Method for preparing thiol compounds

Abstract: Method for preparing thiol compounds by coupling cysteine having the formula HS-CH₂-CH(NH₂)COOH via an -S- bridge to a hydrocarbon compound and subsequently reacting the cysteine conjugate obtained with β-lyase to form the relevant thiol compounds. For instance it is possible to prepare the flavour p-mentha-8-thiol-3-one starting from pulegone as illustrated in the diagram below:

Description: The invention relates to a method for preparing thiol compounds. In Pesticide Biochemistry and Physiology 14 , pp. 50-61 (1980) the in vitro metabolism of pentachloronitrobenzene (PCNB) into pentachloromethylthiobenzene (PCTA) by means of an enzyme system obtained from onions is described. More particularly, this reference relates to the in vitro preparation of PCTA from PCNB at a pH of 7.9 by means of an enzyme system which contains dithiothreitol, glutathione and S-adenosylmethionine. Said enzyme system was prepared from onion roots by ammonium sulphate fractionation and differential centrifugation. The enzyme system contained glutathione-S transferase activity with PCNB, C-S-lyase activity (also termed β-lyase activity) with S-(pentachlorophenyl)cysteine, S-adenosylmethionine-methyl transferase activity with pentachlorothiophenol (PCTP) and probably a few other peptidase activities. The yield of the thiol compound concerned, namely pentachlorothiophenol (PCTP) is, however, negligible in this known method compared with the yield of PCTA (see page 55, right-hand column, lines 10-13 from bottom) so that this method is considered unsuitable for preparing thiol compounds. In Journal of Biological Chemistry, vol.253, 24 , pp.8854-8859 (1978), the cysteine conjugate β-lyase in rat liver is described. This enzyme catalysing cleavage of the thioether linkage in cysteine conjugates has been purified about 500-fold from rat liver cytosol. However, according to the chapter "Assay Methods" (page 8855) the obtained thiol compounds were directly methylated whereafter the methylated derivatives were identified by spectroscopy methods. Microecology and Therapy, vol. 15, 1985, pp. 261-266 discloses a pathway for the introduction of sulfur in the form of thiol-, methylthio-, methylsulfinyl- or methylsulfonyl-groups into xenobiotics. The β-lyase in the gastrointestinal bacteria is a key enzyme in this biotransformation as it converts S-cysteine conjugates into thiol containing metabilites which will usually - on account of the toxic properties of thiols - undergo further metabolism. A method defined in the introduction has now been found which is characterized in that cysteine is coupled by an addition reaction of cysteine to a compound having the formula (R₁)(R₂)C=C(R₃)-CO-R₄ in which the symbols R₁-R₄ represent a hydrogen atom or an optionally saturated and/or heterogeneous hydrocarbon group or, together with the carbon atom to which the symbols are bonded, form one or two, optionally saturated and/or heterogeneous ring systems and subsequently the cysteine conjugate obtained is reacted with a β-lyase to form the relevant thiol compound. For example, in the compound having the formula (R₁)(R₂)C=C(R₃)-CO-R₄ the symbols R₁-R₄ represent a hydrogen atom, an alkyl group containing 1-5 carbon atoms, an alkenylene group containing 2-5 carbon atoms, a cycloalkyl or cycloalkenyl group containing 5-10 carbon atoms or an aryl group containing 6-10 carbon atoms, which abovementioned groups may be substituted by halogen atoms and/or one or more groups containing carbon, nitrogen, sulphur, oxygen and/or halogen atoms. Preferably, the symbols R₂ and R₃ represent a hydrogen atom or an alkyl group containing 1-3 carbon atoms and R₄ an optionally heterogeneous hydrocarbon group bonded via an -O- bridge. For example, unsaturated sugars having the formula in which the symbol R₅ represents a hydrogen atom, an alkyl group containing 1-24 carbon atoms or an alkaline ion and R₆ represents a group consisting of 1-7 monosaccharides selected from the group consisting of glucose, mannose, galactose, arabinose, fucose, xylose, rhamnose, uronic acids and derivatives thereof like the acetates, pyruvates, amines and sulphates are also suitable as starting material for the addition reaction of cysteine. Preferably R₆ represents a glucose-rhamnose-glucose group. The obtained cysteine-conjugates are simply convertable to compounds with the formula having flavouring properties. Many types of cysteine conjugates are known as such from the prior art. For example, the preparation of such a cysteine conjugate is known from Applied and Environmental Microbiology, May 1985. pp. 1146-1153. In this reference, 16-dehydroprogesterone, in particular, is converted with L-cysteine in a non-enzymatical manner into 16-S-cysteinyl-progesterone. Said cysteinyl compound can be converted in the presence of β-lyase into 16-mercaptoprogesterone by means of the second stage of the method according to the invention. The diagram below illustrates the synthesis route described above: The thiol steroid shown above has specific pharmacological properties. The formation of cysteine conjugates of 3-(3,4-dihydroxyphenyl)analine is reported in Biochimica et Biophysica Acta 672 (1981), pp. 151-157. As indicated on page 155 of this reference, polyconjugates can also be obtained in addition to some monoconjugates. These singly or multiply conjugated compounds can also be converted by means of the β-lyase to be used according to the invention into the corresponding mono- or polythiol derivatives. Reference may be made to the following additional references relating to specific cysteine conjugates or derivatives derived therefrom: 1) J.Chem.Soc.Chem.Commun. 1986, pp. 1331-1333; 2) Journal of Food Science, vol. 51, no. 5, 1986, pp. 1191-1194; 3) Planta (1986) 169: 208-215; and 4) Carbohydrate Research 142 (1985), pp. 93-105. The cysteine used in the method according to the invention has the formula HS-CH₂-CH(NH₂)-COOH. In view of the spectrum of activity of the β-lyase to be used in the method according to the invention, L-cysteine is used. The β-lyase (also termed C-S-lyase or cysteine conjugate β-lyase) to be used in the method according to the invention is an enzyme dependent on a pyridoxal 5-phosphate (vitamin B6). In addition to being present in a large number of intestinal bacteria (in 24 out of the 43 intestinal bacteria investigated), the β-lyase is also present in some vegetable and animal cells (Larsen G.L., "Distribution of cysteine conjugate B-lyase in gastro-intestinal bacteria and the environment, Xenobiotica 15, 199-209 (1985)). The bacterial β-lyases are able to convert a wide spectrum of substrates, in particular both S-alkyl- and S-arylcysteine conjugates, whereas the spectrum of activity of β-lyases of vegetable or animal origin is limited. Measured with the cysteinepropachlor conjugate (an S-alkylcysteine conjugate), the β-lyase originating from the anaerobic intestinal bacterium Eubacterium limosum is the most active enzyme and has the lowest substrate specificity (Larsen, loc. cit.). If, however, the conversion of S-(2-benzothiazolyl)cysteine (an S-arylcysteine conjugate) is examined, it emerges that the β-lyase from an anaerobic Fusobacterium species has virtually an identical activity. β-lyase from F.necrophorum and E.limosum differ not only in substrate specificity, but also in size, namely 228 kd and 75 kd (2x38 kd) and also in stability. The enzyme from F.necrophorum requires pyridoxal 5-phosphate for stability but is then also more stable to heat. β-lyases from E.limosum and F.varium exhibit no activity with D-cysteine conjugates and have, in general, a lower activity for S-alkylcysteine conjugates than for S-arylcysteine conjugates. The isolation of β-lyase from both E.limosum and F.varium does not have to be carried out under anaerobic conditions. This indicates that the enzyme is not sensitive to oxygen. It also emerges from the isolation method that the enzyme is located in the cell. The second step described above of the method according to the invention can therefore be carried out with purified/extracted β-lyase or, if the substrates are absorbed by the bacterial cells and are converted therein, with the respective bacteria themselves. The method according to the invention results in many types of thiol compounds with divergent applications. Examples of substances to be prepared pertain to the field of perfumes and flavourings (p-mentha-8-thiol-3-one, damascone derivative), pharmacological steroid compounds and repellants (Warburganal). The invention is explained on the basis of the examples below. Example I In this example, the starting point is pulegone, which is converted via S-cysteinyl-pulegone into p-mentha-8-thiol-3-one. This preparation is illustrated in the diagram below. Stage 1) Preparation of S-cysteinyl-pulegone . 12.2 g of L-cysteine (0.1 mol) (high purity analytical grade supplied by Fluka A.G.), 16.3 ml of pulegone (0.1 mol) and 2.0 g of KHCO₃ (0.02 mol) were stirred for 22 hours in 100 ml of H₂O at room temperature. The yoghurt-like mixture, which was no longer stirrable, was then allowed to stand for 3 days. The product obtained was then filtered off by suction and washed respectively with 100 and 2 x 50 ml of H₂O. After drying over CaCl₂ in vacuo, the product was washed with acetone. The yield was 17.9 g. Appendix 1 shows the 90 MHz H-NMR spectrum of the product obtained. More particularly, an elementary analysis of the product purified by thin-layer chromatography clearly indicates a 1:1 reaction product. Elementary analysis (carried out in duplicate). Found: Calculated (substance + 1/2 mol of H₂O): %C: 54.76 55.29 %H: 8.36 8.57 %N: 5.01 4.96 %O: 19.60 19.83 %S: 11.13 11.35 Stage 2) Splitting of the S-cysteinyl-pulegone . The organism used in this stage was Eubacterium limosum having the ATCC no. 10825. Said organism was cultured under anaerobic conditions at 37°C on a P-medium which had the composition below: Composition of P-medium: Casein peptone (Difco) 10 g/l Beef extract (Difco) 3 g/l Yeast extract (Difco) 3 g/l Glucose (Merck) 2 g/l Tween 80 (Serva) 1 g/l Cysteine-HCl (Fluka) 0.5 g/l Resazurin (Serva) 0.25 g/l Salt solution (analytical grade) 40 ml/l Final pH: 7.2 The salt solution consisted of: CaCl₂ 0.2 g/l MgSO₄.7H₂O 0.2 g/l K₂HPO₄ 1.0 g/l KH₂PO₄ 1.0 g/l NaHCO₃ 10.0 g/l NaCl 2.0 g/l The cell material for producing β-lyase was obtained by culturing E.limosum (3% inoculation) on the abovementioned P-medium in serum bottles having a capacity of 300 ml. By filling the bottle with P-medium to a few centimetres below the rim, the medium became sufficiently low in oxygen as a result of sterilization to make growth of E.limosum possible. After an incubation time of 1 day at 37°C, the cells were harvested by centrifuging them at 50,000 x g for 20 minutes. The cells were subsequently washed twice with a buffer having a pH of 7 which contained 50 mM of phosphate and 50 mM of pyridoxal-HCl. The pellet (approx. 1 g wet weight from 300 ml) was taken up in 10 ml of buffer. S-cysteinyl-pulegone (0.3 g/l = 1.1 mM) was converted in the buffer with the concentrated cell Suspension of E.limosum described above (final concentration: 1.6 mg dry weight/ml). The reaction was carried out for 1 hour at 30°C and was terminated by centrifuging the reaction mixture for 5 minutes at 11,000 x g. As a control, two tests were carried out: a) As a control, boiled cells (denatured enzymes) were used in the test described above. b) In order to be able to assess whether the SH product (p-mentha8-thiol-3-one) had been converted by the S-methyl transferase into the S-methyl product (p-mentha-8-thiomethyl-3-one), the cells were also incubated with p-mentha-8-thio-3-one. The results of gas chromatography analysis of this example (sample no. 1) and the two control tests (samples 2 and 3) are shown in Appendix 2. To carry out the abovementioned gas chromatographic analysis, 1 part of chloroform (CHCl₃) was mixed with 1 part of the reaction mixture obtained. 1 µl of this extract was injected into a gas chromatograph having a 20 M carbowax column, (1.3 m RVS, column temperature: 145°C, injection port and TCD temperature: 160°C). Example II The method according to Example I was repeated, but with the difference that, instead of being carried out on a 1 ml scale, the test was carried out on a 10 ml scale. In this test, the cells were used in a double concentration, viz. 3.2 mg dry weight/ml and the incubation was carried out for 3 hours at 37°C. For a gas chromatographic analysis, a sample (sample B) was taken from this in the following manner. One part of dichloromethane (CH₂Cl₂) was mixed with 4 parts of the reaction mixture. 0.4 µl of this extract was injected into a Varian gas chromatograph in which a 10 % FFAP-chromosorb was provided in a WAW column (2m RVS, i.d. 1/8") (column temperature: 160°C; injection port and FID temperature: 180°C). As a comparison, in addition to the gas chromatogram of sample B shown in Appendix 3 as a control, the gas chromatograms of a) p-mentha-8-thiol-3-one, b) p-mentha-8-thiomethyl-3-one, c) pulegone, and d) S-cysteinyl-pulegone were recorded without cells being used at the same time. It follows from the chromatograms shown in Appendix 3, inter alia, that no detectable p-mentha-8-thiomethyl-3-one is formed (compare 3b with 3e). The pulegone peak in Figures 3d and 3e (retention time 2.7 min.) may be explained by the fact that some of the S-cysteinyl-pulegone dissolves in the extraction agent and is decomposed in the gas chromatograph (160°C). The chromatogram of chemically synthesized p-mentha-8-thiol-3-one (Fig. 3a) reveals an isomer ratio of approximately 2:1. The biologically prepared p-mentha-8-thiol-3-one (Fig. 3e) has a completely different ratio of the two isomers which is approximately 9:1. Example III A commercial cocoa mix was used to prepare two different batches of beverage. The first batch is evaluated without any further addition while p-mentha-8-thiol-3-one prepared according to Example II was added to the second batch in the ratio of 20 µg of said p-mentha-8-thiol-3-one to each kilo of cocoa beverage. The beverage containing p-mentha-8-thiol-3-one has a fuller and richer flavour comparing to the beverage without p-mentha-8-thiol-3-one.

Claims: Method for preparing thiol compounds, characterized in that cysteine is coupled by means of addition to a compound having the formula (R₁)(R₂)C=C(R₃)-Co-R₄ in which the symbols R₁-R₄ represent a hydrogen or an optionally saturated and/or heterogeneous hydrocarbon group or, together with the carbon atoms to which the symbols are bonded, form one or two, optionally saturated and/or heterogeneous hydrocarbon ring systems and that subsequently the cysteine conjugate obtained is reacted with β-lyase to form the relevant thiol compounds. The method according to Claim 1, characterized in that cysteine is coupled by means of addition to a compound having formula (R₁)(R₂)C=C(R₃)-CO-R₄ in which R₁ has the meaning stated in Claim 1, R₂ and R₃ represent a hydrogen atom or an alkyl group containing 1-3 carbon atoms and R₄ represents an optionally heterogeneous hydrocarbon group bonded via an -O- bridge and that subsequently the cysteine conjugate obtained is reacted with β-lyase to form the relevant thiol compounds. The method according to Claim 1, characterized in that cysteine is coupled by means of addition to a compound having the formula: in which the symbol R₅ represents a hydrogen atom, an alkyl group containing 1-24 carbon atoms or an alkaline ion and R₆ represents a group consisting of 1-7 monosaccharides selected from the group consisting of glucose, mannose, galactose, arabinose, fucose, xylose, rhamnose, uronic acids as well as the acetates, pyruvates, amines and sulfates derived therefrom and that subsequently the cysteine conjugate obtained is reacted with β-lyase to form the relevant thiol compounds. Method according to one or more of the Claims 1-3, characterized in that the cysteine conjugate is split by means of bacterial β-lyase. Method according to Claim 4, characterized in that β-lyase from Eubacterium limosum is used. Method according to Claims 1, 4, or 5, characterized in that β-lyase is used in the form of bacterial cells. Method according to one or more of the Claims 1-6, characterized in that the flavouring p-mentha-8-thiol-3 one is prepared starting from pulegone. Method according to one or more of the Claims 1-6, characterized in that the compound 16-mercapto-progesterone is prepared starting from 16-dehydroprogesterone. Flavour composition comprising an effective flavouring amount of one or more flavouring compounds of the formula in which the symbol R₅ represents an hydrogen atom, an alkyl group containing 1-24 carbon atoms or an alkaline ion and R₆ represents a group consisting of 1-7 monosaccharides selected from the group consisting of glucose, mannose, galactose, arabinose, fucose, xylose, rhamnose, uronic acids as well as the acetates, pyruvates, amines and sulfates derived therefrom, prepared according to Claim 3, together with customary ingredients.

Inventors: BERG JAN, KERKENAAR ANTONIUS, SCHMEDDING DIEDERIK JOHANNES M, BERG, JAN, KERKENAAR, ANTONIUS, SCHMEDDING, DIEDERIK JOHANNES MARIA
Applicants: TNO, NEDERLANDSE ORGANISATIE VOOR TOEGEPAST-NATUURWETENSCHAPPELIJK ONDERZOEK TNO
Application year: 1988
Main classification: C12P11
IPC codes: C12P11, C12P33, C12R1, C07C323, A23L1, C12P13