzoisofuranacetates with hydrazines can lead to phthalazinones and/or pyrazolones (Equation (39)). With ethyl esters a mixture of both products may be obtained, and while increased bulk at the a-position leads to the pyrazolone as the sole product, phthalazinones are the exclusive products from /-butyl esters <9lJOC2587). 

Except for t-butyl ester, the increase in the steric bulk has no significant affect (entries 1, 2, 4, Table 26.9). However, increase in the steric bulk of alkyl chain (Table 26.9) results in increase in asymmetric induction. On the other hand, branching of the alkyl chain leads to a drastic decrease in the reaction rate as well as in asymmetric induction (entry 11, Table 26.9). Separating the isopropyl group from the reaction center by a methylene group leads to both increase in the reaction rate and increase in asymmetric induction (entry 12, Table 26.9) [3]. 

Increa sing the bulkiness of the alkyl group from the esterifying alcohol in the ester also restricts the motion of backbone polymer chains past each other, as evidenced by an increase in the T within a series of isomers. In Table 1, note the increase in T of poly(isopropyl methacrylate) over the / -propyl ester and similar trends within the butyl series. The member of the butyl series with the bulkiest alcohol chain, poly(/-butyl methacrylate), has a T (107°C) almost identical to that of poly(methyl methacrylate) (Tg = 105° C), whereas the butyl isomer with the most flexible alcohol chain, poly( -butyl methaciylate), has a T of 20°C. Further increase in the rigidity and bulk of the side chain increases the T. An example is poly(isobomyl methacrylate)... 

Disproportionation increases in the series where the ester is methyl suggesting that this process is favored by increasing the bulk of the ester alkyl group. This trend is also seen for polymeric radicals (Section... 

By employing anionic techniques, alkyl methacrylate containing block copolymer systems have been synthesized with controlled compositions, predictable molecular weights and narrow molecular weight distributions. Subsequent hydrolysis of the ester functionality to the metal carboxylate or carboxylic acid can be achieved either by potassium superoxide or the acid catalyzed hydrolysis of t-butyl methacrylate blocks. The presence of acid and ion groups has a profound effect on the solution and bulk mechanical behavior of the derived systems. The synthesis and characterization of various substituted styrene and all-acrylic block copolymer precursors with alkyl methacrylates will be discussed. 

Also as noted above any substituents present have little effect upon such oxidations. In 2,2 -methylenedifuran (118) the rings are attacked simultaneously giving a tetramethoxy derivative.297 Even the bulk of the fert-butyl group has little effect.298 The only marked substituent effect is that exerted by an aromatic (benzene, thiophene, furan) residue which, if directly attached at the 2-position, promotes elimination instead of the addition of another methoxy group. The net process then becomes one of arylation, as when 2-(2-thienyl)furan (119) is oxidized to 120.298 There are reports that acetyl and carboxy groups can be ejected during oxidation, but that ester groups are usually retained.287... 

In the hydroxycyclopropanation of alkenes, esters may be more reactive than N,N-dialkylcarboxamides, as is illustrated by the exclusive formation of the disubstituted cyclopropanol 75 from the succinic acid monoester monoamide 73 (Scheme 11.21) [91]. However, the reactivities of both ester- as well as amide-carbonyl groups can be significantly influenced by the steric bulk around them [81,91]. Thus, in intermolecular competitions for reaction with the titanacydopropane intermediate derived from an alkylmagnesium halide and titanium tetraisopropoxide or methyltitanium triisoprop-oxide, between N,N-dibenzylformamide (48) and tert-butyl acetate (76) as well as between N,N-dibenzylacetamide (78) and tert-butyl acetate (76), the amide won in both cases and only the corresponding cyclopropylamines 77 and 79, respectively, were obtained (Scheme 11.21) [62,119]. 

Trialkylboranes react rapidly and in high yields with a-halo ketones,1546 a-halo esters,1549 a-halo nitriles,1550 and a-halo sulfonyl derivatives (sulfones, sulfonic esters, sulfonamides)1551 in the presence of a base to give, respectively, alkylated ketones, esters, nitriles, and sulfonyl derivatives.1552 Potassium /-butoxide is often a suitable base, but potassium 2,6-di-f-butyl-phenoxide at 0°C in THF gives better results in most cases, possibly because the large bulk of the two /-butyl groups prevents the base from coordinating with the R3B.1553 The trialkylboranes are prepared by treatment of 3 moles of an alkene with 1 mole of BH,... 

Hydrolysis (or saponification) of n -butyl acetate. Boil 4 5 g. of n-butyl acetate (Section 111,95) with 50 ml. of 10 per cent, sodium hydroxide solution under reflux until the odour of the ester can no longer be detected (about 1 hour). Set the condenser for downward distillation and collect the first 10 ml. of distillate. Saturate it with potassium carbonate, allow to stand for 5 minutes, and withdraw all the liquid into a small pipette or dropper pipette. Allow the lower layer of carbonate solution to run slowly into a test-tube, and place the upper layer into a small test-tube or weighing bottle. Dry the alcohol with about one quarter of its bulk of anhydrous potassium carbonate. Remove the alcohol with a dropper pipette and divide it into two parts use one portion for the determination of the b.p. by the Siwoloboff method (Section 11,12) and convert the other portion into the 3 5-dinitrobenzoate (Section III, 27) and determine the m.p. 

---