The application ﬁelds of lipases are not only those in lipid-modiﬁcation. Lipases are also very powerful tools fo organic synthesis, where non-lipid substrates are reacted. One of such applications is kinetic resolution and asymmetric synthesis, in which enantio- and stereospeciﬁcity of lipases are employed. Here, apart from oil chemistry, the useof lipases for the production chiral chemicals are described.
Figure 7-1-1: Achiral structure. The four identical ligands (white) are attached to the carbon (grey).
Consider an organic compound with tetrahedral structure. If the mirror image of the compound is not superimposable on it’s mirror image, the compound is chiral. If superimposable, thethe compound is achiral. Figure 7-1-1 shows one of the cases of achiral compound. The four ligands (white) attached to the carbon atom (grey) are all identical. Figures 7-1-2 and 7-1-3 show the second and the third cases of achiral structures, respectively. Among the four ligands, three (white) are identical and the rest (red) is diﬀerent from the three (Figure 7-1-2), and two (white) are identicaland the other two (red) are also identical (Figure 7-1-3). The fourth case of achiral structure is shown in Figure 7-1-4. It has two identical (white) substituents and two (red and blue) diﬀerent ones. This structure is a type of achiral, but also called prochiral structure. Prochirality is explained later. 57
Figure 7-1-2: Achiral structure three (white) of the four functional groups areidentical.
Figure 7-1-3: Achiral structure with two (white) identical ligans and other two (red) identical ones among four.
Figure 7-1-4: Achiral structure with two (white) identical ligans and two (red and blue) diﬀerent ones. This is a prochiral compound.
Figure 7-1-5: Chiral structure with four different ligans attached to the stereocenter.
Figure 7-1-5 shows chiral structure. It hasfour diﬀerent (white, red, blue and yellow) ligans. The mirror image can not be superimposed on the original whichever it is turned. In such a case, the original and the mirror image compounds are enantiomer of each other. The carbon atom with the four diﬀerent ligans are attached is called stereocenter .
Notation of enantiomers
Figure 7-1-6: R-conﬁguration.
Conﬁguration of enantiomers are notated by R/S convention as follows. (1) Rank the four ligands according to the priority rules (described later). In Figures 6-1-6, and 6-1-7, suppose that the priority of the ligands is Red → Blue → Yellow → White. (2) Rotate the molecule so that the ligand with the lowest priority is far from you. Look at the molecule from the opposite side of the lowestpriority ligand. (3) Draw a circular arrow from the highest priority ligand to the next highest to the third highest. If the circular arrow is in clockwise, the conﬁguration is R (R-form). If counter clockwise, the molecule is S (S-form).
Figure 7-1-8: Rank the ligands by comparing the atoms directly bound to the stereocenter.
Figure 7-1-9: If some of the directly-bound atoms are thesame, compare the next ones.
Figure 7-1-10: Multiple bonds are regarded as single bonds assuming that the bond-forming atoms were multiply substituted with the other counterpart atoms.
Priority rules (1) Compare the atoms which are directly bound to the stereocenter. Ones with larger atomic numbers have higher priority (Figure 7-1-8). (2) When some of the atoms directly bound to the centerare at the same priority, compare the next atoms (second atoms from the center). If they are still the same, continue comparing similarly the third, fourth ... until a diﬀerence is found (Figure 7-1-9). (3) Double and triple bonds are regarded as single bonds. In this case, each of the two atoms connected by double or triple bonds are considered to be substituted with the other counterpart atom...