Epoxides enzymatic hydrolysis

The /M ) )-nitrite (or formate) esters of v/c-diols obtained via enzymatic ring-opening of epoxides in presence of nitrite (or formate) are unstable and undergo spontaneous (nonenzymatic) hydrolysis to furnish the corresponding diols. This protocol offers a useful complement to the asymmetric hydrolysis of epoxides. Depending on the type of substrate and the enzymes used, enantio-complementary epoxide hydrolysis can be achieved [1851]. 

Enzymatic hydrolysis of A/-acylamino acids by amino acylase and amino acid esters by Hpase or carboxy esterase (70) is one kind of kinetic resolution. Kinetic resolution is found in chemical synthesis such as by epoxidation of racemic allyl alcohol and asymmetric hydrogenation (71). New routes for amino acid manufacturing are anticipated. 

A similar domino process involving the opening of two epoxide moieties after an enzymatic ester hydrolysis has been described by Robinson and coworkers [16]. Treatment of 8-32 with PLE in an aqueous buffer solution at pH 7.5-8 led to 8-34 in 70% yield after formation of 8-33 (Scheme 8.8). 

In addition to the enzymatic hydrolysis of esters, there also ample examples where an epoxide has been cleaved using a biocatalyst. As described by the Faber group [19], reaction of the ( )-2,3-disubstituted ds-chloroalkyl epoxide roc-8-40 with a bacterial epoxide hydrolase (BEH), led to the formation of vie-diol (2 ,3S)-8-41 (Scheme 8.11). The latter underwent a spontaneous cydization to give the desired product (2i ,3i )-8-42 in 92 % ee and 76 % yield. The same strategy was used with the homologous molecule rac-8-43, which afforded the THF derivative (2R,3R)-S-4S in 86% ee and 79% yield. 

Epoxides/arene oxides have varying degrees of chemical reactivity and can be detoxified by hydrolysis to dihydrodiols as shown in Figure 6.8. This can occur either nonenzy-matically, if the epoxide is very reactive, or it can be catalyzed enzymatically by epoxide hydrolase (EH). 

An unusual case of intramolecular competition (chemoselectivity, see Chapt. 1 in [la]) between ester and oxirane occurs in the detoxification of (oxiran-2-yl)methyl 2-ethyl-2,5-dimethylhexanoate (10.49), one of the most abundant isomers of an epoxy resin. The compound is chemically very stable, i.e., resistant to aqueous hydrolysis, but is rapidly hydrolyzed in cytosolic and microsomal preparations by epoxide hydrolase and carboxylesterase, which attack the epoxide and ester groups, respectively [129], The rate of overall enzymatic hydrolysis was species dependent, decreasing in the order mouse > rat > human, but was relatively fast in all tissues examined (lung and skin as portals of entry, and liver as a further barrier). In mouse and rat lung microsomes, ester hydrolysis was 3-4 times faster than epoxide hydration, whereas the opposite was true in human lung microsomes. 

T. Watabe, K. Akamatsu, Enzymatic Hydrolysis of Mono-n-alkyl Substituted Ethylene Oxides and Their Inhibitory Effects on Hepatic Microsomal Epoxide Hydrolase , Chem. Pharm. Bull. 1974, 22, 2155 - 2158. 

G. Bellucci, C. Chiappe, F. Marioni, Enantioselectivity of the Enzymatic Hydrolysis of Cyclohexene Oxide and ( )-l-Methylcyclohexene Oxide A Comparison between Microsomal and Cytosolic Epoxide Hydrolases , J. Chem. Soc., Perkin Trans. 1 1989, 2369 -2373. 

The above-mentioned facts have important consequences on the stereochemical outcome of the kinetic resolution of asymmetrically substituted epoxides. In the majority of kinetic resolutions of esters (e.g. by ester hydrolysis and synthesis using lipases, esterases and proteases) the absolute configuration at the stereogenic centre(s) always remains the same throughout the reaction. In contrast, the enzymatic hydrolysis of epoxides may take place via attack on either carbon of the oxirane ring (Scheme 7) and it is the structure of the substrate and of the enzyme involved which determine the regioselec-tivity of the attack [53, 58-611. As a consequence, the absolute configuration of both the product and substrate from a kinetic resolution of a racemic... 

Although many biochemical reactions take place in the bulk aqueous phase, there are several, catalyzed by hydroxynitrile lyases, where only the enzyme molecules close to the interface are involved in the reaction, unlike those enzyme molecules that remain idly suspended in the bulk aqueous phase [6, 50, 51]. This mechanism has no relation to the interfacial activation mechanism typical of lipases and phospholipases. Promoting biocatalysis in the interface may prove fruitful, particularly if substrates are dissolved in both aqueous phases, provided that interfacial stress is minimized. This approach was put into practice recently for the enzymatic epoxidation of styrene [52]. By binding the enzyme to the interface through conjugation of chloroperoxidase with polystyrene, a platform that protected the enzyme from interfacial stress and minimized product hydrolysis was obtained. It also allowed a significant increase in productivity, as compared to the use of free enzyme, and simultaneously allowed continuous feeding, which further enhanced productivity. 

Petri, A., Marconcini, P. and Salvadori, P. (2005) Efficient immobilization of epoxide hydrolase onto silica gel and use in the enantioselective hydrolysis of racemic para-nitrostyrene oxide. J. Mol. Catal. B Enzymatic, 32, 219. 

Preparation of enantiometrically pure aldehyde substrates for DHAP-dependent aldolase reactions has been accomplished by a combination of enzymatic and chemical methods. The lipase-catalyzed resolution of racemic aldehyde precursors has been accomplished by enantioselective acetate hydrolysis, as exhibited in the preparation of enantiomerically pure R- and S -glycidaldehyde acetals (Scheme 5.10).31 Regioselective ring opening of the epoxides, followed by acetal hydrolysis, generated the aldehydes in enantiomerically pure form. 

To prove the above hypothesis of precocene mode of action, it was required to synthesize the corresponding 3,4-epoxyprecocenes and to study its chemical reactivity. When this was accomplished in two different laboratories (16,18) the lability of these epoxides towards nucleophilic or electrophilic attack was confirmed.In each case the chemical hydrolysis of these compounds gave the same isomer ratio of diols observed in the enzymatic metabolic process. 

Chiappe, C., Leandri, E., Lucchesi, S., Pieraccini, D., Hammock, B.D., and Morisseau, C. 2004. Biocatalysis in ionic liquids The stereoconvergent hydrolysis of trans- -methylstyrene oxide catalyzed by soluble epoxide hydrolase. Journal of Molecular Catalysis B Enzymatic, 27 243 8. 

Whilst ring opening of epoxides (Volume 6, Chapter 1.3) is really beyond the scope of this review, two recent papers are noteworthy poorly nucleophilic amines can be reacted very cleanly as their di-ethylaluminum derivatives,and a start has been made on chiral induction of opening of epoxides (e.g. cyclohexene oxides).Amino alcohols have been resolved by enantioselective enzymatic hydrolysis of their acetates. Ring opening of phthalimidoaziridines has bwn achieved with water, phenol and tosic acid, amongst other nucleophiles,giving products of formal N—O addition to the double bond. 

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