Acylation of alcohols

The traditional method for transforming carboxylic acids into reactive acylating agents capable of converting alcohols to esters or amines to amides is by formation of the acyl chloride. Molecules devoid of acid-sensitive functional groups can be converted to acyl chlorides with thionyl chloride or phosphorus pentachloride. When milder conditions are necessary, the reaction of the acid or its sodium salt with oxalyl chloride provides the acyl chloride. When a salt is used, the reaction solution remains essentially neutral. 

This reaction involves formation of a mixed anhydride-chloride of oxalic acid, which then decomposes, generating both C02 and CO. 

Treatment of carboxylic acids with half an equivalent of oxalyl chloride can generate anhydrides.  

Carboxylic acids can be converted to acyl chlorides and bromides by a combination of triphenylphosphine and a halogen source. Triphenylphosphine and carbon tetrachloride convert acids to the corresponding acyl chloride.100 Similarly, carboxylic acids react with the triphenyl phosphine-bromine adduct to give acyl bromides.101 Triphenviphosphine-iV-hromosuccinimide also generates acyl bromide in situ.102 All these reactions involve acyloxyphosphonium ions and are mechanistically analogous to the alcohol-to-halide conversions that are discussed in Section 3.1.2. 

Acyl chlorides are highly reactive acylating agents and react very rapidly with alcohols and other nucleophiles. Preparative procedures often call for use of pyridine as a catalyst. Pyridine catalysis involves initial formation of an acyl pyridinium ion, which then reacts with the alcohol. Pyridine is a better nucleophile than the neutral alcohol, but the acyl pyridinium ion reacts more rapidly with the alcohol than the acyl chloride.103 

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Acylation of alcohols is often performed in the presence of an organic base such as pyridine. The base serves two purposes. It neutralizes the protons generated in the reaction and prevents the development of high acid concentrations. Pyridine also becomes directly involved in the reaction as a nucleophilic catalyst (see Section 8.5). 

Chiral N,N-disubstituted 2-(aminomethyl)pyrrolidines as catalysts for asymmetric acylation of alcohols 99YGK598. 

Figure 6.45 Activated acyl donors for irreversible acylation of alcohols.

Gotor-Fernandez, V., ReboDedo, F. and Gotor, V. (2007) Preparation of chiral pharmaceuticals through enzymatic acylation of alcohols and amines, in Biocatalysis in the Pharmaceutical and Biotechnology Industry, (ed. R.M. Patel), Dekker, Taylor and Francis, New York, Chapter 7, pp. 203-248. 

Sparteine 1 was also used in a palladium complex-catalyzed enantios-elective benzoylation of alcohols using monoxide and the organobismuth(V) compound (Scheme 37). The carbonylative acylation of alcohols using carbon monoxide (CO) is known to be an alternative tool for the prepar-... 

The reactive 1-acyl-3-alkylimidazolium species also plays a role in acylation of alcohols with carboxylic anhydrides or carboxylic acid chlorides using 1-substituted imidazoles as catalysts.[145] In this case the reactive species is formed in situ ... 

In related work a library of 1,458 peptide ligands and various metal salts was tested in hydrolysis reactions of (p-nitrophenyl)phosphates.35 An active substructure composed of polymer-bound histidine in combination with Eu3+ was identified by further dissecting the original hit structure. It needs to be pointed out that catalytically active polymer beads can also be tested for catalytic activity using IR-thermography. In a seminal paper this was demonstrated using 7,000 encoded polymer beads prepared by split-and-pool methods, specifically in the metal-free acylation of alcohols.36... 

To a much smaller extent non-enzymic processes have also been used to catalyse the stereoselective acylation of alcohols. For example, a simple tripeptide has been used, in conjunction with acetic anhydride, to convert rram-2-acctylaminocyclohexanol into the (K),(R)-Qster and recovered (S),(S)-alcohol[17]. In another, related, example a chiral amine, in the presence of molecular sieve and the appropriate acylating agent, has been used as a catalyst in the conversion of cyclohexane-1(S), 2(/ )-diol into 2(S)-benzoyloxy-cyclohexan-1 f / j-ol1 IS]. Such alternative methods have not been extensively explored, though reports by Fu, Miller, Vedejs and co-workers on enantioselective esterifications, for example of 1-phenylethanol and other substrates using /. vo-propyl anhydride and a chiral phosphine catalyst will undoubtedly attract more attention to this area1191. 

It must further be mentioned that the acylation of alcohols, phenols, and amines with acid chlorides (and also anhydrides) is now frequently carried out in pyridine solution instead of according to the older Schotten-Baumann method (action of acid chloride in aqueous-alkaline suspension). The hydrogen chloride is fixed by the pyridine. 

Although in recent years transesterification processes of racemic alcohols have received major attention, enzymatic acylation of amines for synthetic purposes is also being employed as a conventional tool for the synthesis of chiral amines and amides [31], using CALB as the biocatalyst in the majority of these reactions [31a]. The main difference between enzymatic acylation of alcohols and amines is the use of the corresponding acyl donor, because activated esters which are of utility... 

Dumeunier R, Marko IE (2004) On the role of triflic acid in the metal triflate-catalysed acylation of alcohols. Tetrahedron Lett 45 825-829... 

Other authors have described the lipase-catalyzed chemoselective acylation of alcohols in the presence of phenolic moities [14], the protease-catalyzed acylation of the 17-amino moiety of an estradiol derivative [15], the chemoselectivity in the aminolysis reaction of methyl acrylate (amide formation vs the favored Michael addition) catalyzed by Candida antarctica lipase (Novozym 435) [16], and the lipase preference for the O-esterification in the presence of thiol moieties, as, for instance, in 2-mercaptoethanol and dithiotreitol [17]. This last finding was recently exploited for the synthesis of thiol end-functionalized polyesters by enzymatic polymerization of e-caprolactone initiated by 2-mercaptoethanol (Figure 6.2)... 

The seminal work of Klibanov in the early 1980s [46,47] made it clear that enzymes can be used in hydrophobic organic solvents, although at the price of a severely reduced reaction rate [48, 49]. Indeed, many Upases, as well as some proteases and acylases, are so stable that they maintain their activity even in anhydrous organic solvents. This forms the basis for their successful application in non-hydrolytic reactions, such as the (enantioselective) acylation of alcohols and amines, which now are major industrial applications [50]. 

The effect of organic solvents on the lipase PS-mediated acylation of alcohol 40... 

Both aliphatic alcohols and phenols have been immobilized as esters of support-bound carboxylic acids. The esterification can be achieved by treatment of resin-bound acids with alcohols and a carbodiimide, under Mitsunobu conditions, or by acylation of alcohols with support-bound acyl halides (see Section 13.4). 

Standard solid-phase peptide synthesis requires the first (C-terminal) amino acid to be esterified with a polymeric alcohol. Partial racemization can occur during the esterification of N-protected amino acids with Wang resin or hydroxymethyl polystyrene [200,201]. /V-Fmoc amino acids are particularly problematic because the bases required to catalyze the acylation of alcohols can also lead to deprotection. A comparative study of various esterification methods for the attachment of Fmoc amino acids to Wang resin [202] showed that the highest loadings with minimal racemization can be achieved under Mitsunobu conditions or by activation with 2,6-dichloroben-zoyl chloride (Experimental Procedure 13.5). iV-Fmoc amino acid fluorides in the presence of DMAP also proved suitable for the racemization-free esterification of Wang resin (Entry 1, Table 13.13). The most extensive racemization was observed when DMF or THF was used as solvent, whereas little or no racemization occurred in toluene or DCM [203]. 

SCHOTTEN-BAUMANN REACTION. Acylation of alcohols with acyl halides in aqueous alkaline solution. 

Naemura, K. Fukuda, R. Konishi, M. Hirose, K. Tobe, Y. Lipase YS-catalyzed acylation of alcohols a predictive active site model for lipase YS to identify which enantiomer of a primary or a secondary alcohol reacts faster in this acylation. J. Chem. Soc. Perkin Trans 1, 1994, 1253. 

Scheme 7.15. Acylation of alcohols with electron-withdrawing groups in the p position [54, 55],...

Sano, T. Ohashi, K. Oriyama, T. Remarkably fast acylation of alcohols with PhCOCl promoted by TMEDA. Synthesis 1999, 1141-1144. Carrigan, M. D. Freiberg, D. A. Smith, R. C. Zerth, H. M. Mohan, R. S. A simple and practical method for large-scale acetylation of alcohols and diols using Bi(OTf)3. Synthesis 2001, 2091-2094. 

Ishihara, K. Kubota, M. Kurihara, H. Yamamoto, H. Scandium triflate as an extremely active Lewis acid catalyst in acylation of alcohols with acid anhydrides and mixed anhydrides./. Org. Chem. 1996, 61, 4560-4567. Zhao, H. Pendri, A. Greenwald, R. B. General procedure for acylation of tertiary alcohols Sc(OTf)3/DMAP reagent. /. Org. Chem. 1998, 63, 7559-7562. 

Orita, A. Tanahashi, C. Kakuda, A. Otera, J. Highly powerful and practical acylation of alcohols with acid anhydrides catalyzed by Bi(OTf)3. J. Org. Chem. 2001, 66, 8926-8934. 

Li, T.-S. Li, A.-X. Montmorillonite day catalysis. Part 10. K-10 and KSF-catalyzed acylation of alcohols, phenols, thiols and amines scope and limitation. J. Chem. Soc. Perkin Trans. 1 1998, 1913-1917. 

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