Читать книгу Enzyme-Based Organic Synthesis - Cheanyeh Cheng - Страница 17

Some of the enzymes may selectively catalyze one functional group at certain region of the molecule among several similar functional groups located at different regions of the same molecule. This kind of substrate specificity is called regiospecificity or diastereospecificity [13]. Example of regiospecificity in organic synthesis has been found since 1986 that the regioselective deacylation of methyl 2,3,4,6‐tetra‐O‐acyl‐D‐hexopyranosides gives the 6‐OH derivatives in high yields using the lipase from Candida cylindracea [14]. The enzyme‐catalyzed regioselective O‐acylation of ribo‐, arabino‐, xylo‐, rhamnopyranosides, and aryl pyranosides is reviewed, and the methodology is applied to the total synthesis of the naturally occurring rhamnopyranoside by Bashir et al [15]. Regioselective biotransformation of dinitrile compounds 2‐, 3‐, and 4‐(cyanomethyl) benzonitrile can be performed by whole bacterium cell Rhodococcus rhodochrous to the corresponding 2‐(cyanophenyl) acetic acid, 3‐ or 4‐(cyanomethyl) benzoic acid with high yield [16]. Whole cell bacterium Bacillus cereus has also been used for regioselectively converting 2‐phenylenediamine to 2‐aminoacetanilide with a76% molar yield [17]. Other than bacterium, monooxygenase in the phytopathogenic fungi Colletotrichum gloeosporioides and Botrytis cinerea has been found having the ability of regioselective hydroxylation of the C‐H bonds to yield the corresponding diols [18]. Enzyme in cultured plant cells of Phytolacca Americana can reduce, and regioselectively hydroxylate and glucosylate, raspberry ketone and zingerone to their β‐glycosides [19]. Ginkgo biloba cell suspension cultures were used to regio‐ and stereoselectively convert sinenxan A, 2α,5α,10β,14β‐tetra‐acetoxy‐4(20), 11‐taxadiene, a taxoid isolated from callus tissue cultures of Taxus spp., in Taxol^® synthesis [20]. The regioselective oxidation of (–)‐verbenone, an important component of the essential oil from rosemary, to (–)‐10‐hydroxyberbenone with human liver microsomes has been investigated by Miyazawa et al [21]. Although regioselective oxidation of terpenoids is difficult by chemical methods, regioselective oxidation of (+)‐ and (–)‐citronellene was recently performed with Spodoptera litura, a larvae of common cutworm, to (2R,3S)‐3,7‐dimethyl‐6‐octene‐1,2‐diol (yield: 89.7%) and (2S,3R)‐3,7‐dimethyl‐6‐octene‐1,2‐diol (yield: 56.3%) [22]. These examples of enzyme‐catalyzed regiospecificity clearly elucidates that the specific enzyme–substrate binding configuration at the active site allows only one of several similar function groups in different regions of the substrate molecule reacts to produce the product. The number of binding points of enzyme–substrate complex for regioselective molecular recognition must be a “multi‐point” binding case to match the complexity of the substrate molecule.