Title of Invention

A PROCESS FOR THE MANUFACTURE OF RETINYL ACYLATE

Abstract

A process for the manufacture of a retinyl acylate of the formula wherein R1 signifies optionally substituted C1-23-alkyl; C2-23-alkenyl featuring 1 to 5 double bonds; optionally aromatically substituted phenyl-C1-6-alkyl; or optionally substituted phenyl, is characterized by treating a compound of the formula wherein R1 has the significance given above and R2 signifies hydrogen or COR1, with an agent which is an acid anhydride or a complex of sulphur trioxide, as specified in the description, in the presence of dimethylformamide as a solvent and simultaneously as a weak base. In addition to dimethylformamide an aprotic organic solvent may optionally be present, particularly an aliphatic hydrocarbon, an optionally alkyl-substituted alicyclic hydrocarbon, an aromatic hydrocarbon, a halogenated aliphatic hydrocarbon, a nitrated aliphatic hydrocarbon, an aliphatic ether, a cyclic ether, an aliphatic nitrile, an aliphatic amine, an aliphatic or alicyclic amide, dimethyl sulphoxide, tetramethylene sulphone, or a mixture of one or more of such solvents. The products are useful as intermediates for the manufacture of compounds of the vitamin A group or in certain cases as the compounds themselves.

Full Text

The present invention concerns a novel process for the manufacture of cycloalkenyl-polyene esters, in particular retinyl acylates, i.e. 3,7-dimethyl-9-(2',6',6'-trimethylcyclohex-r-en-l'-yl)-nona-2,4,6,8-tetraenyl acylates, which are of commercial interest as intermediates for the manufacture of compounds of the vitamin A group or in certain cases as the compounds themselves. The process involves an elimination and an isomerization of the corresponding 3,7-dimethyl-6-hydroxy-9-(2',6',6'-trimethylcyclohex-r-en-r-yl)-nona-2,4,7-trienyl acylate or of a 6-acylated derivative thereof using certain acid anhydrides or complexes thereof previously unknown as agents for this purpose. Accordingly, the present invention provides a process for the manufacture of a retinyl acylate of the general formula I wherein R1 signifies optionally substituted C1-23-alkyl; C2-23-alkenyl featuring 1 to 5 double bonds; optionally aromatically substituted phenyl-C1-6-alkyl; or optionally substituted phenyl, characterized by treating a compound of the general formula wherein R has the significance given above and R signifies hydrogen or COR , with an agent which is an acid anhydride or a complex of sulphur trioxide and which is selected from trifluoroacetic anhydride; a C1-6-alkanesulphonic acid anhydride; trifluoro-methanesulphonic acid anhydride; optionally substituted benzenesulphonic acid anhydride; phosphorus pentoxide; sulphur trioxide; and a complex of sulphur trioxide with a tri(C1-6-alkyl)amine, with a nitrogen-containing heteroaromatic compound or with a di(C1-6-alkyl)formamide, in the presence of dimethylformamide as a solvent and simultaneously as a weak base. Under the term "C1-23-alkyl" or "C2-23'alkenyl featuring 1 to 5 double bonds" (significances of R ) there are to be understood, depending on the number of carbon atoms, not only straight-chain but also branched alkyl or alkenyl groups. Examples of C1. 23-alkyl groups are methyl, ethyl, propyl, pentyl, heptyl, undecyl, pentadecyl and heptadecyl, and examples of C2-23-alkenyl groups are 8-heptadecenyl and heptadeca-8,11-dienyl. The corresponding alkanoyl and alkenoyl groups (COR1) are acetyl, propionyl, butyryl, caproyl, capryl, dodecanoyl, palmitoyl and stearoyl and, respectively, oleoyl and linoleyl. An especially preferred meaning for R1 as C1.23-alkyl is methyl. In the case where C1-23-alkyl is substituted, the substituents are especially up to three C1.4-alkoxy groups, which can be in each case straight-chain or branched. Two or three alkoxy substituents may be the same or different. The term "optionally aromatically substituted phenyl" means that the phenyl group is either unsubstituted or substituted with one or more substituents selected from for example alkyl, alkoxy and nitro groups and halogen atoms. Said substituents are suitably up to three, and selected from 1 to 3 Ci.4-alkyl groups, 1 to 3 C1-.4-alkoxy groups, 1 to 2 nitro groups and 1 to 3 halogen atoms. This applies equally to the "optionally substituted phenyl" group. Any alkyl or alkoxy substituent with 3 or 4 carbon atoms can be straight-chain or branched, and any halogen atom can be fluorine, chlorine, bromine or iodine. In respect of the agents used in the process of the present invention for promoting the elimination and isomerization reaction involved, the C1-6-alkanesulphonic acid anhydride can be any one of methane-, ethane-, propane-, butane-, pentane- and hexane-sulphonic acid anhydride, of which those featuring an alkane moiety with three to six carbon atoms can have a straight-chain or branched alkane moiety. If "optionally substituted benzenesulphonic acid anhydride" is substituted, then the substituents are one or more selected from for example alkyl, alkoxy and nitro groups and halogen atoms, whereby the or each halogen atom can be fluorine, chlorine, bromine or iodine. Said substituents are suitably up to three, and selected from 1 to 3 C1-.4-alkyl groups, 1 to 3 C1.4-alkoxy groups, 1 to 2 nitro groups and 1 to 3 halogen atoms. Any alkyl or alkoxy group with three or four carbon atoms can be straight-chain or branched. In the case of a complex of sulphur trioxide with a tri(C1-6-alkyl)amine or a di(C1-6-alkyl)formamide the alkyl groups may be straight-chain or branched, and the same or different. Examples of such alkyl groups are methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert.butyl, n-pentyl and n-hexyl. Examples of the trialkylamines themselves - and at the same time preferred trialkylamines - are trimethylamine, triethylamine and N-ethyldiisopropylamine. An example of the dialkylformamides, and the preferred one, is dimethylformamide. In complexes of sulphur trioxide with a nitrogen-containing heteroaromatic compound such compounds are especially heterocycles featuring at least one ring nitrogen atom. Pyridine and pyridine derivatives, such as picoline and quinoline, are examples. Such nitrogen-containing heteroaromatic compounds can also be bonded to a polymeric carrier ("polymer-bound"). Pyridine is the most preferred of these compounds. An example of pyridine bonded to a polymeric carrier is poly-(4-vinyl-pyridine), the adduct of which with sulphur trioxide is commercially available. The use of a complex of sulphur trioxide as the agent in the process of the present invention is advantageous by virtue of its simple production from the educts sulphur trioxide and the nitrogen containing compound as well as the easier handling compared with the aggressive sulphur trioxide when this is used alone. The complexes are in part known and in some cases are commercially available. They can be produced readily by introducing sulphur trioxide into the diluted trialkylamine, nitrogen-containing heteroaromatic compound or dialkylformamide, methylene chloride, for example, being used as the diluent. Preferred agents for use in the process of the present invention are phosphorus pentoxide, sulphur trioxide and complexes of sulphur trioxide. The process in accordance with the present invention is effected in the presence of dimethylformamide, which acts simultaneously as a solvent and a weak base. However, an additional solvent can be present which is generally an aprotic organic solvent. Said solvent may be a polar or an apolar aprotic organic solvent and is suitably an aliphatic hydrocarbon with 5 to 10 carbon atoms, such as pentane, hexane, heptane or octane; an optionally alkyl-substituted alicyclic hydrocarbon with up to 10 carbon atoms, such as cyclohexane, methylcyclohexane or decalin; an aromatic hydrocarbon, such as benzene or toluene; a halogenated aliphatic hydrocarbon, such as methylene chloride, chloroform, carbon tetrachloride or dichloroethane; a nitrated aliphatic hydrocarbon, such as nitro-methane; an aliphatic ether, such as diethyl ether, diisopropyl ether, tert. butyl methyl ether or 1,2-dimethoxyethane; a cyclic ether, such as tetrahydrofuran, methylfuran or 1,4-dioxan; an aliphatic nitrile, such as acetonitrile; an aliphatic amine, such as triethylamine or N-ethyldiisopropylamine; an aliphatic or alicyclic amide (in addition to dimethyl-formamide itself), such as N,N-dimethylacetamide or l-methyl-2-pyrrolidone, respectively; dimethyl sulphoxide; tetramethylene sulphone (sulfolane); or a mixture of one or more of the aforementioned solvents. Preferred additional solvents are aliphatic ethers, cyclic ethers, especially tetrahydrofuran, and dimethyl sulphoxide. If a mixture of dimethylformamide with an aprotic organic solvent is used, the volume of the dimethylformamide in relation to the total volume of the solvent mixture is conveniently at least about 10 % of the whole, and preferably at least about 20 %. Preferred solvent mixtures are those containing dimethylformamide and, as the aprotic organic solvent, tetrahydrofuran or dimethyl sulphoxide. The amount of agent used in accordance with the process of the present invention for promoting the elimination and isomerization reaction involved is suitably from about 1 to about 3 molar equivalents per molar equivalent of starting material of formula II used. Preferably this amount is about 2 to about 3 molar equivalents. The amount of dimethylformamide used in relation to the amount of starting material of formula II is conveniently about 1 to about 10 litres per mole. The process of the present invention is conveniently effected at temperatures from about -50°C to about +50°C, preferably in the temperature range from about -35°C to about +20°C. The process of the present invention is conveniently effected by adding the agent which promotes the elimination and isomerization to the starting material of formula II in the dimethylformamide or solvent mixture containing dimethylformamide at the desired temperature. During the addition and the subsequent reaction the reaction mixture is conveniently stirred. Furthermore, the addition and subsequent reaction is conveniently carried out under an inert gas atmosphere, such as under nitrogen or argon. Periodic checks of the progress of the reaction may be made using such known analytical techniques as reversed phase high pressure liquid chromatography (RP-HPLC) and normal phase HPLC (NP-HPLC). After completion of reaction, which normally is achieved within 20 hours, more especially within 10 hours, the mixture is conveniently quenched with an organic base, such as a trialkylamine, preferably triethylamine, or a-solid or aqueous inorganic base. If desired, the quenched mixture is held for several hours, and subsequently extracted, e.g. with cold hexane, to isolate the product, which may then be purified, as necessary, using conventional methodology. The starting material of formula II may be used in the process of the present invention as a single compound of formula II or as a mixture of two or more such compounds, both in respect of the isomeric form and in respect of the significance of R" (hydrogen or COR'). For example, a mixture of a compound of formula II in which R signifies hydrogen with one in which R1 signifies C0R1e.g. acetyl, may be used as the starting material. In principle the starting material of formula II may be in any isomeric form, and in practice the starting materials utilized feature various combinations of 2(Z)-, 4(Z)-, 7(Z)-, 2(E)-, 4(E)- and 7(E)-configurated double bonds as well as of 6(S)- and 6(R)-configurated carbon atoms (the 6-carbon atom bears the hydroxyl or acyloxy group OR ). In view of the availability of the starting materials normally as mixtures of various isomeric forms or in particular isomeric forms, e.g. featuring 2(Z)-, 4(Z)- and 7(E)-configurated double bonds, such mixtures or particular isomeric forms are generally employed. The (all-E) isomers are more desirably used as the starting materials if readily available. However, an advantage of the process of the present invention resides in the result that whatever isomeric form of the starting material of formula II is used the produced retinyl acylate of formula I features a high proportion of the most desirable (all-E) isomer. In this connection it has been established that the temperature at which the process is performed exerts a significant influence on the selectivity in favour of said (all-E) isomer: in general, the lower the temperature, the higher the proportion of (all-E) isomer is produced in any instance. 16 hours. After that reaction time the yield of (all-E)-vitamin A acetate was calculated on the basis of RP-HPLC to be 917.6 mg (94.9% yield based on the amount of starting material used; content: 83.8% all-E, 15.3% 2Z, 1.0% 2Z,4Z). The mixture was then quenched by adding rapidly 3.0 ml (21.52 mmol) of triethylamine at 0°C, left to stir (350 rpm) at 0°C for 15 minutes and then poured into approx. 70 ml of n-hexane and rapidly extracted. The layer separation was effected rapidly in order to keep the dimethyl-formamide phase cold during the extraction. This process was repeated six times with a total volume of approx. 450 ml of n-hexane. To ensure complete extraction, the remaining dimethylformamide phase was analysed by RP-HPLC. The combined extracts were then concentrated in vacuo at 40-45' The residue (approx. 10 ml; n-hexane takes approx. 3% (v/v) dimethylformamide during extraction) still containing dimethylformamide was brought to exactly 20 ml with dimethylformamide and the (all-E)-vitamin A acetate content was checked (calibrated RP-HPLC). The yield of (all-E)-vitamin A acetate was 880.2 mg (91.0% yield based on the amount of starting material used; content: 83.6% all-E, 15.4% 2Z, 1.0% 2Z,4Z). A UV determination at 325 nm in n-hexane confirmed this result. The dimethylformamide was then removed by evaporation in vacuo at 50°C under light protection to yield 966.8 mg (99.9% w/w from starting material, 967.0 mg = 100% w/w) of product as a yellow oil. Example 2 Synthesis of (all-E)-3,7-dimethvl-9-(2',6',6'-trimethvlcvclohex-l'-en-r-vl l)-nona-2.4.6.8-tetraenyl acetate [(all-E)-vitamin A acetate] from (2Z.4Z)-3,7-dimethvl-6-hvdroxv^-9-(2\6',6'-trimethylcyclohex-r-en-r-yl)-nona-2,4,6>8--tetraenyl acetate In a 100 ml round-bottomed flask equipped with a magnetic bar stirrer and flushed with argon were introduced 1020 mg (2.94 mmol) of (2Z,4Z)-3,7-dimethyl-6-hydroxy-9-(2',6',6'-trimethylcyclohex-l'-en-l'-yl)-nona-2,4,6,8-tetraenyl acetate and 30 ml of dimethylformamide. The stirred (350 rpm) homogeneous solution was cooled down to 0°C (ice bath), and 900.0 mg (5.88 mmol) of l:l-dimethylformamide-sulphur trioxide-complex were rapidly added to the mixture. Then the reaction mixture was stirred (350 rpm) at 0°C (ice bath) for about 20 hours. After that reaction time the yield of (all-E)-vitamin A acetate was calculated on the basis of RP-HPLC to be 926.0 mg (95.8% yield based on the amount of starting material used; content: 83.0% all-E, 15.7% 2Z, 1.2% 2Z,4Z). The mixture was then quenched by adding rapidly 2.0 ml (14.35 mmol) of cold triethylamine at 0°C, left to stir (350 rpm) at 0°C for 15 minutes and then poured into approx. 70 ml of n-hexane and rapidly extracted. The layer separation was effected rapidly in order to keep the dimethylformamide phase cold during the extraction. This process was repeated six times with a total volume of approx. 450 ml of n-hexane. To ensure complete extraction, the remaining dimethylformamide phase was analysed by RP-HPLC. Example 4 Synthesis of (all-E)-3J-dimethvl-9-(2\6\6'-trimethvlcvclQhex-l'-en-l'-vl)-nona-2 tetraenyl acetate [(all-E)-vitamin A acetate] from (2Z,4Z)-3 J-dimethyl-6-hydroxv-9-(2',6\6'-trimethylcyclohex-r-en-r-yl)-nona-2,4,6.8-tetraenyl acetate In a 100 ml round-bottomed flask equipped with a magnetic bar stirrer and flushed with argon were introduced 1020 mg (2.94 mmol) of (2Z,4Z)-3,7-dimethyl-6-hydroxy-9-(2',6',6'-trimethylcyclohex-r-en-r-yl)-nona-2,4,6,8-tetraenyl acetate and 30 ml of dimethylformamide. The stirred (350 rpm) homogeneous solution was cooled down to -45 to -50°C (ice bath), and 534 |il (913.6 mg, 3.24 mmol) of trifluoromethanesulphonic acid anhydride were added to the mixture within 3-5 minutes. Then the reaction mixture was stirred (350 rpm) at -45 to -50°C for about 6 hours. After that reaction time the yield of (all-E)-vitamin A acetate was calculated on the basis of RP-HPLC to be 908.0 mg (93.9% yield based on the amount of starting material used; content: 88.3% all-E, 11.3% 2Z, 0.4 % 2Z,4Z). The mixture was then quenched by adding rapidly 1 ml (7.17 mmol) of triethylamine at-45 to -50°C, well mixed and left to stir (350 rpm) at 0°C for 20 minutes (until the internal temperature reached 0°C) and then poured into approx. 70 ml of n-hexane and rapidly extracted. The layer separation was effected rapidly in order to keep the dimethylformamide phase cold during the extraction. This process was repeated six times with a total volume of approx. 450 ml of n-hexane. To ensure complete extraction, the remaining dimethylformamide phase was analysed by RP-HPLC. The combined extracts were then concentrated in vacuo at 40-45°C. The residue (approx. 10 ml; n-hexane takes approx. 3% (v/v) dimethylformamide during extraction) still containing dimethylformamide was brought to exactly 20 ml with dimethylformamide and the (all-E)-vitamin A acetate content was checked (calibrated RP-HPLC). The yield of (all-E)-vitamin A acetate was 840.8 mg (87.0% yield based on the amount of starting material used; content: 88.0% all-E, 11.5% 2Z, 0.5% 2Z,4Z). A UV determination at 325 nm in n-hexane confirmed this result. The dimethylformamide was then removed by evaporation in vacuo at 50°C under light protection to yield 962,8 mg (99.6% w/w from starting material, 967.0 mg = 100% w/w) of product as a yellow oil. Claims 1. A process for the manufacture of a retinyl acylate of the general formula wherein R1 signifies optionally substituted C1-C23-alkyl; C2-23-alkenyl featuring 1 to 5 double bonds; optionally aromatically substituted phenyl-C1-6-alkyl; or optionally substituted phenyl, characterized by treating a compound of the general formula wherein R1 has the significance given above and R1 signifies hydrogen or C0R' with an agent which is an acid anhydride or a complex of sulphur trioxide and which is selected from trifluoroacetic anhydride; a C1-6-alkanesulphonic acid anhydride; trifluoro-methanesulphonic acid anhydride; optionally substituted benzenesulphonic acid anhydride; phosphorus pentoxide; sulphur trioxide; and a complex of sulphur trioxide with a tri(C1-6-alkyl)amine, with a nitrogen-containing heteroaromatic compound or with a di(C1-6-aIkyl)formamide, in the presence of dimethylformamide as a solvent and simultaneously as a weak base. 2. The process according to claim 1, characterized in that R1 signifies methyl. 3. The process according to claim 1 or claim 2, characterized in that the agent is phosphorus pentoxide, sulphur trioxide or a complex of sulphur trioxide. 4. The process according to any one of claims 1 to 3, characterized in that in addition to dimethylformamide an aprotic organic solvent is present. 5. The process according to claim 4, characterized in that the aprotic organic solvent is an aliphatic hydrocarbon with 5 to 10 carbon atoms; an optionally alkyl-substituted alicyclic hydrocarbon with up to 10 carbon atoms; an aromatic hydrocarbon; a halogenated aliphatic hydrocarbon; a nitrated aliphatic hydrocarbon; an aliphatic ether; a cyclic ether; an aliphatic nitrile; an aliphatic amine; an aliphatic or alicyclic amide; dimethyl sulphoxide; tetramethylene sulphone; or a mixture of one or more of the aforementioned solvents. 6. The process according to claim 5, characterized in that the aprotic organic solvent is an aliphatic ether, a cyclic ether, especially tetrahydrofuran, or dimethyl sulphoxide. 7. The process according to any one of claims 4 to 6, characterized in that the volume of the dimethylformamide in relation to the total volume of the solvent mixture of dimethylformamide and aprotic organic solvent is at least about 10 % of the whole, and preferably at least about 20 %. 8. The process according to any one of claims 1 to 7, characterized in that the amount of agent used is from about 1 to about 3 molar equivalents per molar equivalent of starting material of formula II used, preferably about 2 to about 3 molar equivalents. 9. The process according to any one of claims 1 to 8, characterized in that the amount of dimethylformamide used in relation to the amount of starting material of formula II is about 1 to about 10 litres per mole. 10. The process according to any one of claims 1 to 9, characterized in that the process is effected at temperatures from about -50°C to about +50°C, preferably in the temperature range from about -35° C to about +20°C.

Documents:

142-mas-2000-abstract.pdf

142-mas-2000-claims.pdf

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142-mas-2000-description complete.pdf

142-mas-2000-form 1.pdf

142-mas-2000-form 26.pdf

142-mas-2000-form 3.pdf

142-mas-2000-form 5.pdf

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