Tetrahedron report number 203

Enzymes as Catalysts in Synthetic Organic Chemistry [New Synthetic Methods (53)]

Asymmetric bioreduction of activated C=C bonds using enoate reductases from the old yellow enzyme family.

The old yellow enzyme (OYE) family is a large group of flavin-dependent redox biocatalysts with major applications in the industrial reduction of activated alkenes. These enzymes have broad specificity, are relatively stable, and have been made available in large quantities by using conventional genetic methods. The catalytic cycle comprises two half-reactions: reduction of flavin mononucleotide by NAD(P)H followed by flavin oxidation through stereospecific reduction of the CC bond of a wide range of activated alkenes. Recent years have witnessed extensive investigation of these reactions, aided by knowledge of atomic resolution structures for selected family members. In turn, this has led to deep understanding of the stereochemical course of substrate reduction and expansion of the biocatalytic versatility of this enzyme family through engineering approaches. We provide an overview of the structures, mechanisms, and chemical specificity of the reactions catalyzed by the OYE members. We provide an overview of the biocatalytic potential of this family of enzymes and illustrate the value of combining mechanistic and structural studies of biocatalysts to guide future exploitation of these enzymes in industrial biocatalysis.

Biocatalytic Reductions and Chemical Versatility of the Old Yellow Enzyme Family of Flavoprotein Oxidoreductases

It has been known for many decades that chiral compounds can be obtained by stereospecific biocatalytic reduction. Further significant methodological developments in this field have, however, only been made during the past ten years; they include the application of previously unused microorganisms and electron donors, the discovery of additional substrates for the known reductases, the development of methods for regenerating reduced pyridine nucleotides, and the discovery of new reductases which were sought for specific preparative purposes. Many chiral compounds can now be synthesized by microbial hydrogenation using H2 and hydrogenase-containing microorganisms as well as by electromicrobial or electroenzymatic reduction. In the two latter methods, anaerobic or aerobic organisms are supplied with electrons from electrochemically reduced, artificial mediators, e.g., methyl viologen. Reductases that do not require pyridine nucleotides and can accept electrons directly from reduced viologens are especially useful. Two examples of this type of enzyme are described which are of preparative interest. Many cells contain methyl viologen-dependent NAD(P) reductases, a large number of which have still not been characterized. A productivity number is proposed which allows different methods of bioconversion with microorganisms to be compared. The productivity numbers of compounds synthesized by the methods described in this review are often 10- to 100-fold higher than those of substances obtained by conventional techniques.

Chiral Compounds Synthesized by Biocatalytic Reductions [New Synthetic Methods (51)]

Total Synthesis of Natural α-Tocopherol. I. Preparation of Bifunctional Optically Active Precursors for the Synthesis of the Side Chain by Means of Microbiological Transformations

Our concept for a new total synthesis of natural α-tocopherol includes the synthesis of a corresponding (3 R, 7 R)-configurated C15 side chain to be built up by using twice an optically active C5 unit together with an achiral C5 end part. (S)-3-methyl-γ-butyrolactone (11) and (S)-2-methyl-γ-butyrolactone (9) represent suitable bifunctional C5-precursors for this purpose. These two key compounds have been prepared by fermentative transformation including the enantioselective hydrogenation of the double bond of ethyl-4, 4-dimethoxy-3-methylcrotonate (5) by bakers yeast (yielding 11 after ester hydrolysis and cyclization of the fermentation product) and (E)-3-(1′, 3′-dioxolan-2′-yl)-2-buten-1-ol (8) by the fungus Geotrichum candidum (yielding directly 9).

Totalsynthese von natürlichem α‐Tocopherol. 1. Mitteilung. Herstellung bifunktioneller, optisch aktiver Synthesebausteine für die Seitenkette mit Hilfe mikrobiologischer Umwandlungen

Total Synthesis of Natural α-Tocopherol



Two independent syntheses of (S)-6-hydroxy-2,5,7,8-tetramethylchroman-2-yl-methanol (8b), (Scheme 6 resp. 9) as optically active chroman moiety for the preparation of natural vitamin E via (S)-6-acetoxy-2,5,7,8-tetramethylchroman-2-carboaldehyde (2a) (Scheme 1) and a corresponding side chain are described. Both reaction sequences use trimethyl-hydroquinone as starting material; one approach employs an optically active C4 unit (10a) (Schemes 5 and 6) to introduce the required configuration at C(2), the other uses an optically active C5-synthon (11a) (Schemes 8 and 9) to build the optically active chroman unit. The correct configuration and optical purity of the chroman synthesized is established by correlation with optically pure material of known configuration from which natural vitamin E had already been derived [2].

Totalsynthese von natürlichem α-Tocopherol. 4. Mitteilung. Aufbau des Chromanringsystems aus Trimethylhydrochinon und einem optisch aktiven C4- bzw. C5-Synthon†

Total Synthesis of Natural α-Tocopherol



A short and efficient route to optically pure (+)-(3 R, 7 R)-trimethyldodecanol (14) is demonstrated, 14 serving as side chain unit in the preparation of natural vitamin E. The synthesis of 14 is based on the concept of using a single optically active C5-synthon of suitable configuration and functionalization to introduce both asymmetric centres in 14. (−)-(S)-3-Methyl-γ-butyrolacton (1) and ethyl (−)-(S)-4-bromo-3-methylbutyrate (2), respectively, is used in a sequence of either two Grignard C,C-coupling reactions 5 8 and 12 13 or two Wittig reactions 17a 18 and 20 21 to achieve this goal. 14 is converted to (2 R, 4′R, 8′R)-α-tocopherol (= vitamin E) by coupling with a chroman unit in known manner. Optical purity of products and intermediates is established.

Totalsynthese von natürlichem α-Tocopherol. 2. Mitteilung. Aufbau der Seitenkette aus (−)-(S)-3-Methyl-γ-butyrolacton

All four stereoisomers of (E)-vitamin Kb i. e. (21E, 7R, 111R)-l (= 1a), (21E, 71 R, l11S)-1 (= 1b), (21E, 71 S, 111S) 1 ( = 1c), and (21E, 71S, 111R)-l ( = Id), have been synthesized in a state of high chemical and stereoisomeric purity. The synthesis of stereoisomers lb-d relied on the use of the optically active Cf1* and C*10-building blocks (R)- or (S)-4-(benzyloxy)-3-methylbutanal ((R)- or (S)-2) and (R)- or (S)-citronellal ((R)- or (S)-3) which had been secured by the Rh1-catalyzed allylamine-to-enamine isomerization technology. For the synthesis of the natural (E)-vitamin-K1 stereoisomer 1a, a new route starting from natural phylol was developed, based on an O-alkylation/rearrangement procedure. A HPLC method was developed which separates with remarkable efficiency all four stereoisomers of (E)- as well as three out of the four stereoisomers of (Z)-vitamin K1 on optically active poly(trityl methacrylate) as the chiral stationary phase supported on Nucleosil. By this method, the stereoisomeric content of the stereoisomers 1b-d synthesized was shown to be in the range of 96-98 %, while the natural isomer 1a was configurationally uniform. The biological activity of the four (E)-vitamin-K1 stereoisomers was determined by means of the curative prothrombin time test with vitamin-K-depleted chicks. A high precision of the results was obtained with the recently introduced up-and-down organization of the test and the statistical evaluation according to an estimation procedure. With the natural (E)-vitamin-K1 stereoisomer 1a as standard (set at 1. 0), activities of 0. 93, 1. 19, and 0. 99 were found for stereoisomers 1b, 1c, and 1d, respectively. Within the confidence limits, these activity ratios can be regarded as identical, A very similar efficacy was obtained by comparison of (E, all-rac )-vitamin K1 ((21E, RS, 11′ RS)-1; equimolar mixture of the four stereoisomers 1a-d) with the natural (E)-vitamin-K1 stereoisomer 1a). A synergistic effect was not detectable, as was the case with the eight α-tocopheryl-acetate stereoisomers.

Synthesis of All Four Stereoisomers of (E)-Vitamin KT (Phylloquinone), Analysis of Their Diastereoisomeric and Enantiomeric Purities and Determination of Their Biopotencies†‡

Steroselective Total Synthesis of Natural Phytol and Derivatives thereof; Use of these Compounds in the Synthesis of Natural Vitamin K1



The Li2CuCl4-catalyzed couplings of the easily accessible bifunctional C5 allylic acetates (E)-18a and (E)-18b with racemic hexahydrofarnesylmagnesium bromide ((3 RS/RS, 7 RS/SR)-19a) proceed with high chemo- and stereoselectivity (≥98% (E)-retention) to give the (2E, 7 RS/RS, 11 RS/SR)-phytol derivatives 1a and 1b, respectively, in yields of 72–80% (Scheme 5). The same couplings performed with optically active hexahydrofarnesylmagnesium bromide (3 R, 7 R)-19a yielded the (E)-phytol derivatives of the natural series (7 R, 11R)-1a and (7 R, 11 R)-1b. Acid-catalyzed hydrolysis of(2 E, 7 R, 11 R)-1b gave natural phytol((2 E, 7 R, 11 R)-1c) Friedel-Crafts alkylation of ‘menadiol monobenzoate’ 11b with (2 E, 7 R, 11 R)-1a or (2 E, 7 R, 11 R)-1b gave the dihydrovitamine K1 derivative (2 E/Z, 7′ R, 11′R)-12b ((E/Z)≈ 9:l). Conversion of configurationally pure (2 E, 7′ R, 11′ R)-12b (yield 73%; obtained after chromatographic removal of the (Z)-isomer) into natural vitamine K1 ((2 E,7′ R, 11′ R)-2) was achieved in the usual way by saponification and oxidation with air.



Some further investigations of the coupling reactions of bifunctional C5 allylic synthons with hexahydrofarnesylmagnesium bromide (3 RS/RS, 7 RS/SR)-19a showed the outcome of these reactions to be critically dependent on the nature of the leaving group, the double-bond geometry and the nature and concentration of the catalyst. Thus, the Li2CuCl4-catalyzed couplings of (3 RS/RS,7 RS/SR)-19a with the allylic halides 29a and 29c as well as with p-toluenesulfonate 29b yielded besides the phytol derivatives 1a and 1b - also the SN2′-type products 30a and 30b (Scheme 8, Table 2); the same result was found for the coupling with the cis-configurated allylic acetates (Z)-18a and (Z)-18b (Table 3). A similar loss of chemo selectivity as well as the loss of stereoselectivity in the coupling reactions of 19 with the bifunctional (E)-olefins of type 18 was observed when the Li2CuCl4-catalyst concentration was increased from 0.2 to 25 mol-% or upon substitution of Li2CuCl4 by copper (I) chloride or iodide (Table 4).

Max Schmid

Papers

Totalsynthese von natürlichem α‐Tocopherol. 1. Mitteilung. Herstellung bifunktioneller, optisch aktiver Synthesebausteine für die Seitenkette mit Hilfe mikrobiologischer Umwandlungen

Totalsynthese von natürlichem α-Tocopherol. 4. Mitteilung. Aufbau des Chromanringsystems aus Trimethylhydrochinon und einem optisch aktiven C4- bzw. C5-Synthon†

Totalsynthese von natürlichem α-Tocopherol. 2. Mitteilung. Aufbau der Seitenkette aus (−)-(S)-3-Methyl-γ-butyrolacton

Synthesis of All Four Stereoisomers of (E)-Vitamin KT (Phylloquinone), Analysis of Their Diastereoisomeric and Enantiomeric Purities and Determination of Their Biopotencies†‡

Stereoselektive Totalsynthese von natürlichem Phytol und Phytolderivaten und deren Verwendung zur Herstellung von natürlichem Vitamin K1