In view of the additional steps required for the introduction of the protection groups,
their removal and finally their product purification, it was decided to study in detail
the possibilities for using Strategy 2, i.e., the direct enzymatic acylation of glycerol.
In principle, it should be straightforward to extend the above-described method to
the production of regioisomerically pure 1(3)-sn-monoacylglycerols. Unfortunately,
however, under these conditions the products were produced in only low to moderate
yields. Initially, and in view of the high regioselectivities displayed by many lipases
with regard to the primary positions of glycerides, it seemed possible to increase the
yield of 1(3)-sn-monoacylglycerols simply by employing an excess of glycerol, and
an increase in yield to 60–70 % was indeed achieved in this manner (Figure 13).
Nonetheless, even with a 10- fold excess of glycerol, the content of 1(3)-sn-mono-
acylglycerols in the reaction mixture did not exceed 70 %. Clearly, the primarily
produced monoacylglycerols are excellent substrates for the lipase catalysts and
are rapidly converted into the corresponding 1,3-sn-diacylglycerols. Another disad-
vantage is that no triacylglycerols, i.e. natural fats and oils can be employed as acyl
donors because in this way, mixtures of regioisomeric monoglycerides would be
produced automatically. It became apparent that a practical and useful method
for the preparation of regioisomerically pure 1(3)-sn-monoacylglycerols had to ful-
fil the requirements outlined in Figure 14.
108 6 Lipase-Catalyzed Synthesis of Regioisomerically Pure Mono- and Diglycerides
Figure 13. Approaches to 1(3)-sn-monoglycerides.
Figure 14. Isomerically pure 1(3)-sn-monoacylglycerols: process requirements.