Yeast-mediated reactions

Finally, an interesting and potentially useful baker s-yeast-mediated reaction has been demonstrated with the conversion of squalene oxide (17) to lanosterol (18) (Scheme 5.18). Two facets of this reaction are noteworthy Firstly, if racemic squalene oxide is used, then the lanosterol... 

It seems that the enantioselectivity of the reduction of keto esters with the keto group being part of a five-membered ring proceeds better than that of open-chain keto esters substituted at carbon C-2 [225, 226]. The reduction of 50 (Figure 21.16) gave 80% of (1R,2S)-51 showing both an ee and a de of 100%. In another experiment, however, 51 was obtained with a de of only 60% [217, 227]. Several mold strains have been shown to perform the same reduction with high enanho- and dias-tereoselection, whereas from the baker s yeast-mediated reactions quite often mixtures of the stereoisomeric products were obtained [227]. More complex substrates have also been studied [228-232], but yields and /or stereoselectivity were often only moderate. 

The versatility of these yeast-mediated reactions is exemplified by the synthesis of masked 1,2- or 1,4-diols by the Eridania-me ated bioi uction of 67 to afford (5)-con-figurated alcohols 68 (Fig. 19) possessing e.e. values of 90-95% [99] as well as by the convenient access to the aggregation pheromone sulcatol [( -69] that has been obtained by the reduction of 6-methyl-hept-5-en-2-one (70) in 80% yield and 94% e.e. [100],... 

An IL solvent system is applicable to not only lipase but also other enzymes, though examples are still limited for hpase-catalyzed reaction in a pure IL solvent. But several types of enzymatic reaction or microhe-mediated reaction have been reported in a mixed solvent of IL with water. Howarth reported Baker s yeast reduction of a ketone in a mixed solvent of [hmim] [PFg] with water (10 1) (Fig. 16). Enhanced enantioselectivity was obtained compared to the reaction in a buffer solution, while the chemical yield dropped. 

The yeast-mediated enzymatic biodegradation of azo dyes can be accomplished either by reductive reactions or by oxidative reactions. In general, reductive reactions led to cleavage of azo dyes into aromatic amines, which are further mineralized by yeasts. Enzymes putatively involved in this process are NADH-dependent reductases [24] and an azoreductase [16], which is dependent on the extracellular activity of a component of the plasma membrane redox system, identified as a ferric reductase [19]. Recently, significant increase in the activities of NADH-dependent reductase and azoreductase was observed in the cells of Trichosporon beigelii obtained at the end of the decolorization process [25]. 

Reaction of lithiodithianes with acyl chlorides, esters or nitriles leads to the fOTination 1,2-dicarbonyl compounds in which one of the carbonyl groups is protected as the thioacetal. d76043j44 Optically active amino ketones of type (69) are inepared via acylation of dithiane with an oxazoline-protected (5)-serine methyl ester (Scheme 41). Optically active (5)-2-alkoxy-l-(l,3-dithian-2-yl)-l-propanones were prepaid by the reaction of the corresponding methyl (5)-lactate with 2-lithio-l,3-enantioselective synthesis of (-)-trachelanthic acid. Enantioselective synthesis of L-glyceraldehyde involves the acylation of a dithiane glycolic acid derivative followed by bikers yeast mediated reduction. ... 

The Mitsunobu reaction is also the basic step in the synthesis of 65. The starting homochiral hydroxy ester 62 is readily available by yeast-mediated reduction of ethyl 2-oxocyclopentanecarboxylate. Treatment of 62 with hydrazoic acid under Mitsunobu conditions resulted in inversion to the azide 63, which was hydrogenated in the presence of di-ter/-butyl... 

Epoxide opening reactions have also been employed in the synthesis of southern half fragments. For example, the synthesis of optically active model system 82 by Barret and Capps [125] includes addition of a lithio-dithiane to ketone 81 and a subsequent intramolecular epoxide opening as key steps. Chirality was introduced in this case by a known yeast mediated reduction of ethyl 2-oxocyclohexane-carboxylate to afford optically active 80. 

Endogenous N-nitrosation may result from cell-mediated reactions, involving alveolar and peritoneal macrophages, bacteria, and yeasts [5,58,59]. In earlier investigation, dimethylamine was used to determine... 

A familiar example of a stereoselective reaction would be the formation of a higher yield of f rans-2-butene than cis-2-butene in an E2 reaction, no matter whether the starting material is (R)- or (S)-2-bromobutane. The addition of bromine to trans-2-butene to produce mcso-2,3-dibromobutane or the addition of bromine to the cis isomer to produce an equimolar mixture of the two enantiomers of 2,3-dibromobutane is a stereospecific reaction. Note that a reaction that gives only one of a pair of eirantiomers is not necessarily stereospecific. A yeast-mediated reduction of 3-chloropropiophenone gives (S)-3-chloro-l henylpropan-l-ol, with no evidence for formation of the R enantiomer. Because the reactant cannot exist as stereoisomers, it is not possible for stereoisomerically different reactants to give stereoisomerically different products, and the reaction can only be considered stereoselective, not stereospecific. 

The hydrolysis of an ester moiety is regarded by many scientists as a most simple reaction, even sometimes armoying. This was also true for baker s yeast-mediated hydrolyses [307,308]. The first report of an ester hydrolysis has been published in the steroid field, and it was reported as an undesired side reaction [309]. Many different enzymes of baker s yeast may be responsible for the hydrolysis of ester moieties not... 

It must be pointed out that yeast-mediated biotransformations may be complicated by side reactions that interfere or even dominate the desired conversion workup may be somewhat time consuming and messy due to the separation of the product from the huge amount of biomass. The ideal interactions between the yeast and the substrate are scarcely found in praxi. Ideal interactions between substrate yeast and product include that both substrate and product are able to pass the cell membrane, should be soluble in the fermentation medium, and must not inactivate the catalytic activity of the involved microbial enzymes. Finally, high turnover rates and high regio- and stereospecificity are also of high practical relevance. Some advice on how to deal with these basic problems often encountered in such biotransformations is provided in Fig. 1 [9,11,13,19,20]. 

Yeast-mediated deacylations have been regarded merely as simple and more or less useless reactions. Originally observed in the steroid field [454], yeast-mediated hydrolyses and the enzymes involved in these processes have been investigated very carefully. Thus, proteinase [455-457], esterase [458-467], phospholipase [468-470], lipase [468,471], tributy-linase [472,473], as well as triacylglycerol lipase activities [471,474-476] have been de-... 

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