S -Ethyl 3-hydroxybutanoate

Optically pure a, . (R)-3-hydroxybutanoates can be obtained by alcoholysis of poly-(R)-3-hydroxybutanoate, a fermentation product of fructose by Alcaligones eutrophus.4 (S)-Ethyl 3-hydroxybutanoate in 84-87% ee can be synthesized in 57-67% yield on a decagram-scale by an Organic Syntheses procedure6 using bakers yeast reduction of ethyl 3-oxobutanoate with the aid of sucrose.7 In order to obtain enantioselectivity as high as 95-97% ee, the substrate concentration should be kept below 1 g/L.6... 

T Kometani, H Yoshii, Y Takeuchi, R Matsuno. Large-scale preparation of (S)-ethyl 3-hydroxybutanoate with a high enantiomeric excess through baker s yeast-mediated bioreduction. JFerm Bioeng 76 33-37, 1993. 

Dieter Seebach, Marius A. Sutter, Roland H. Weber, and Max F. Ziger 1 YEAST REDUCTION OF ETHYL ACETOACETATE (S)-(+)-ETHYL 3-HYDROXYBUTANOATE... 

S)-( -)-ETHYL 3-HYDROXYBUTANOATE (Butanoic acid, 3-hydro yr-, etHyl ester, (S))... 

The procedure of enriching the (S) —(+)-enantiomer to 100% enantiomeric excess by the previously described crystallization method 1s tedious.4 It provides optically pure ethyl (S)-(+)-3-(3, 5 -dlnltro-benzoyloxy)butanoate of [ ]q5 +26.3° (chloroform, a 2), which after cleavage gives enantiomerically pure ( )-(+)-ethyl 3-hydroxybutanoate of [< ](j5 + 43.5° (chloroform, a 1.0). This optically pure compound has recently become commercially available from Fluka AG, CH-9470 Buchs (Switzerland), but it is very expensive. After submission and checking of this procedure, it was shown that the ee of the product can be increased to >95% by working under aerobic conditions and by adding the ketoester more slowly. 

NATURAL PRODUCTS FROM (S)- OR (R)-ETHYL 3-HYDROXYBUTANOATE The Skeleton of Ethyl 3-Hydroxybutanoate is Indicated by Heavy Lines... 

Preparaton of the Enantiomers of Ethyl 3-Hydroxybutanoate. Reduction of ethyl acetoacetate with yeast yields ethyl (S)-3-hydroxybutanoate (15) as shown in Figure 4 (12-14). Purification of crude 15 as its crystalline 3,5-dinitrobenzoate gives (S)-15 of 100% e.e. (14,15). 

Ethyl 3-hydroxybutanoate, isolated from yellow passion fruit, mainly consisted of the (S)-enantiomer (82 %), comparable to the product, obtained by yeast reduction of the corresponding 3-ketoacid ester (see above). In contrast, ethyl 3-hydroxybutanoate in purple passion fruit and mango mainly consisted of the (R)-enantiomer (69 % and 78 %). 

Related Reagents. (25, 45)-3-Benzoyl-2-r-butyl-4-methyl-l,3-oxazolidin-5-one (,S )-4-Benzyl-2-oxazolidinone (R,R)-2-t-Butyl-5-methyl-l,3-dioxolan-4-one 10,2-(2amphorsultam 10-Dicyclohexylsulfonamidoisobomeol (R,/J)-2,5-Dimethylboro-lane Ethyl 3-Hydroxybutanoate 2-Hydroxy-l,2,2-triphenyl-ethyl Acetate (5)-4-Benzyl-2-oxazolidinone (RJi)-l,2-Dipheny 1-1,2-diaminoethane NA( -Bis[3,5-bis(trifluoromethyl)-benzenesulfonamide] 2,2,6-Trimethy 1-4//-1,3-dioxin-4-one. 

In this experiment we will use the enzymes found in baker s yeast to reduce ethyl acetoacetate to 5( + )-ethyl 3-hydroxybutanoate. This compound is a very useful synthetic building block. At least eight chiral natural product syntheses are based on this hydroxyester. ... 

Treatment of the derivative with excess acidified ethanol will regenerate 100% ee 5(+)-ethyl 3-hydroxybutanoate. If several crops of crude product are pooled, distillation at reduced pressure (see Chapter 7) can give chemically pure ethyl 3-hydroxybutanoate (bp 71-73° at 12 mm). Both the R and S enantiomers have the same boiling points, so the optical purity will not be changed by distillation. 

Day 2 of fhe experiment is used to isolate the chiral ethyl 3-hydroxybutanoate. After this has been isolated, each student s product is analyzed by chiral gas chromatography and polarimetry to determine the percentages of each of the enantiomers. As an optional experiment (Experiment 28B), the products can also be analyzed by NMR using a chiral shift reagent to determine the percentages of each of the enantiomers present in the ethyl 3-hydroxybutanoate produced in the chiral reduction. 

Chiral gas chromatography will provide a direct measure of amounts of each stereoisomer present in your chiral ethyl 3-hydroxybutanoate sample. A Varian CP-3800 equipped with an Alltech Cyclosil B capillary column (30 m, 0.25-mm ID, 0.25 /xm) provides an excellent separation of (R) and (S)-enantiomers. Set the FID detector at 270°C and the injector temperature at 250°C, with a 50 1 split ratio. Set the column oven temperature at 90°C and hold at that temperature for 20 minutes. The helium flow rate is 1 mL/min. The compounds elute in the following order ethyl (S)-3-hydroxybutanoate (14.3 min) and the (R)-enantiomer (15.0 min). Any remaining ethyl acetoacetate present in the sample will produce a peak with a retention time of 14.1 minutes. Your observed retention times may vary from those given here, but the order of elution will be the same. Calculate the percentages of each of the enantiomers from the chiral gas chromatography results. Usually, about 92-94% of the (S)-enantiomer is obtained from the reduction. 

Fill a 0.5-dm polarimeter cell with your chiral hydroxyester (about 2 mL required). You may need to combine your product with material obtained by one other student in order to have enough material to fill the cell. Determine the observed optical rotation for the chiral material. Your instructor will show you how to use the polarimeter. Calculate the specific rotation for your sample using the equation provided in Technique 23. The concentration value, c, in the equation is 1.02 g/mL. Using the published value for the specific rotation of ethyl (S)-(-l-)-3-hydroxybutanoate of [ d ] = +43.5°, calculafe fhe optical purity (enantiomeric excess) for your sample (see Technique 23, Section 23.5). Report the observed rotation, the calculated specific rotation, the optical purity (enantiomeric excess), and the percentages of each of the enantiomers to the instructor. How do the percentages of each of the enantiomers calculated from the polarimeter measurement compare to the values obtained from chiral gas chromatography ... 

In Experiment 28A, the yeast reduction of ethyl acetoacetate forms a product that is predominantly the (S)-enantiomer of ethyl 3-hydroxybutanoate. In this part of the experiment, we will use NMR to determine the percentages of each of the enantiomers in the product. The 300 MHz proton NMR spectrum of racemic ethyl 3-hydroxybutanoate is shown in Figure 1. The expansions of fhe individual patterns from Figure 1 are shown in Figure 2. The methyl protons (HJ appear as a doublet at 1.23 ppm, and the methyl protons (H, ) appear as a triplet at 1.28 ppm. The methylene protons (H and H ) are diastereotopic (nonequivalent) and appear at 2.40 and 2.49 ppm (each a doublet of doublefs). The hydroxyl group appears at about 3.1 ppm. The quartet at 4.17 ppm results from the methylene protons (HJ split by the protons (Hjj).The methane proton (H ) is buried under the quartet at about 4.2 ppm. 

Ferrulactone II Grandisol c/s-Pityol Figure 38 Compounds synthesized by starting from ethyl 3-hydroxybutanoate. 

The lithium alkoxyenolatcs derived from ( + )-ethyl (S)-3-hydroxybutanoate and (-)-dimethyl (S)-2-hydroxybutanedioate, respectively, undergo addition to nitroethene with good diastereose-lection28. 

Lactobacillus kefir was also employed as the whole-cell biocatalyst for the asymmetric reduction of ethyl 4-chloroacetoacetate to ethyl (.S )-4-chloro-3-hydroxybutanoate, the chiral... 

Amidjojo, M. and Weuster-Botz, D. (2005) Asymmetric synthesis of the chiral synthon ethyl (S)-4-chloro-3-hydroxybutanoate using Lactobacillus kefir. Tetrahedron Asymmetry, 16 (4), 899-901. 

Yamamoto, H., Matsuyama, A. and Kobayashi, Y. (2002) Synthesis of ethyl (R)-4-chloro-3-hydroxybutanoate with recombinant Escherichia coli cells expressing (S)-specific secondary alcohol dehydrogenase. Bioscience Biotechnology and Biochemistry, 66 (2), 4814-83. 

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