Front Matter

the case of glucose, maltose, trehalose or sucrose, only the primary 6–and/or 6’-

positions were acylated. A regioselective acylation of a secondary hydroxyl in su-

crose, the 2-OH, was recently reported by Plou et al. (1999). Using vinyl laurate and

the lipase ofHumicola lanuginosaadsorbed to Celite in DMSO, the main product

was 2-O-lauroylsucrose. However, the authors found also that the same reaction

occurred partially in the presence of Celite, Eupergit, and even the simple

Na 2 PO 4 alone. These chemical acylations must therefore be taken into account in

acylations of hydroxyl-containing compounds with enol esters in polar solvents

using immobilized enzymes.

Subsequently, an enzymatic fructose transesterification was reported and per-

formed recently in a 2-L pilot-scale reactor (Coulon et al., 1999). After initial stu-

dies to remove the strong inhibitors of sugar ester synthesis, water and methanol as

byproducts (through raising the temperature, though that may cause stability pro-

blems), 2-methyl-2-butanol (b.p. 102 8 C) was used as solvent and the pressure

was reduced to 200 mbar. Using immobilizedCandida antarcticalipase as bioca-

talyst and oleic acid methyl ester as acyl donor, more than 90 % of fructose was

acylated, compared to only 50 % acylation at atmospheric pressure. This is ex-

plained by the evaporation of the reaction byproduct (methanol), which shifted

the equilibrium. After drying over molecular sieves in a bypass, the solvent could

be recycled in the reactor. Moreover, synthesis performed with an equimolar mixture

of both substrates also gave promising results: although the reaction rate was slower

than synthesis performed with an excess of fatty acid, the fructose monooleate con-

centration was still high, 44 g L–1rather than 56 g L–1. The concentration of residual

acyl donor was very low. Downstream processes for the recovery of pure fructose

monooleate were simplified in this case.Candida antarcticalipase also catalyzed

sugar synthesis very efficiently using rapeseed oil as acyl donor.

17.2.2 Lipase-mediated catalysis in the presence of adjuvants

A method for the synthesis of sugar fatty acid esters has been reported which is based

on a mainly solid-phase system. The acylation of a solid sugar with a fatty acid was

performed via lipase-mediated catalysis in the presence of a very small amount of

organic solvent (e.g., acetone, tetrahydrofuran ort-butanol) serving mainly as adju-

vant (see Table 2). In a typical experiment, the reaction mixture consisted of equi-

molar (typically 0.5 mmol) free carbohydrate and fatty acid and an organic solvent as

adjuvant (in a concentration of 100–300 %, w/w), to maintain a small catalytic

liquid phase. In hydrophobic-solvent systems mentioned in the previous chapter,

the liquid phase volume was several magnitudes larger. Although water generated

during esterification was removed by the addition of activated molecular sieves, this

method seemed unsuitable from an economic point of view.

With regard to solvent-free systems, to date, water removal is easier and has been

achieved in open test tubes, evacuationin vacuo, pervaporation using special mem-

branes, and dry gas bubbling. However, when organic solvents are present in the

reaction medium, the exclusive removal of water becomes difficult: low-boiling-

point solvents such as acetone,t-butanol or 2-methyl-2-butanol would also be re-

368 17 Enzymatic Synthesis and Modification of Glycolipids