OYE-catalyzed reduction

A systematic study on enzymatic catalysis has revealed that isolated enzymes, from baker s yeast or old yellow enzyme (OYE) termed nitroalkene reductase, can efficiently catalyze the NADPH-linked reduction of nitroalkenes. Eor the OYE-catalyzed reduction of nitrocyclohexene, a catalytic mechanism was proposed in which the nitrocyclohexene is activated by nitro-oxygen hydrogen bonds to the enzymes His-191 and Asn-194 [167, 168]. Inspired by this study Schreiner et al. 

Figure 5.1 Mechanism of OYE-catalyzed reduction of an alkene activated by an electron-withdrawing group (EWG = carbonyl, nitro).

Baylis-Hillman adducts are readily available building blocks, which upon stereoselective OYE-catalyzed reduction afford synthetically important chiral products [42]. The first example to be reported involves the formation of the Roche ester (4a) from the Baylis-Hillman adduct 3a especially the allyl and benzyl ethers (3b and 3c, respectively) can be reduced stereoselectively using several different OYEs, with results (99% ee) rivaling and surpassing synthetic catalysts (Eigure 5.6) [42]. 

Figure S.6 OYE-catalyzed reduction of the Roche ester (3a) and O-protected forms 3b and 3c leading to chiral products 4a-c [42],...

A bioreduction system might be applied to many NAD(P)H-dependent enzyme reactions other than carbonyl reduction. Recently, two novel old yellow enzymes (OYEs) catalyzing the asymmetric hydrogenation of C=C bonds were found and applied to a bioreduction system for the production of double chiral compounds. 

Table 5.2 OYE-catalyzed stereoselective reduction of Baylis-Hillman adducts in the presence of an NADPH regeneration system [36],...

Alternative to the Nef pathway, some OYEs can bioreduce p,p-disubstituted nitroalkenes to oxazetes. This reaction proceeds through reduction of the nitro group to the nitroso alkene, which spontaneously forms 1,2-oxazete derivatives. At elevated temperatures by a retro-[2 + 2]-cycloaddition, HCN and the ketone are formed as final products. The bioreduction pathway was determined by the OYE used. Thus, using xenobiotic reductase A oxazete formation was predominant, while the PETN reductase-catalyzed process favored the Nef pathway (Scheme 2.15). 

A spectacular example of the combination of an ene reductase with other enzymatic activities is about the cascade biosynthesis of artemisinin 43 (Scheme 3.11), natural drug possessing the most rapid action against Plasmodium fcdciparum (malaria). Artemisinic acid, precursor of 43, has been produced biotedmologically on g-scale by means of an engineered yeast [68]. Then, the C(11)=C(13) bond reduction of artemisinic aldehyde 44 was catalyzed by the cloned Dbrl (double bond reductase 1) protein from Artemisia annua to yield the (llS)-dihydroartemisinic aldehyde 45 precursor. It is interesting to note that the stereochemical outcome of this ene reductase is opposite to that observed for the reduction of the very similar substrate 23 with OYEs [69]. 

OYEs are oxidoreductases that catalyze the addition of the elements of hydrogen to an electron-poor olefinic double bond in a stereospecific trans-manner [6]. The hydride is delivered from reduced flavin mononucleotide (FMNH2) in a Michael-type process, the proton being donated by a tyrosine residue [8] or by water [9,10] in the so-called oxidative half-reaction (Figure 5.1). NAD(P)H in the reductive half-reaction ensures reduction of the flavin [6]. Mechanistically... 

However, the most extensively investigated class of ERs is members of the OYE family of flavin oxidoreductases (EC 1.6.99.1). There is detailed information known about OYEs, such as their structure, reaction mechanism, substrate scope, kinetic properties, and biocatalytic approaches. Therefore, this chapter will focus on this latter class of enzymes. They have been intensively studied over the past decade in view of their applicability in preparative-scale biotransformations [1, 8-12]. These FMN-containing enzymes catalyze the asymmetric reduction of a,p-unsaturated... 

---