PROCESS FOR ISOMERIZING A PENT-1-EN-3-OL

The present invention relates to a process for isomerizing a pent-l-en-3-ol to a mixture of the isomers Z-pent-2-en-l-ol and E-pent-2-en-l-ol.

Allyl alcohols, especially tertiary allyl alcohols, are important intermediates in industrial organic chemistry. It is known that allyl alcohols isomerize with acid catalysis. This isomerization corresponds to a 1,3 migration of the hydroxyl group and a corresponding shift of the double bond. The migration of a double bond and of a substituent is known for allyl compounds and is generally referred to as an allylic rearrangement. Allylic rearrangements of allyl alcohols are equilibrium reactions.

A tertiary allyl alcohol of particular interest in industrial organic chemistry is 3- methylpent-l-en-4-yn-3-ol. Its allylic rearrangement results in about 85 % of Z-3- methylpent-2-en-4-yn-l-ol and about 15 % of E-3-methylpent-2-en-4-yn-l-ol. The Z- isomer is a useful intermediate, e.g. for the manufacture of Vitamin A, and the E-isomer is also a useful intermediate, e.g. for the manufacture of astaxanthin, zeaxanthin and further carotenoids. The isomers may be separated from each other by physical means, e.g. by fractional distillation.

EP-A-O 860 415 generally describes the isomerization of allyl alcohols to the Z- and E- isomers. The isomerization reaction is performed in an aqueous solution in the presence of protonic acids at a pH in the range of from 2 to 5. The process is predominantly used to convert primary or secondary allyl alcohols to tertiary allyl alcohols. It is further described that a solvent may be present, e.g. in case an allyl alcohol of low solubility in water is used. However, the solvent must be miscible with water in the mixing ratio employed.

This method does not seem appropriate to isomerize 3-methylpent-l-en-4-yn-3-ol as this is a tertiary allyl alcohol.

Czech patent application publication no. 269 025 is concerned with a method for producing Z-3-methylpent-2-en-4-yn-l-ol by means of the acidically catalyzed allylic rearrangement of 3-methylpent-l-en-4-yn-3-ol. The isomerization is conducted in an organic solvent in the presence of a strongly acidic cation exchanger swollen by water. The organic solvent employed is an aromatic hydrocarbon, e.g. benzene, or a halogenated aliphatic hydrocarbon. However, the reaction is performed at incomplete conversion of no more than 70 % and the maximum yield of 3-methylpent-2-en-4-yn-l-ol reported in the examples is only about 52 %.

In Chem. Papers 43 (6) 743-751 (1989) A. Skoda et al. report on a kinetic study of the allyl rearrangement of 3-methylpent-l-en-4-yn-3-ol catalyzed by sodium hydrogensulfate in aqueous solution. This publication only describes scientific experiments and does not give an hint on a possible realization of a corresponding technical process for large scale production. Moreover, the separation of the catalyst for further reuse is difficult.

It is the object of the present invention to provide an alternative process for isomerizing a pent-l-en-3-ol, especially 3-methylpent-l-en-4-yn-3-ol, to a mixture of Z-pent-2-en-l-ol and E-pent-2-en-l-ol, which process provides the isomers in good yields at high conversion and/or with high selectivity.

The object is met by a process for isomerizing a pent-l-en-3-ol according to formula (1)

to a mixture of the stereoisomers Z-pent-2-en-l-ol (2) and E-pent-2-en-l-ol (3) according to formulae (2) and (3)

wherein R ¹ is selected from alkyl, aryl, and alkylaryl radicals, R ² is selected from H, alkyl, and aryl radicals; which process comprises reacting the pent-l-en-3-ol (1) in a multiphase system comprising an aqueous phase and an organic solvent phase, and in the presence of an acid catalyst which is not a cation exchanger, wherein the organic solvent phase comprises a water-immiscible organic solvent.

The present invention is also directed to a process of preparing an isoprenoid selected from vitamin A, vitamin A derivatives, and carotenoids, which process comprises isomerizing a pent-l-en-3-ol as described above, isolating the Z-pent-2-en-l-ol and E- pent-2-en- 1 -ol, and converting the Z-pent-2-en- 1 -ol or E-pent-2-en- 1 -ol to the corresponding isoprenoid.

The formulae throughout the present application are represented by conventional line representation.

The present invention further relates to a process of preparing an isoprenoid selected from vitamin A, vitamin A derivatives, and carotenoids, which process comprises isomerizing 3-methylpent-l-en-4-yn-3-ol as described above, isolating the Z-3-methylpent-2-en-4-yn- l-ol and E-3-methylpent-2-en-4-yn-l-ol, and converting the Z-3-methylpent-2-en-4-yn-l- ol or E-3-methylpent-2-en-4-yn-l-ol to the corresponding isoprenoid.

According to a preferred embodiment, R ¹ in formulae (1), (2) and (3) is selected from Ci to Cj ₅ alkyl radicals, preferably Ci to C ₅ alkyl radicals, and a phenyl radical; R ² is selected

from H, Ci to Ci ₅ alkyl radicals, preferably Ci to C ₅ alkyl radicals, and a phenyl radical. According to the most preferred embodiment, R ¹ is a methyl radical and R ² is H. In this case, the pent-l-en-3-ol is 3-methylpent-l-en-4-yn-3-ol according to formula (4)

and the corresponding isomers are Z-3-methylpent-2-en-4-yn-l-ol according to formula (5) and E-3-methylpent-2-en-4-yn-l-ol according to formula (6)

Contrary to the isomerization reaction disclosed in EP-A-O 860 415 which is conducted in the aqueous phase, the present process involves reacting the pent-l-en-3-ol in a multiphase system comprising an aqueous phase and an organic solvent phase. Preferably, the multiphase system is a two-phase system.

The organic solvent may be any water-immiscible organic solvent. Mixtures of various water-immiscible organic solvents may also be employed. The term "water-immiscible" solvent means any solvent that maintains during the isomerization reaction a distinct organic solvent phase in addition to the aqueous phase. Typically, the water-immiscible solvent is miscible in water to an extent of less than 3 g per 100 g of water.

Preferably, the water-immiscible organic solvent is a non-halogenated organic solvent. More preferably, the water-immiscible organic solvent is toluene or a non-aromatic organic solvent. In one embodiment the non-aromatic organic solvent may be selected from ethers, ketones, saturated aliphatic hydrocarbons and mixtures thereof. Exemplary organic solvents include diisopropylether, diethyl ether, tetrahydrofuran, methyl isobutyl ketone, hexane, heptane, and paraffin oil. Preferably, the non-aromatic organic solvent is

selected from diisopropylether, methyl isobutyl ketone, and paraffin oil. Most preferably, the organic solvent is diisopropylether or toluene.

The type of acid catalyst used in the present process is not critical. Mixtures of various acid catalysts may also be used. The acid catalyst may be a Brønsted acid or a Lewis acid.

Preferably, the Brønsted acid for use in the present invention has a pk _s value of less than 4, more preferably of less than 2, even more preferably of less than 1.5 and most preferably of less than 1. The Brønsted acid may be an inorganic acid or organic acid. Examples of Brønsted acids that are useful in the present process are sulfuric(IV) acids; perchloric acid; phosphoric acids, preferably orthophosphoric acid; heteropoly acids having the "Keggin structure" according to the general formula H _8-n [Y ⁿ (M ₃ Oi ₀ ) ₄ ] wherein M is Mo or W; n is the valence of Y, and Y is selected from B ^UI , Si™, Ge™, P ^v , As ^v , A ¹¹¹ , Fe ¹¹¹ , Co ¹¹ , Co ¹¹¹ , Cu ¹ , Cu ¹¹ , Zn", Cr ¹¹¹ , Mn™, Ti™, Zr™, preferably Y is Si™ or P ^v , e.g. molybdatophosphoric acid H ₃ [P(Mo ₃ Oi _O ) ₄ ] (H ₂ O) _x , molybdatosilicic acid H ₄ [Si(Mo ₃ Oi _O ) ₄ ] -(H ₂ O) _x ; tungstophosphoric acid H ₃ [P(W ₃ Oi _O ) ₄ ] (H ₂ O) _x , tungstosilicic acid H ₄ [Si(W ₃ Oi _O ) ₄ KH ₂ O) _x with x being 1 to 100, typically 5 to 50; sulfonic acids, e.g. methanesulfonic acid, toluenesulfonic acid, preferably para-toluenesulfonic acid (p-TsOH), trifluormethane sulfonic acid, methane trisulfonic acid (SO ₃ H) ₃ CH, methane trisulfonic acid methyl ester (CH ₃ SO ₃ ) ₃ CH; oxalic acid, hydrogen bis(oxalato)borate, hydrogen tris(oxalato)phosphate.

Preferably, the Brønsted acid is selected from perchloric acid and sulfuric(VI) acids, more preferably, the Brønsted acid is sulfuric acid.

Examples of Lewis acids that are useful in the present process are vanadium oxotrichloride VOCl ₃ , (THF) ₃ VCl ₃ , and VCl ₃ , vanadium oxotrichloride being preferred.

According to the most preferred embodiment, sulfuric acid is used as the acid catalyst in the present process.

Although it is not mandatory, a stabilizer, such as hydroquinone, may be added to the reaction mixture.

Typically, the molar ratio of acid catalyst to pent-l-en-3-ol educt ranges from 0.001 : 1 to 10:1, depending on the type of pent-l-en-3-ol and type of acidic catalyst used. In case sulfuric acid is used as the acid catalyst, the molar ratio of acid catalyst to pent-l-en-3-ol educt typically ranges from 0.1 : 1 to 10: 1 , preferably from 0.15: 1 to 5: 1 , more preferably from 0.2:1 to 2.5:1 and most preferably from 0.2:1 to 1:1. In case of a Lewis acid catalyst, such as vanadium oxotri chloride, it is typically used in a molar ratio of acid catalyst to pent-l-en-3-ol ranging from 0.01:1 to 10:1, preferably from 0.02:1 to 5:1 more preferably from 0.03:1 to 3:1 and most preferably from 0.04: 1 to 1 : 1. In case a Brønsted acid which is not sulfuric acid is used as the acid catalyst, the molar ratio of acid catalyst to pent-l-en-3-ol typically ranges from 0.001 :1 to 1 : 1 , preferably from 0.01 :1 to 0.5:1 more preferably from 0.015:1 to 0.3:1, and most preferably from 0.02: 1 to 0.2: 1.

Typically, the acid catalyst is dissolved in water and added to the process as an aqueous solution. Its concentration in water may vary considerably depending on the type of acid catalyst used and it is within the ordinary skill of the expert to select the appropriate concentration. In case sulfuric acid is used as the acid catalyst its concentration in water typically ranges from 3 to 73 % by weight, preferably from 10 to 35 % by weight, more preferably from 12 to 30 % by weight, and most preferably from 15 to 25 % by weight. Compared to the prior art methods the separation and recycling of the catalyst is much more convenient in the present process as the catalyst largely stays in the aqueous phase of the multiphase system.

Usually, the pent-l-en-3-ol is soluble in the water-immiscible non-aromatic non- halogenated organic solvent and thus, it is added to the process as a solution in the organic solvent. In one embodiment, the concentration of the pent-l-en-3-ol in the organic solvent ranges from 5 to 80 % by weight, preferably from 15 to 60 % by weight, more preferably from 20 to 50 % by weight, and most preferably from 25 to 45 % by weight.

If used, the stabilizer, e.g. hydroquinone, is typically added in an amount of from 1 to 10,000 ppm by weight, preferably from 5 to 1,000 ppm by weight, more preferably from 7 to 500 ppm by weight, and most preferably from 10 to 100 ppm by weight, each based on the total weight of the multiphase system.

The various components may be added in any order. For example, the aqueous solution of the acid catalyst may be loaded into the reaction vessel and then the solution of the pent-1-en- 3-ol in the organic solvent may be added or vice versa. It is also possible to divide the total amount of a component to be added and add the partial amounts at various stages in the process.

In order to ensure thorough mixing of the reaction components the reaction mixture is typically agitated, e.g. stirred. Preferably, the isomerization reaction is conducted in a stirred tank reactor.

The isomerization reaction is typically conducted at a temperature in the range of from 20 to 80°C, depending on the type of the reaction components used and the other reaction conditions applied. At atmospheric pressure the reaction temperature preferably ranges from 30 to 70 ⁰ C, more preferably from 35 to 65 ⁰ C, and most preferably from 40 to 60 ⁰ C.

Although the isomerization reaction is typically conducted at atmospheric pressure, it may also be conducted at subatmospheric or superatmospheric pressure.

Depending on the nature of the reaction components and the other reaction conditions applied, the reaction time of the isomerization is typically from 1 to 25 h. At a typical temperature range the reaction time is preferably from 1.5 to 8 h, more preferably from 2 to 6 h, and most preferably from 2 to 5 h.

Typically, the process according of the present invention is conducted at a pent-l-en-3-ol conversion of at least 60 %, preferably at least 71 %, more preferably at least 80 %, even more preferably at least 85 % and most preferably at least 90 %.

In a preferred embodiment of the present invention 3-methylpent-l-en-4-yn-3-ol is isomerized to a mixture of Z-3-methylpent-2-en-4-yn-l-ol and E-3-methylpent-2-en-4-yn-l- ol using the reaction components, amounts and reaction conditions described above, including all the preferred embodiments.

In a more preferred embodiment of the present invention 3-methylpent-l-en-4-yn-3-ol is isomerized to a mixture of Z-3-methylpent-2-en-4-yn-l-ol and E-3-methylpent-2-en-4-yn-l- ol employing diisopropylether as the solvent and using the other reaction components, amounts and reaction conditions described above, including all the preferred embodiments.

In another more preferred embodiment of the present invention 3-methylpent-l-en-4-yn-3-ol is isomerized to a mixture of Z-3-methylpent-2-en-4-yn-l-ol and E-3-methylpent-2-en-4-yn- l-ol employing sulfuric acid as the acid catalyst and using the other reaction components, amounts and reaction conditions described above, including all the preferred embodiments.

In an even more preferred embodiment of the present invention 3-methylpent-l-en-4-yn-3- ol is isomerized to a mixture of Z-3-methylpent-2-en-4-yn-l-ol and E-3-methylpent-2-en-4- yn-l-ol employing diisopropylether as the solvent and sulfuric acid as the acid catalyst and using the other reaction components, amounts and reaction conditions described above, including all the preferred embodiments.

The process of the present invention may further comprise the isolation of the Z-pent-2- en-l-ol and E-pent-2-en-l-ol. The mixture of the isomers obtained from the isomerizing step may be separated into the Z- and E-isomers according to conventional separation methods. Typically, the isolation comprises one or more of the following separation steps: extraction, distillation, including fractionated distillation and rectification.

The reaction mixture obtained from the isomerization step comprises an aqueous phase and an organic solvent phase, the organic solvent phase containing the major amount of both of the isomers. Thus, a phase separation step typically will be the first step of the

separation procedure. As the organic solvent phase usually still comprises minor amounts of the acidic catalyst, neutralization of the organic phase is typically performed prior to or after the phase separation step. Neutralization is achieved by adding an appropriate base, e.g. sodium carbonate.

The further working-up of the organic solvent phase and recovery of the distinct Z- and E- isomers may be carried out as it is known in the art. Typically, the lower boiling components (e.g. organic solvent and unreacted pent-l-en-3-ol) and higher boiling components will be separated from the isomer mixture in one or more, preferably at least two devolatilization and/or distillation steps. Typically, the Z-pent-2-en-l-ol is then separated from the E-pent-2-en-l-ol by one or more final rectification steps.

The present invention also relates to a process of preparing an isoprenoid selected from vitamin A, vitamin A derivatives, and carotenoids, which process comprises isomerizing a pent-l-en-3-ol as described above, isolating the Z-pent-2-en-l-ol and E-pent-2-en-l-ol, and converting the Z-pent-2-en-l-ol or E-pent-2-en-l-ol to the corresponding isoprenoid.

Z-3-methylpent-2-en-4-yn-l-ol obtained in major amounts by a preferred embodiment of the present isomerization reaction is an intermediate product for the syntheses of vitamin A or vitamin A derivatives. It can be converted to vitamin A or a vitamin A derivative by various process steps well known to a person skilled in the art. One of the most economically successful processes for the preparation of vitamin A and its derivatives is the Isler synthesis of 1948. The conversion of Z-3-methylpent-2-en-4-yn-l-ol to vitamin A or a vitamin A derivative is for example described in US-A-2,451,739 to Otto Isler. Z-3- methylpent-2-en-4-yn-l-ol is coupled with the Ci ₄ component 2-methyl-4-(2,6,6-trimethyl- l-cyclohexen-l-yl)-2-buten-l-al via a Grignard reaction to result in l-hydroxy-3,7- dimethyl-6-hydroxy-9-trimethyl-cyclohexenyl-nonadiene-(2,7)- yne (oxenyne). The oxenyne is first subjected to partial hydrogenation at the triple bond, preferably Lindlar hydrogenation; then esterification at the terminal hydroxyl group, preferably acetylation with acetic anhydride; followed by dehydration and allylic rearrangement. The resulting crude vitamin A ester, preferably vitamin A acetate, is then purified, preferably by

crystallization. Thereafter, the vitamin A ester may be further reacted to obtain the desired vitamin A derivative, e.g. it may be hydrolyzed to obtain vitamin A.

E-3-methylpent-2-en-4-yn-l-ol obtained in minor amounts by the preferred embodiment of the present isomerization reaction also is a useful intermediate. It may be used to synthesize astaxanthin, zeaxanthin and further carotenoids. The conversion from E-3- methylpent-2-en-4-yn-l-ol to the desired carotenoid is carried out in various process steps as it is well known to the person skilled in the art. For example, E-3-methylpent-2-en-4-yn- l-ol is the C ₆ starting component in the synthesis of astaxanthin wherein E-3-methylpent-2- en-4-yn- 1 -ol which is optionally protected is first reacted with a Cg component, two equivalents of the resulting Ci ₅ components are then reacted with a Ci ₀ component to form astaxanthin (C ₄₀ ). An illustrative synthesis of astaxanthin is described in EP-A-O 005 748. A different synthesis of astaxanthin also using E-3-methylpent-2-en-4-yn-l-ol as starting C ₆ component is taught in CN-A-166 803. E-3-methylpent-2-en-4-yn-l-ol may also be used as the C ₆ starting component in the synthesis of zeaxanthin: An exemplary technical preparation of zeaxanthin according to a 2(C ₉ + C ₆ ) + Ci ₀ = C ₄₀ construction wherein the E-3- methylpent-2-en-4-yn-l-ol is employed in its IPM-protected form is described by E. Widmer et al. in HeIv. Chim. Acta 1990, 73, 861.

The invention will now be further illustrated in the following non-limiting examples.

EXAMPLES

In the following examples the terms "yield" and "selectivity" both refer to the sum of the desired products, i.e. the sum of the Z- and E-isomers.

Example 1

15 g OfH ₂ SO ₄ (97 %, 148.3 mmol) in 68.3 g of H ₂ O were placed in a 350 ml 4-necked flask equipped with a stirrer, temperature measurement device, condenser, argon atmosphere, and oil bath. 43.33 g of 3-methylpent-l-en-4-yn-3-ol (450.8 mmol) dissolved in 83.3 g of diisopropylether were added under stirring at 25°C. After addition of 0.017 g (0.15 mmol) of hydroquinone the reaction mixture was heated up to 52°C within 15 min and stirred for 6 h. After cooling to 25°C the phases were separated, the organic layer was washed with 5 ml of saturated sodium carbonate until pH 8 was reached, and concentrated at 15 mbar, 40°C. 42 g of crude product were obtained as orange brown oil, containing 75.4 % of Z-3-methylpent-2-en-4-yn-l-ol and 14.7 % of E-3-methylpent-2-en-4-yn-l-ol (conversion 99.45 %, yield 87.3 %, selectivity 88 %). The crude product was purified by distillation in a 100 ml three-necked round bottom flask equipped with magnetic stirrer, temperature measurement device, and 15 cm Vigreux column combined with a Liebig condenser (internal temperature 54°C-78°C, head temperature 37°C, 3.3 mbar). Z-3- methylpent-2-en-4-yn-l-ol (31.52 g) and E-3-methylpent-2-en-4-yn-l-ol (6.03 g) were obtained as slightly yellow oil in 86.65 % yield. The purity of the products was 100 %, analyzed by gas chromatography.

Examples 2 to 10

The isomerization reactions of the Examples 2 to 10 were carried out according to the above procedure with the modifications reported in the Table below. The experiments were carried out without final purification.

Table

s/c molar ratio of substrate (3-methylpent-l-en-4-yn-3-ol) to catalyst w/w weight ratio of organic solvent to water temp, reaction temperature ret. time reaction time conv. conversion ether diisopropylether MIBK methyl isobutyl ketone