Alcohols acylations, 4-dimethylaminopyridine

However, this method is appHed only when esterification cannot be effected by the usual acid—alcohol reaction because of the higher cost of the anhydrides. The production of cellulose acetate (see Fibers, cellulose esters), phenyl acetate (used in acetaminophen production), and aspirin (acetylsahcyhc acid) (see Salicylic acid) are examples of the large-scale use of acetic anhydride. The speed of acylation is greatiy increased by the use of catalysts (68) such as sulfuric acid, perchloric acid, trifluoroacetic acid, phosphoms pentoxide, 2inc chloride, ferric chloride, sodium acetate, and tertiary amines, eg, 4-dimethylaminopyridine. 

AC2O, AcCl, Pyr, DMAP, 24-80°, 1-40 h, 72-95% yield. The use of DMAP increases the rate of acylation by a factor of 10. These conditions will acylate most alcohols, including tertiary alcohols. The use of DMAP (4-N,N-dimethylaminopyridine) as a catalyst to improve the rate of esterification is quite general and works for other esters as well. 

Acyl triflamides are excellent acylating agents for alcohols and amines [52], Classical methods for carboxyl activation include formation of anhydrides, either homo- or mixed (e.g. phosphoryl, sulfonyl, etc.). Further activation is possible by adding 4-dimethylaminopyridine as catalyst [53], The active acylating agents are throught to be... 

Having proved the synthetic utility of their system, the authors subsequently evaluated a second supported base, 4-dimethylaminopyridine (AO-DMAP) 118, toward the acylation of 2° alcohols (Scheme 30). Employing a premixed solution of phenyl-l-ethanol 119, Et3N 14 and acetic anhydride 37 (0.33, 0.50, and 0.50 M) in hexane, reactions were conducted at room temperature and the effect of residence time evaluated (10-50 s). Using a 60 cm packed bed, the authors were able to obtain near quantitative conversions to 120 employing residence times <20 s, with flow reactions providing superior results to those obtained in analogous batch reactions. 

In general reaction of an alcohol with the appropriate anhydride or acid chloride in pyridine at 0-20 JC is sufficient. In the case of tertiary alcohols, acylation is very slow in which case a catalytic amount of 4-dimethylaminopyridine (DMAP) can be added to speed up the reaction by a factor of 10,000. Reaction of polyols with acyl chlorides (1.2 equiv) in the presence of hindered bases (2.0equiv) such as 2,4,6-collidine, diisopropylethylamine or 1,2,2,6,6-penta-methylpiperidine in dichloromethane at -78 °C leads to selective acylation of a primary alcohol. Primary alcohols can also be acylated selectively with isopro-penyl acetate or acetic anhydride in the presence of a catalytic amount of 1,3-dichlorotetrabutyldistannoxane 325.1 [Scheme 4.325].602 The catalyst 325.1 is available commercially or can be easily prepared by simply mixing dibutyltin oxide and dibutyldichlorostannane. No aqueous workup is necessary since the catalyst can be removed by simple chromatography. 

Acyl halides such as acetyl chloride react with water to regenerate the starting acid and they react with alcohols to yield esters. The reaction is often facilitated by the presence of a tertiary amine catalyst. Although this may function purely as a base to neutralize the hydrochloric acid which is formed in the reaction, bases such as dimethylaminopyridine can also activate the carboxyl group via the formation of the intermediate shown in Scheme 3.66. 

Dialkylaminopyridine (DAAP) catalysts are effective catalysts for acylation reactions. The acetylation of 1- methylcyclohexanol by acetic anhydride in the presence of triethylamine (TEA) was used as a test reaction for the polymer-bond DAAP. The conversion of the starting alcohol was measured by GC analysis using ethyl benzene as an internal standard. The initial rates were studied and a first order analyses showed that the rate constant for reaction (1) was 0.178 h for 4-dimethylaminopyridine (DMAP), and 0.120 h for PNIPAM-DAAP-MR. 

The reaction of acylphosphonate diesters with alcohols was reported early to lead to dialkyl hydrogenphosphonates and carboxylate esters In a more recent systematic study, conditions were developed to use acylphosphonates for the acylation of alcohols. It was found that 1,5-diazabicyclo [5.4.0] undec-7-ene (dbu) is a highly effective catalyst for acylation of alcohols by acylphosphonates. Two special aspects deserve mention (1) /er butyl alcohol could be acylated with diethyl benzoylphosphonate, in the presence of dbu and 4-dimethylaminopyridine, to give r r/-butyl benzoate in 57% yield (2) the primary hydroxy group of a diol (e.g. butane 1,3-diol) could be acylated fairly selectively in the presence of a secondary hydroxy group by this methodology (ratio of mono to diacyl product =88 12). 

With pyridine, acid chlorides and acid anhydrides yield A-acyl pyridinium salts 6 which are very reactive and sensitive to hydrolysis, unlike the quaternary salts 2-5 they are involved in the acylation of alcohols and amines in pyridine as solvent (Einhorn variant of the Schotten-Baumann reaction). However, 4-dimethylaminopyridine 12 (Steglich reagent) and 4-(pyrrolidin-l-yl)pyridine 13 are better acylation catalysts by a factor of 10 [46a]. Reactions with sulfonyl chlorides in pyridine proceed via A-sulfonylpyridinium salts 14. 

Carboxylic, and arylsulfonic acid halides react rapidly with pyridines generating 1-acyl- and 1-arylsulfonylpyridinium salts in solution, and in suitable cases some of these can even be isolated as crystalline solids. The solutions, generally in excess pyridine, are commonly used for the preparation of esters and sulfonates from alcohols and of amides and sulfonamides from amines. 4-Dimethylaminopyridine (DMAP) is widely used (in catalytic quantities) to activate anhydrides in a similar manner. The salt derived from DMAP and t-butyl chloroformate is stable even in aqueous solution at room temperature. " ... 

The basic nucleophilic poly(iV-alkylacrylamide)-immobilized catalyst 95 was prepared from the active estercopolymer 94 according to the chemistry shown in Eq. 37 [131]. This polymercontained a ca. 8 mol% loading of an analog of ATjAT-dimethylaminopyridine, anucleophilic catalyst that others had immobilized on insoluble cross-linked resins previously. This soluble version was shown to be effective as a catalyst for acylation of hindered alcohols and phenols (Eqs. 38 and 39). This catalyst contained an azo dye as a marker, which facilitated analysis of the phase separation of the polymer-immobilized catalyst. 

Vedejs and Chen [39] described an efficient non-enzymatic system able to approach the efficiency of some of the lipase methods in enantioselectivity. The reaction was carried out in a 2 1 ratio racemic secondary alcohol acylating agent, in contrast to Evans procedure. The pyridinium salt 8 was prepared by reaction of the chiral 4-dimethylaminopyridine (DMAP) 6 with the commercially available chloroformate 7. This pyridinium salt proved to be unreactive to secondary alcohols. The reactivity was found only upon strict experimental conditions addition of a Lewis acid, then the racemic alcohol, followed by addition of a tertiary amine gave the carbonate 9. Under these conditions (using MgBr2 and triethylamine), (2-naphthyl)- -ethanol was converted (room temperature, 20 h and 54% conversion) into the (S)-carbonate (82% ee). The recovered alcohol showed 83% ee, revealing a stereoselectivity s=39 for the process. A number of 1-arylalkanols have been resolved by this procedure in 20-44% yield (based on the racemic material) and 80-94% ee. For the use of this system in enantiodivergent reactions, see Schemes 6.1 and 6.32. 

Pyridine is more nucleophilic than an alcohol toward the carbonyl center of an acyl chloride. The product that results, an acylpyridinium ion, is, in turn, more reactive toward an alcohol than the original acyl chloride. The conditions required for nucleophilic catalysis therefore exist, and acylation of the alcohol by acyl chloride is faster in the presence of pyridine than in its absence. Among the evidence that supports this mechanism is spectroscopic observation of the acetylpyridinium ion. An even more effective catalyst is 4-dimethylaminopyridine (DMAP), which functions in the same way but is more reactive because of the electron-donating dimethylamino substituent. ... 

Lovastatin can be prepared by a fermentation process in the presence of a specific microorganism. Lovastatin can be converted to simvistatin. Hydrolysis of the ester followed by reclosure of the lactone gives the diol. The less-hindered alcohol can be selectively protected using the bulky t-butyldimethylchlorosilane. The free alcohol can be esterified by the acid chloride in the presences of dimethylaminopyridine acylation catalyst. The silyl ether can be selectively removed by treatment with tetrabutyl ammonium fluoride. The fluoride anion reacts at the silicon without hydrolyzing the lactone or ester. 

Esterification.—iVAWW -Tetramethylchloroformamidinium chloride, which is readily prepared from iVAWW -tetramethylurea and oxalyl chloride, is an efficient reagent for the esterification of carboxylic acids with alcohols yields of between 66 and 97% are obtained, and the method has also been applied to macrolide synthesis. A modified one-pot procedure for the esterification of carboxylic acids, using phenyl dichlorophosphate-dimethylformamide complex, has appeared. A simple method of activation of carboxylic acids, using methanesulphonyl chloride and triethylamine followed by addition of the alcohol and 4-dimethylaminopyridine, leads to esters in 57— 96% yield for thirteen examples. 0-Methylcaprolactim reacts with carboxylic acids to give methyl esters in 73—91 % yield for seven examples and 2-iodoethyl esters are prepared from acyl chlorides, ethylene oxide, and sodium iodide. Transesterification, catalysed by titanium(iv) alkoxides, provides an effective method for synthesis of esters. Diethyl trichloromethylphosphonate reacts with carboxylic acids to give ethyl esters via transesterification, in 52 to 98 % yield. ... 

Under these conditions, tertiary alcohols are not acylated. Most alcohols, including tertiary alcohols, can be acylated by the addition of DMAP (4-dimethylaminopyridine) and acetyl chloride to the reaction containing acetic anhydride and pyridine. In general, the addition of DMAP increases the rate of acylation by lO (eq 2) ... 

The mechanism of catalysis by dimethylaminopyridine is considered to involve an A -acylpyridinium ion. However, the identity of the anion also affects the reactivity so that a complete formulation requires attention to the ion pair characteristics of the acylpyridinium ion. Interestingly, in the presence of 4-dimethylaminopyridine, acetic anhydride is a more reactive acylating agent than acetyl chloride. This is a reversal of their normal reactivity. This reversal can be explained if the counterion acetate, a stronger base than chloride, is involved in deprotonating the alcohol. " ... 

Ester and Thioester Formation. These reactions occur through the same O-acylurea or anhydride active intermediate as in the amide coupling reactions, and the discussion of associated problems applies here as well. In general, alkyl and (particularly) aryl thiols can be efficiently coupled to carboxylic acids using DCC. Reactions of primary and secondary alcohols proceed reliably, but require the presence of an acylation catalyst. This is usually 4-Dimethylaminopyridine (DMAP), " (see also 1,3-Dicyclohexylcarbodiimide—4-Dimethylaminopyridine), but others have been used including 4-pyrrolidinopyridine and pyridine (.solvent) with catalytic p-Toluenesulfonic Acid The acylation of more hindered alcohols often re.sults in reduced yields however, even f-butanol can be acylated, providing a useful route to t-butyl esters. Various other carbodiimide derivatives have also been used in the preparation of esters. As with amides, which are not limited to intermolecular reactions, a wide variety of lactones can also be synthesized. ... 

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