Temperature acylation

During this study it was however found that the multiple esterification processes were highly dependent on the acyl reagent used. Different protection patterns could be acquired from the same starting material by control of temperature, acyl reagents, reagent mole ratio, and solvent polarity (Scheme 8, 9,10). 

Mizugaki et al. studied the effects of the reaction temperature, acylating agents, bases, and the molar ratio of IV-acetylcysteamine (303) with respect to acid to prevent the formation of the trans derivative 305 (82CPB206). With DBU and a mixed anhydride of ds-2-octenoic acid (302), only small amounts of cis-304 and trans esters 305 and 306 were obtained, and the extent of isomerization was more than 50%. 

The process has been extended to the synthesis of diethyl l-fluoro-2-oxoalkylphosphonates. Thus, low-temperature acylation of diethyl l-(hydroxycarbonyl)-l-fluoromethylphosphonate with carboxylic acid chlorides in the presence of -BuLi in THF followed by decarboxylation provides an expeditious route for the synthesis of diethyl l-fluoro-2-oxoalkyIphosphonates in good yields (66-78%, Scheme 7.27). ° ... 

The reaction occurs at room temperature or in refluxing benzene or toluene and yields are generally in the range 65-90%. The reagent also decarbonylates aroyl chlorides to aryl chlorides (ArCOCl — ArCI) at relatively low temperatures. Acyl halides of type (1) are converted into olefins 13... 

Reduction of azides. Thi temperature. Acyl azides give Reductive amination. ... 

More complex alkoxy substituents may be introduced by an acylation-alcoholysis sequence (Scheme 2), [3] Typically, this method involves the conversion of the lithium acyl metalate into the tetramethylammonium salt 5 which upon low-temperature acylation affords the thermolabile acyloxycarbene... 

We have previously shown that plant acyl-ACP desaturases can function in E. coli to produce novel monounsaturated fatty acids (3, 5). In addition, we have found that an E. coli unsaturated fatty acid auxotroph can be complemented by the expression of acyl-ACP desaturases, and the effectiveness of this complementation is dependent on the fatty acid chain-length specificity of the acyl-ACP desaturase. In these experiments, an E.coli fab A (Ts), fadR strain was used, which requires exogenous unsaturated fatty acids for growth at all temperatures. Acyl-ACP desaturases were co-expressed with a plant-type ferredoxin in the E. coli unsaturated fatty acid auxotroph. The expression of plant-type ferredoxin has been demonstrated to stimulate the in vivo activity of an acyl-ACP desaturase in wild-type E. coli (5). 

Acetic acid and other carboxylic acids are protonated in superacids to form stable carboxonium ions at low temperatures. Cleavage to related acyl cations is observed (by NMR) upon raising the temperature of the solutions. In excess superacids a diprotonation equilibrium, indicated by theoretical calculations, can play a role in the ionization process. 

Chiral 2-oxazolidones are useful recyclable auxiliaries for carboxylic acids in highly enantioselective aldol type reactions via the boron enolates derived from N-propionyl-2-oxazolidones (D.A. Evans, 1981). Two reagents exhibiting opposite enantioselectivity ate prepared from (S)-valinol and from (lS,2R)-norephedrine by cyclization with COClj or diethyl carbonate and subsequent lithiation and acylation with propionyl chloride at — 78°C. En-olization with dibutylboryl triflate forms the (Z)-enolates (>99% Z) which react with aldehydes at low temperature. The pure (2S,3R) and (2R,3S) acids or methyl esters are isolated in a 70% yield after mild solvolysis. 

The tryptophan side-chain can also be introduced using methyl 2-ethoxy-1-nitroacrylate as an clcctrophile[l 7b]. Vinylation occurs at room temperature by addition elimination. Reduction by SnCl2 followed by acylation generates A -acyl-o(, 3-dehydrotryptophans. 

This experiment examines the effect of reaction time, temperature, and mole ratio of reactants on the synthetic yield of acetylferrocene by a Eriedel-Crafts acylation of ferrocene. A central composite experimental design is used to find the optimum conditions, but the experiment could be modified to use a factorial design. 

Acylated Corticoids. The corticoid side-chain of (30) was converted iato the cycHc ortho ester (96) by reaction with a lower alkyl ortho ester RC(OR )2 iu benzene solution ia the presence of i ra-toluenesulfonic acid (88). Acid hydrolysis of the product at room temperature led to the formation of the 17-monoesters (97) ia nearly quantitative yield. The 17-monoesters (97) underwent acyl migration to the 21-monoesters (98) on careful heating with. In this way, prednisolone 17a,21-methylorthovalerate was converted quantitatively iato prednisolone 17-valerate, which is a very active antiinflammatory agent (89). The iatermediate ortho esters also are active. Thus, 17a,21-(l -methoxy)-pentyhdenedioxy-l,4-pregnadiene-liP-ol-3,20-dione [(96), R = CH3, R = C Hg] is at least 70 times more potent than prednisolone (89). The above conversions... 

Recently, the use of Hpase enzymes to iateresterify oils has been described (23). In principle, if a 1,3-speciftc Hpase is used, the fatty acid ia the 2 position should remain unchanged and the randomization occur at the terminal positions. However, higher temperatures, needed to melt soHd fats, may cause a 1,2-acyl shift and fatty acids are scrambled over all positions. 

MPD-1 fibers may be obtained by the polymeriza tion of isophthaloyl chloride and y -phenylenediamine in dimethyl acetamide with 5% lithium chloride. The reactants must be very carefully dried since the presence of water would upset the stoichiometry and lead to low molecular weight products. Temperatures in the range of 0 to —40° C are desirable to avoid such side reactions as transamidation by the amide solvent and acylation of y -phenylenediamine by the amide solvent. Both reactions would lead to an imbalance in the stoichiometry and result in forming low molecular weight polymer. Fibers are dry spun direcdy from solution. 

Carboxylic acid hydiazides are prepared from aqueous hydrazine and tfie carboxylic acid, ester, amide, anhydride, or halide. The reaction usually goes poody with the free acid. Esters are generally satisfactory. Acyl halides are particularly reactive, even at room temperature, and form the diacyl derivatives (22), which easily undergo thermal dehydration to 1,3,4-oxadiazoles (23). Diesters give dihydtazides (24) and polyesters such as polyacrylates yield a polyhydrazide (25). The chemistry of carboxyhc hydrazides has been reviewed (83,84). 

Reaction 21 is the decarbonylation of the intermediate acyl radical and is especially important at higher temperatures it is the source of much of the carbon monoxide produced in hydrocarbon oxidations. Reaction 22 is a bimolecular radical reaction analogous to reaction 13. In this case, acyloxy radicals are generated they are unstable and decarboxylate readily, providing much of the carbon dioxide produced in hydrocarbon oxidations. An in-depth article on aldehyde oxidation has been pubHshed (43). 

Physical Properties. Almost all Hquid diacyl peroxides (20) and concentrated solutions of the soHd compounds are unstable to normal ambient temperature storage many must be stored well below 0°C. Most of the soHd compounds are stable at ca 20°C but many are shock-sensitive (187). Other physical constants and properties have been reviewed (187,188). The melting poiats and refractive iadexes of some acyl peroxides are Hsted ia Tables 10-12. 

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