Esters, preparation from alcohols

In the case of alcohols 12 and 13, the compounds were previously enantioresolved by means of CSP acid (IS,2R,4R)-(—)-2, and their ACs were determined by X-ray crystallography (entries 2 and 3 )- So, the ACs of CSDP acid esters prepared from alcohols 12 and 13 were established by chemical correlation. Notably, the CSDP acid esters of 12 and 13 were more effectively separated by HPLC on silica gel than the corresponding CSP acid esters ... 

Section 20 7 Esters occur naturally or are prepared from alcohols by Fischer estenfi cation or by acylation with acyl chlorides or acid anhydrides (see Table 20 3)... 

Plasticizers. About 2.5% of U.S. adipic acid consumed in 1988 was used in two basic types of adipic ester based plasticizers (195). Simple adipate esters prepared from Cg—alcohols are used especially as PVC plasticizers (qv). Eor special appHcations requiring low volatility or extraction resistance, polyester derivatives of diols or polyols are preferred. 

Aiyl esters, prepared from the phenol and an acid chloride or anhydride in the presence of base, are readily cleaved by saponification. In general they are more readily cleaved than the related esters of alcohols, thus allowing selective removal of phenolic esters. 9-Fluorenecarboxylates and 9-xanthenecarboxylates are also cleaved by photolysis. To permit selective removal, a number of carbonate esters have been investigated aryl benzyl carbonates can be cleaved by hydrogenolysis aryl 2,2,2-trichloroethyl carbonates, by Zn/THF-H20. 

Dimethoxybenzyl esters prepared from the acid chloride and the benzyl alcohol are readily cleaved oxidatively by DDQ (CH2CI2, H2O, rt, 18 h, 90-95% yield). A 4-methoxybenzyl ester was found not to be cleaved by DDQ. The authors have also explored the oxidative cleavage (ceric ammonium nitrate, CH3CN, H2O, 0°, 4 h, 65-97% yield) of a variety of 4-hydroxy- and 4-amino-substituted phenolic esters. ... 

Sulfonate esters are especially useful substrates in nucleophilic substitution reactions used in synthesis. They have a high level of reactivity, and, unlike alkyl halides, they can be prepared from alcohols by reactions that do not directly involve bonds to the carbon atom imdeigoing substitution. The latter aspect is particularly important in cases in which the stereochemical and structural integrity of the reactant must be maintained. Sulfonate esters are usually prepared by reaction of an alcohol with a sulfonyl halide in the presence of pyridine ... 

An advantage that sulfonate esters have over alkyl halides is that their preparation from alcohols does not involve any of the bonds to carbon. The alcohol oxygen becomes the oxygen that connects the alkyl group to the sulfonyl group. Thus, the configuration of a sulfonate ester is exactly the same as that of the alcohol from which it was prepared. If we wish to study the stereochemistry of nucleophilic substitution in an optically active substrate, for example, we know that a tosylate ester will have the same configuration and the same optical purity as the alcohol from which it was prepared. 

The most common plasticisers for PVC are phthalate esters prepared from aliphatic C8 alcohols. For high temperature applications higher molecular weight esters are used, e.g., from mellitic acid, which are resistant to volatilisation and are used for PVC interiors in cars (to prevent fogging of windscreens). 

The difficulties attendant with the direct preparation from alcohol and phosphorus halide of partial ester/partial halide systems may be overcome through the use of a variety of alternative approaches. We will briefly consider a few of these here. 

Both the alkyl halide and the alkoxide ion are prepared from alcohols. The problem then becomes one of preparing the appropriate alcohol (or alcohols) from the starting ester. This is readily done using lithium aluminum hydride. 

Fig (20) Bromide (172), prepared from alcohol (171) on alkylation yields (173) which is converted to (174) without any difficulty. Conversion of (174) to homoallylic bromide (175) is accomplished by the method of Julia. On alkylation followed by cyclization generated the tricyclic alcohol which undergoes dehydration to yield unsaturated ester (177) which is converted to unsaturated lactone (146) by epoxidation and elimination. 

Silane reacts with methanol to give the alkoxysUanes (MeO) SiH4 (n = 2, 3, 4) and with higher alcohols in the presence ofbase to give tetraalkoxysilanes (equation 8). This type of reaction is, however, rarely carried out as the alkoxysi-lanes produced are more conveniently prepared from alcohols and SiCU (which is more readily available than SiELi and much more easily handled). Similarly, silyl esters, Si(OCOR)4, are also usually prepared from polyhalogenosUanes rather than from silane and a carboxylic acid. 

Vinyl sulfones were prepared from PhS02Na and vinyl iodinium salts C=C—I Ph BF4 Sulfinate esters (RS(=0)0R were prepared from alcohols and sulfinyl chlorides, in the presence of Proton Sponge . " ... 

Phosphonates have also been prepared from alcohols and (Ar0)2P(=0)Cl, NEts and a TiCl4 catalyst. ° The reaction of (R0)2P(=0)H and aryl iodides with a Cul catalyst leads to aryl phosphonates. " Polymer-bound phosphonate esters have been used for olefination. Dienes are produced when allylic phosphonate esters react with aldehydes. 

Esters can be prepared from alcohols and activated forms of carboxylic acids by a variety of meth-ods. Carboxylic acid esters are mainly base labile and, therefore, they complement the acid sensitive ether protecting groups. A variety of esters can be used for simpler and for more complex problems. Here, only the most practical groups will be treated. The application of esters as protected forms is frequently complicated by the possibility of acyl migration which may occur under basic, acidic and also neutral conditions. ... 

The HPLC separation data of diastereomeric esters prepared from other racemic alcohols 24, 36, 38, 39, and 57-63 with MaNP acid (.S)-(+)-3 are listed in Table 9.3. It should be emphasized that for most alcohols, their diastereomeric MaNP esters are clearly separated with a values of 1.10-1.88. Phenylacetylene alcohol 57 was separable as the MaNP esters 65a/65b (a = 1.30, entry 3). Substituted cyclohexanols 58 and 59 were also effectively separated as MaNP esters (entries 4 and 5). Especially, the a value of trans-2-isopropylcyclohexanol MaNP esters 66a/66b is as large as 1.88, which is comparable to that of fhe menthol case. On the ofher hand, in the case of trans-2-methylcyclohexanol MaNP esters 67a/67b, the a value is relatively small, a = 1.21. These results indicate that the combination of a longer and larger alkyl group on one side and a smaller alkyl group on fhe ofher side leads to better separation of two diastereomers, as seen in 2-hexadecanol esters 54a/54b (see Fig. 9.15) and tro s-2-isopropylcyclohexanol MaNP esters 66a/66b. 

Dimethoxybenzyl esters prepared from the acid chloride and the benzyl alcohol are readily cleaved oxidatively by DDQ (CH2CI2, Fl20, rt, 18h, 90-95% yield). A... 

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