Regioselective esterification

The primary OH group can be selectively blocked by the bulky triphenyl-methyl (trityl) moiety, followed by esterification at the secondary OH groups and removal of the protecting trityl group. Thus 2,3-di-O-acetyl cellulose has been obtained by this procedure. Moreover, regioselectively substituted mixed cellulose esters, acetate/propionate, were prepared by subsequent acy-... 

Figure 12 Lipase-catalyzed regioselective esterification of glucose in a mixed...

The esterification can also be carried out as a one-pot reaction without isolation of the azolide for example, an 80% yield of crocetin bis(/ -D-glucosyl ester) was obtained via the triazolide. 1953 Reactions of 8 -apo-/ -carotene-8 -oic acid with D-galactose or lactose were claimed to proceed also with a high degree of regioselectivity.11963... 

The regioselective esterification at position 6 of a-D-glucose and a-D-glucopyrano-sides with fatty acids [81] is readily achieved by use of Novozym 435, in accordance with Scheme 8.56. 

A unique Pd(II)-promoted ortfto-esterification of 2,5-diphenyloxazole has been described [26], When 2,5-diphenyloxazole was heated with 2.5 equivalents of Pd(OAc>2 in acetic acid and CCLt, regioselective palladation took place, giving rise to arylpalladium(n) o-complex 31 in almost quantitative yield. The regioselectivity observed reflects the strong coordination ability of the oxazole nitrogen atom to the palladium atom. Complex 31 was then dissolved in MeOH-THF (1 1) and the solution was stirred at 0 °C to produce 2-(5 -phenylthiazol2-yl)-benzoate 32. A... 

At first sight, it appears that it should be feasible to prepare such esters regioselectively using a similar biocatalytic approach to that employed for the 6- and 7-amino acylation of 6-APA and 7-ADCA shown above. Unfortunately, owing to the poor nucleophilicity of alcohols, biocatalytic esterification in aqueous media is far more challenging than amida-tion. Therefore, it was not until the pioneering work of Klibanov and co-workers, who first demonstrated the use of enzymes in neat organic solvents, that this option became viable (see Section 1.4). 

Scheme 1.20 Regioselective esterification of lobucavir (PCL Pseudomonas cepacia lipase, now known as Burkholderia cepacia)...

Regioselective esterification of the 8-hydroxyl group of accumulated monacolin J, produced using a truncated lovastatin biosynthetic pathway, could provide a viable biocatalytic route to simvastatin. With this aim, Xie and Tang cloned and expressed the acyl transferase LovD from the lovastatin biosynthetic pathway into E. colv, they found... 

III. Reagent-Dependent Regioselective Control in Multiple Carbohydrate Esterifications... 

Gu J, Ruppen ME, Cai P. (2005) Lipase-catalyzed regioselective esterification of rapamycin Synthesis of temsirolimus (CCl-779). Org Lett 7 3945-3948. 

CRL, HLL AOT-isooctane Regioselective esterification of butane-1,3-diol with oleic acid [32]... 

Saponins (e.g., asiaticoside, 17) are widely distributed in plants as well as in some lower marine animals and possess a broad range of biological activities [62a], Examples of acylation of these complex molecules have been reported by different authors. For instance, the regioselective esterification of 17 by CalB was quite remarkable, and only the 4" -0-acetate (17a) was isolated in 59% yields [66]. 

The regioselective esterification of other groups of saponins, e.g., a series of diosgenins [70], the haemolytic compound digitonin [71], and the so-called cardiac glycosides [72], have also been investigated by different groups. 

Flavonoid glycosides and their esters are an important group of natural compounds isolated from plants. They are widely used in pharmaceutical, cosmetic, and food preparations, and it is therefore not surprising that the search for new derivatives with improved antioxidant and antimicrobial activity, or-more sim-ply-with more suitable physico-chemical properties for specific applications (i.e., increased stability and/or solubility), has identified enzyme-catalyzed regioselective esterification as a particularly promising approach to the target molecules [21a],... 

Following the first examples of the acylation of adenosine and uridine [2], the regioselective lipase-catalyzed esterification of these important molecules has been extensively investigated by Gotor s group [21g]. Attention has been mainly focused on deoxynucleosides - compounds of special interest in medicinal chemistry because of their antiviral and antitumoral properties. Some significant examples are discussed below. 

Finally, as a specific example, the regioselective esterification of ribavirin (24) deserves to be discussed (Figure 6.6) [83]. This compound is a powerful antiviral agent used to treat hepatitis C. In order to overcome some significant side effects, it has been suggested that its administration in the form of a prodrug might improve its pharmacokinetic profile. Indeed a series of predinical evaluations showed that the alanine ester 24a possesses improved bioavailability. In... 

Despite the simple structure of a glucopyranoside linked to a colchicine or thio-colchicine moiety, subtilisin-catalyzed acylation of colchicoside and thiocolchico-side still represents the only example of regioselective esterification of alkaloid... 

More recently, a similar approach was proposed to explain the regioselectivity displayed by Candida antartdca lipase A in the esterification of quinic and shikimic acid derivatives [99]. 

Regioselective esterification and acetylation reactions of lactose were described by Thelwall (1982). These selective reactions produced partially protected derivatives which were of value in the further modifica-... 

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